ACS1790T
PLL Frequency Synthesizer with Integrated VCO
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ADVANCED COMMUNICATIONS PRODUCT GROUP
Introduction
Features
The ACS1790T is a high-performance, low phase noise,
programmable frequency synthesizer with ultra-fine
dynamic output frequency control. It can be used as a
companion to a compatible ToPSync™ device to
generate outputs locked to an external reference source
or standalone for standard frequency generation for
common applications. The circuit includes an integrated
VCO, loop filter, phase-and-frequency detector and
output dividers. The default power-up mode is definable
by pin settings and once operational the ACS1790T is
fully-programmable via an I2C interface. The ACS1790T
requires only a low-speed external clock input for
operation.
When used in conjunction with a compatible ToPSync®
device in a Synchronous Ethernet system the ACS1790T
allows the Ethernet transmit clocks to be derived initially
from the local reference oscillator and subsequently
frequency-locked to an external reference source once
the system is fully configured, simplifying system design
and reducing component count and BOM cost.
High frequency LVPECL output:
10 MHz – 200 MHz, 1 ppb step
Low frequency LVCMOS output:
2 kHz – 125 MHz
1.8V, 2.5V and 3.3V operation
Very-low frequency feedback clock output for
connection to ToPSync® or external PFD
Tunable over +/- 500 ppm range without loss of
lock
Integrated VCO, PFD and loop filter
2.3 – 2.7 GHz VCO frequency
Typical RMS jitter performance for target
application masks:
OC-48, STM-16 (ANSI T1.105.03 & ITU-T G.813)
0.56 ps (12 kHz – 20 MHz)
1G Ethernet (IEEE 802.3-2008 38 & 39)
0.15 ps (637 kHz – 20 MHz)
XAUI (IEEE 802.3-2008 Clause 47)
0.11 ps (1.875 MHz – 20 MHz)
10G Ethernet (IEEE 802.3-2008 53 & 54)
0.29 ps (4 MHz – 80 MHz)
VDDF
MODE
IC
VSSF
LD
OEB
Pin Diagram
24
19
RESETB 1
OSCFSEL0
18 VDDA
CLK
OSCFSEL1
ACS1790T
VDDD
VSSA
VSSD2
13 OUT2
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Reference spurs: < -67 dBc
10, 12.8, 20 or 25 MHz input clock
Operating voltage: 3.0 - 3.6V
I2C -bus interface
Four selectable slave addresses to allow
multiple devices to be used with a single
controlling master
Lock detect output
Pin and register output enable control
Temperature range: -40 to 85C
4 x 4 mm QFN 24 package
Pb-Free, Halogen free, RoHS/WEEE compliant
product
12
A1
VDDD2
SCL
SDA
7
A0
FBCLK
VSSD 6
OUT1P
OUT1N
Page 1
Optimised for Synchronous Ethernet, SONET and
SDH operation
Meets RMS jitter requirements of Gigabit
Ethernet, 10 Gigabit Ethernet and OC-48 / STM16
Default options for 25 MHz & 125 MHz or
25 MHz & 156.25 MHz outputs at reset
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Table of Contents
INTRODUCTION .................................................................................................................................................................................1
PIN DIAGRAM ....................................................................................................................................................................................1
FEATURES .........................................................................................................................................................................................1
BLOCK DIAGRAM ..............................................................................................................................................................................5
PIN DESCRIPTION .............................................................................................................................................................................6
DEVICE OPERATION ..........................................................................................................................................................................7
THE VCO PLL BLOCK ................................................................................................................................................................................7
THE OUTPUT DIVIDER BLOCK........................................................................................................................................................................8
THE OUTPUT STAGE ....................................................................................................................................................................................8
THE CONTROL BLOCK .................................................................................................................................................................................8
Reset ..................................................................................................................................................................................................9
USING THE ACS1790T ......................................................................................................................................................................9
HARDWARE DEVICE CONFIGURATION .............................................................................................................................................................9
Input clock frequency selection .......................................................................................................................................................9
Default output frequency selection............................................................................................................................................... 10
REGISTER-BASED DEVICE CONFIGURATION .................................................................................................................................................. 10
I2C slave address selection ........................................................................................................................................................... 10
I2C Write .......................................................................................................................................................................................... 11
I2C Read .......................................................................................................................................................................................... 11
Register map .................................................................................................................................................................................. 12
Register Descriptions ..................................................................................................................................................................... 13
VCO CALIBRATION AND LOCK DETECTION ................................................................................................................................................... 18
FRACTIONAL DIVIDER DITHERING ............................................................................................................................................................... 18
OUTPUT ENABLE CONTROL AND SQUELCHING .............................................................................................................................................. 19
OUT1 and OUT2 .............................................................................................................................................................................. 19
FBCLK ............................................................................................................................................................................................. 19
OUT2 SLEW-RATE AND VOLTAGE CONTROL ................................................................................................................................................. 20
ELECTRICAL SPECIFICATIONS ........................................................................................................................................................ 21
MAXIMUM RATINGS ................................................................................................................................................................................ 21
OPERATING CONDITIONS ......................................................................................................................................................................... 21
DC CHARACTERISTICS ............................................................................................................................................................................. 22
RF CHARACTERISTICS ............................................................................................................................................................................. 24
APPLICATIONS INFORMATION ........................................................................................................................................................ 28
USING THE ACS1790T WITH TOPSYNC ................................................................................................................................................... 28
USING THE ACS1790T IN A STANDALONE APPLICATION .............................................................................................................................. 30
USING THE ACS1790T AS PART OF A PHASE-LOCKED LOOP ......................................................................................................................... 30
CONFIGURATION FOR A PARTICULAR OUTPUT FREQUENCY .............................................................................................................................. 31
Programming for the OUT1 output ................................................................................................................................................ 31
Programming for the OUT2 output ................................................................................................................................................ 35
Programming to use both outputs ................................................................................................................................................ 37
Programming the feedback clock output ..................................................................................................................................... 38
Configuration for common output frequencies ............................................................................................................................ 39
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DESIGN CONSIDERATIONS ........................................................................................................................................................................ 43
PCB placement ............................................................................................................................................................................... 43
Output clock termination ............................................................................................................................................................... 43
Power supplies and filtering .......................................................................................................................................................... 44
APPLICATION EXAMPLE ............................................................................................................................................................................ 45
PACKAGING INFORMATION ............................................................................................................................................................ 46
REVISION STATUS/ HISTORY ......................................................................................................................................................... 47
ORDERING INFORMATION ............................................................................................................................................................. 48
DISCLAIMERS ......................................................................................................................................................................................... 48
CONTACTS ............................................................................................................................................................................................. 48
List of Tables
Table 1 : ACS1790T Pin Description ........................................................................................................................................................6
Table 2 : Input Clock Frequency Selection ..............................................................................................................................................9
Table 3 : Default Output Frequency Selection ..................................................................................................................................... 10
Table 4 : I2C Interface Slave Address Selection ................................................................................................................................... 10
Table 5 : ACS1790T Register Map ........................................................................................................................................................ 12
Table 6 : OUT1 and OUT2 Output Control ............................................................................................................................................. 19
Table 7 : FBCLK Output Control ............................................................................................................................................................ 20
Table 8 : OUT2 Drive-Type Setting ........................................................................................................................................................ 20
Table 9 : Absolute Maximum Ratings ................................................................................................................................................... 21
Table 10 : Operating Conditions ........................................................................................................................................................... 21
Table 11 : DC Characteristics ................................................................................................................................................................ 22
Table 12 : RF Characteristics ................................................................................................................................................................ 24
Table 13 : I2C Timing Specification ....................................................................................................................................................... 25
Table 14 : OUT1 Output Division Ratios ............................................................................................................................................... 32
Table 15 : VCO PLL Prescaler Division Ratios ...................................................................................................................................... 32
Table 16 : VCO PLL Reference Frequencies ......................................................................................................................................... 33
Table 17 : Register Settings for 155.52 MHz Output .......................................................................................................................... 34
Table 18 : OUT2 Output Division Ratios ............................................................................................................................................... 35
Table 19 : Register Settings for 10.24 MHz Output ............................................................................................................................ 37
Table 20 : Register Settings for Ethernet (25MHz input) .................................................................................................................... 38
Table 21 : Feedback Clock Output Division Ratios .............................................................................................................................. 39
Table 22 : Register Settings for Common Frequencies (Input clock 10 or 20 MHz) ......................................................................... 40
Table 23 : Register Settings for Common Frequencies (Input clock 12.8 MHz) ................................................................................ 41
Table 24 : Register Settings for Common Frequencies (Input clock 25 MHz) ................................................................................... 42
Table 25 : Ordering Information ............................................................................................................................................................ 48
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List of Figures
Figure 1 : ACS1790T Block Diagram .......................................................................................................................................................5
Figure 2 : ACS1790T VCO PLL ..................................................................................................................................................................7
Figure 3 : ACS1790T Output Divider Structure .......................................................................................................................................8
Figure 4 : I2C Write Operation................................................................................................................................................................ 11
Figure 5 : I2C Read Operation................................................................................................................................................................ 11
Figure 6 : I2C Start and Stop Timing...................................................................................................................................................... 25
Figure 7 : I2C Data Timing ...................................................................................................................................................................... 25
Figure 8 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 155.52 MHz) ..................................................................................................... 26
Figure 9 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 125 MHz) ........................................................................................................... 26
Figure 10 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 156.25 MHz) ................................................................................................... 27
Figure 11 : OUT2 Phase Noise (fin = 12.8 MHz, fout = 25 MHz) ........................................................................................................... 27
Figure 12 : ToPSync and ACS1790T Operation with Separate Oscillators ......................................................................................... 28
Figure 13 : ToPSync and ACS1790T Operation with a Shared Oscillator ........................................................................................... 29
Figure 14 : ToPSync and ACS1790T Operation with a Shared VCXO ................................................................................................. 30
Figure 15 : Standalone ACS1790T Operation ...................................................................................................................................... 30
Figure 16 : Using the ACS1790T as Part of a PLL ............................................................................................................................... 31
Figure 17 : Simplified View of OUT1 Output Path ................................................................................................................................ 31
Figure 18 : Simplified View of OUT2 Output Path ................................................................................................................................ 35
Figure 19 : Simplified View of Both Output Paths ................................................................................................................................ 37
Figure 20 : FBCLK Output Path ............................................................................................................................................................. 38
Figure 21 : Recommended LVPECL DC Termination ........................................................................................................................... 43
Figure 22 : Recommended LVPECL AC Termination ............................................................................................................................ 43
Figure 23 : Recommended Power Supply Filtering .............................................................................................................................. 44
Figure 24 : Example Synchronous Ethernet Application...................................................................................................................... 45
Figure 25 : QFN-24 Physical Package Detail ........................................................................................................................................ 46
Figure 26 : Recommended PCB Land Pattern ..................................................................................................................................... 46
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Block Diagram
VDDA
VDDF
VDDD
POWER
MANAGEMENT
RESETB
VDDD2
LD
OEB
LOCK
DETECTOR
OUT2
‘P’
DIVIDER
‘R’
DIVIDER
CLK
VCO
‘O’
DIVIDER
OUT1P
‘B’
DIVIDER
OUT1N
‘N’
INTEGER
DIVIDER
OSCFSEL0
OSCFSEL1
CONTROL
MODE
‘T’
DIVIDER
‘K’
FRACTIONAL
DIVIDER
FBCLK_
SELECT
FEEDBACK DIVIDER
A0
A1
SCL SDA
FBCLK
VSSA
VSSF
VSSD
VSSD2
Figure 1 : ACS1790T Block Diagram
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Pin Description
The ACS1790T uses a QFN 24 package with 24 device pins and a center thermal pad that is connected to the device ground.
Table 1 shows the pin definitions.
Table 1 : ACS1790T Pin Description
Number
Name
Type
Description
1
RESETB
I
Active Low Reset
2
OSCFSEL0
IPD
Input Clock Frequency Selection
3
CLK
I
Input Clock
4
OSCFSEL1
IPD
Input Clock Frequency Selection
5
VDDD
Power
Digital Power Supply for Internal Logic and FBCLK Output
6
VSSD
Ground
Digital Ground for Internal Logic and FBCLK Output
7
SCL
I
I2C Clock (requires external pull up resistor)
8
SDA
I/O
I2C Data (requires external pull up resistor)
9
A0
IPD
I2C Address Selection
10
FBCLK
0
Feedback Clock to ToPSync (LVCMOS)
11
A1
IPD
I2C Address Selection
12
VDDD2
Power
Digital Power Supply for OUT2 Clock Output
13
OUT2
O
Single-ended Output Clock (LVCMOS)
14
VSSD2
Power
Digital Ground for OUT2 Clock Output
15
VSSA
Ground
Analogue Ground
16
OUT1N
O
Differential Output Clock (LVPECL)
17
OUT1P
O
Differential Output Clock (LVPECL)
18
VDDA
Power
Analogue Supply
19
VDDF
Power
RF Supply
20
VSSF
Ground
RF Ground
21
IC
-
Internally connected pin – do not connect
22
MODE
IPD
Initial Mode Select
23
OEB
I
Output Enable
24
LD
O
Lock Detect (Open Drain)
25
GND
Ground
Center Body Contact. Connect to ground
I
O
I/O
IPD
– Input
– Output
– Bidirectional pin
– Input with internal pull-down
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Device Operation
Figure 1 shows the internal architecture of the ACS1790T. The ACS1790T can be divided into a number of main blocks – the
voltage-controlled oscillator phase-locked-loop (VCO PLL) block, the output divider block, the output stage and the control block.
The VCO PLL block
The VCO PLL block is a fully self-contained phase-locked loop that generates an output frequency of between 2.3 and 2.7 GHz
that is phase-locked to the clock input. This block is shown in Figure 2.
VCO
fPFD
fVCO
‘N’
INTEGER
DIVIDER
‘K’
FRACTIONAL
DIVIDER
FEEDBACK DIVIDER
Figure 2 : ACS1790T VCO PLL
The VCO frequency, fVCO is determined by a divider in the feedback path that supports fractional division ratios and that locks
the VCO output to a multiple of the reference frequency, fPFD, which is the input frequency after passing through the R-divider
shown in Figure 1. For input frequencies of 10 and 12.8 MHz the R-divider is in bypass mode, making fPFD equal to 10 or 12.8
MHz respectively. For input frequencies of 20 and 25 MHz the R-divider is set to divide by two, making fPFD equal to 10 or 12.5
MHz respectively. The high resolution of the fractional feedback divider provides ultra-fine control of the VCO output. Any
frequency within the VCO operating range can be achieved with an accuracy of better than 1 ppb. The design of the VCO and the
corresponding phase-and-frequency detector ensure that the output has very low jitter and is not adversely affected by noise
and jitter on the input clock that falls above the ACS1790T’s PLL bandwidth.
Locking of the VCO to an arbitrary frequency, including immediately after reset, is a two stage process. Firstly, the ACS1790T
performs a calibration operation to select the optimal VCO operating point for the selected frequency. When this operation is
complete the ACS1790T transitions to a “fine-lock” state in which the VCO output frequency is tuned until it is phase-locked to
the reference clock. Once in the fine-lock state, the output frequency of the ACS1790T can be adjusted by up to +/- 500 ppm
from the frequency at which the calibration operation was performed without requiring further calibration. However, if the
frequency is moved outside of this range then a new calibration cycle must be initiated. At reset the VCO frequency is set to 2.5
GHz. Therefore, once calibrated, the actual output frequency can be set to any value between 2.49875 GHz and 2.50125 GHz
without needing to reinitiate the calibration process. This +/- 500 ppm adjustment range should be adequate for any application
in which the output frequency is set and subsequently fine-tuned. Refer to the Applications Information section for further
details of setting the VCO output frequency and initiating recalibration.
The VCO PLL block provides a lock detect signal, LD, to indicate when lock has been achieved after a recalibration cycle. This is
an open-drain pin that is pulled low until lock is established, at which time it is released and pulled high by an external pull-up
resistor. This allows the LD pins of multiple ACS1790T devices to be connected together to provide a single overall lock
indication.
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The output divider block
The output of the VCO PLL block drives the divider block that consists of a number of cascaded dividers, and a multiplexer, as
shown in Figure 3.
fVCO
‘P’
DIVIDER
LOW-SPEED
OUTPUT (OUT2)
‘B’
DIVIDER
HIGH-SPEED
OUTPUT (OUT1)
‘O’
DIVIDER
‘T’
DIVIDER
FEEDBACK
OUTPUT (FBCLK)
T-SEL
Figure 3 : ACS1790T Output Divider Structure
The O-divider is a shared prescaler whereas the B- and P-dividers are specific to the OUT1 and OUT2 outputs respectively. The Tdivider further divides down either the OUT1 or OUT2 output to generate the feedback clock that is used when the ACS1790T
forms the controllable oscillator portion of an external phase-locked loop, such as when the ACS1790T is used in conjunction
with a suitable ToPSync device.
The O, P, B and T-dividers are individually programmable, and together with the frequency of the VCO, determine the ACS1790T
output frequencies. Additionally, the P and B-dividers can be individually disabled in the case that only a single output is being
used to reduce power consumption and device noise. The T-divider, which is responsible for generating the low-speed feedback
output, FBCLK, can be driven from the output of either the P or B-divider through the multiplexer shown in Figure 3.
Refer to the Using the ACS1790T section for further information on the output dividers.
The output stage
The ACS1790T provides a total of three clock outputs – a high-speed LVPECL output, OUT1; a low-speed LVCMOS output, OUT2
and; a low-speed LVCMOS feedback clock, FBCLK.
The OUT2 output features a separate power supply pin for the output buffer, allowing operation at 1.8V, 2.5V or 3.3V. There is
also a slew-rate limiting option on the OUT2 output to slow the edge-rate of the output clock. The FBCLK output operates at a
fixed 3.3V level.
An active-low output enable signal, OEB, is provided to disable the OUT1 and OUT2 outputs, and these outputs can also be
disabled under software control.
The control block
The control block is responsible for control of the other parts of the ACS1790T and includes a set of registers accessible to an
external device through a standard I2C interface. These registers allow configuration of the VCO PLL, output dividers and output
stage, as well as monitoring of the ACS1790T’s status. The registers are fully documented in the section entitled Register-based
Device Configuration.
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Reset
The control block is also responsible for handling device reset. The ACS1790T includes a power monitor to reset the device
automatically at power-up, and an external, active-low, reset pin, RESETB.
During reset, all the registers are reset to their default values, the I2C interface is reset and the clock outputs are held inactive.
Immediately after reset, the registers are loaded with default values dependent on the state of the MODE and OSCFSEL[1:0]
pins (see the section entitled Hardware Device Configuration) to enable generation of frequencies for 1G or 10G Ethernet
designs. The VCO will automatically enter a calibration cycle during which the lock-detect pin, LD, is driven low. Once the
calibration cycle is complete the LD pin is released and the outputs are activated (unless disabled). The ACS1790T is now fully
functional and can be programmed under software control.
Using the ACS1790T
There are two methods of programming the ACS1790T. The initial operating mode is determined by the state of various device
pins that are sampled at power-up and reset. Subsequent control, and status monitoring, is provided through a set of registers
accessible to an external microprocessor through an I 2C-compatible interface.
Hardware device configuration
There are three device pins that determine the initial operating state at power-up and reset – OSCFSEL0, OSCFSEL1 and MODE.
The state of these pins is sampled during reset and the pins have internal pull-down mechanisms to ensure that valid inputs are
sensed even if the pin is left floating. In a typical application these configuration pins would be connected to a static level, either
by tying them directly to ground or 3.3V as appropriate, or connecting them to ground or 3.3V through a pull-up/pull-down
resistor. The presence of the internal pull-down means that care needs to be taken when using an external pull-up resistor to
ensure that the voltage at the pin exceeds the minimum V IH threshold. A single 4.7Kohm resistor per pin, or a 1Kohm resistor
shared between up to three pins, is recommended to ensure this requirement is met.
Should dynamic control of the hardware configuration pins be required then they can be actively driven from logic. However,
since the state of these pins is only sampled at reset, it is necessary to reset the ACS1790T, by asserting RESETB, whenever the
state of the configuration pins is changed.
Input clock frequency selection
The two OSCFSEL pins determine the frequency of the ACS1790T’s input clock. Table 2 shows the acceptable frequencies.
Table 2 : Input Clock Frequency Selection
OSCFSEL1
OSCFSEL0
Input Frequency of CLK
0
0
20 MHz
0
1
10 MHz
1
0
12.8 MHz
1
1
25 MHz
The 10, 12.8 and 20 MHz options correspond to the supported reference clock frequencies of the ToPSync family allowing a
single oscillator and OSCFSEL setting to be shared between the two parts when the ACS1790T is used as a companion to a
ToPSync device. The 25 MHz input frequency is not supported by the ToPSync but is a frequency commonly found in Ethernet
applications and easily generated using very-low cost oscillators.
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Default output frequency selection
The MODE pin determines the initial output frequencies from the ACS1790T as shown in Table 3.
Table 3 : Default Output Frequency Selection
MODE
OUT1
OUT2
Typical Application
0
125 MHz
25 MHz
Gigabit Ethernet
1
156.25 MHz
25 MHz
10 Gigabit Ethernet
When the ACS1790T is used in a Synchronous Ethernet application the MODE pin will typically be set to generate the
appropriate output frequencies for the desired Ethernet speed. This allows the ACS1790T’s outputs to provide clocks to the
Ethernet circuitry immediately after reset prior to any additional configuration being performed.
When the ACS1790T is used for an application other than Synchronous Ethernet, for example as a Sonet clock generator, it is
unlikely that either setting of the MODE pin will provide the desired output frequency. In this case, the MODE pin setting is
arbitrary and the ACS1790T output frequency must be set via the I2C interface, either from a microprocessor or a companion
ToPSync, prior to the output clocks being used.
Register-based device configuration
The ACS1790T incorporates eleven user-accessible registers accessed by a microprocessor or companion ToPSync through an
industry-standard I2C interface. This section documents these registers and the method of accessing them. Since the I2C bus is
a widely-adopted standard, non-ACS1790T specific details, such as the general I2C protocol, are not included here. For this
information refer to the I2C specification, which can be obtained from NXP Semiconductors.
The ACS1790T I2C interface meets the requirements of an F/S-mode I2C device as defined in the I2C specification. The
ACS1790T is able to complete register accesses faster than the I 2C bus and therefore cycle-stretching is unnecessary and SCL
is an input only to the ACS1790T. For full electrical and timing characteristics of the I2C interface refer to the Electrical
Specifications section.
I2C slave address selection
The ACS1790T provides two address selection pins, A0 and A1, to allow one of four different I 2C slave addresses to be selected
as shown in Table 4. This allows a single device, such as a ToPSync, to control up to four associated ACS1790T devices.
Table 4 : I2C Interface Slave Address Selection
A1
A0
I2C Slave Address (0x0)
0
0
C0 (b’1100 000x)
0
1
C2 (b’1100 001x)
1
0
C4 (b’1100 010x)
1
1
C6 (b’1100 011x)
The A0 and A1 address pins should be connected to static levels on the PCB – either directly to power or ground or through
4.7K-ohm, or lower, resistors. The state of these pins should not be changed during operation.
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I2C Write
Figure 4 shows the process used to write one or more registers. The operation is initiated by the host generating a start, or
repeated start, condition and then writing the appropriate slave address (with bit 0 clear) followed immediately by the eight bit
address of the first register to be written and then one or more data bytes containing the desired register contents. After each
register write the register address pointer is automatically incremented unless it has reached the address of the highest
register, 0x0A. This allows multiple consecutive registers to be written simply by sending additional data bytes. The host must
generate either a stop or a repeated start condition to terminate the write operation. The ACS1790T acknowledges the slave
address, and all subsequent bytes written, in accordance with the I2C specification.
Figure 4 : I2C Write Operation
I2C Read
Figure 5 shows the process used to read one or more registers. It starts off in the same way as a register write operation, with
the host writing the slave address (with bit 0 clear) followed by the address of the register to access. However, the host must
then issue a repeated start condition, or optionally a stop followed by a start, followed by a write of the slave address with bit 0
set to initiate a read operation. The host can then read consecutive registers as required until generating a final stop condition.
As for write operations, the auto-increment of the address stops at the last register, allowing this register to be polled with
repeated reads without needing to re-write the register address. In accordance with the I2C specification, the host must
acknowledge receipt of each byte read, by driving SDA low during the ninth bit time of the byte, except for the last byte that must
not be acknowledged (by allowing SDA to float). This signals the ACS1790T to not drive SDA during the next bit time so that the
host can generate the stop condition.
Figure 5 : I2C Read Operation
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Register map
Table 5 shows the ACS1790T register map.
Table 5 : ACS1790T Register Map
Address
Default
Bit significance
reg_status
00
*
mode
oscfsel1
oscfsel0
a0
a1
oeb
lock
reg_output
01
6C
fbclk_
out1/2_
fbclk_
out1_
out2_
out2_
out2_
auto_
auto_
enable
enable
enable
slew-
drive
squelch
squelch
Register name
7
(MSB)
6
5
4
3
2
1
0
(LSB)
rate
reg_n_div
02
*
integer portion of VCO feedback divider
reg_k_div_hi
03
*
bits [23:16] of fractional portion of VCO feedback divider
reg_k_div_med
04
00
bits [15:8] of fractional portion of VCO feedback divider
reg_k_div_lo
05
00
bits [7:0] of fractional portion of VCO feedback divider
reg_o_div
06
*
reg_b_div
07
*
reg_t_div
08
00
prescaler division ratio
out1 division ratio
fbclk_
feedback divider ratio
select
reg_p_div
09
*
reg_vco
0A
01
out2 division ratio
vco_
calibrate
reg_dither
1B
*
(preserve)
dither
(preserve)
Address and default values are in hexadecimal
* indicates that the default value of this register is determined by the state of the hardware configuration pins at reset
Other register addresses are reserved for factory test purposes. For correct operation undocumented addresses must not be
read or written.
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Register Descriptions
Status Register
Description
Bit
mode
Default value
Read-only
0*** ***0
Bit description
oscfsel1
oscfsel0
3
a0
2
a1
1
0
Access
00
Bit name
6
4
Address (0x0)
reg_status
Status register
7
5
Software name
Value
lock
Reset
not used
-
-
0
Indicates the state of the MODE pin as
sampled at power-up or last reset
0
MODE is low (gigabit Ethernet mode)
*
1
MODE is high (10-gigabit Ethernet mode)
Indicates the state of the OSCFSEL1 pin
as sampled at power-up or last reset
0
OSCFSEL1 is low
1
OSCFSEL1 is high
Indicates the state of the OSCFSEL0 pin
as sampled at power-up or last reset
0
OSCFSEL0 is low
1
OSCFSEL0 is high
Indicates the state of the A0 I2C address
pin as sampled at power-up or last reset
0
A0 is low
1
A0 is high
Indicates the state of the A1
address
pin as sampled at power-up or last reset
0
A1 is low
1
A1 is high
Indicated the current state of the output
enable pin
0
OEB is low (outputs are enabled)
1
OEB is high (outputs are disabled)
VCO PLL lock status
0
VCO PLL is not locked
1
VCO PLL is locked
I 2C
oeb
Bit settings
*
*
*
*
*
0
Output control register
Description
Bit name
7
-
6
Access
Default value
reg_output
01
Read-write
0110 1100
Bit description
Value
Bit settings
Reset
not used
-
-
0
fbclk_auto_
squelch
Controls whether the feedback clock
output is deactivated (held low) when the
VCO PLL is out not locked
0
FBCLK toggling when VCO PLL not locked
1
1
FBCLK low when VCO PLL not locked
out1/2_
Controls whether the OUT1 and OUT2
clock outputs are deactivated (held low)
when the VCO PLL is out not locked
0
OUT1/2 toggling when VCO PLL not locked
1
OUT1/2 low when VCO PLL not locked
Controls whether the feedback clock
output is enabled
0
FBCLK is held low
1
FBCLK is driven by output of T-divider
auto_
squelch
4
Address (0x0)
Output control register
Bit
5
Software name
fbclk_
enable
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
3
out1_
enable
2
out2_
enable
1
out2_slew_
0
out2_
FINAL DATASHEET
Controls whether the OUT1 clock output
is enabled
0
OUT1 is held low
1
OUT1 is driven by output of B-divider
Controls whether the OUT2 clock output
is enabled
0
OUT2 is held low
1
OUT2 is driven by output of P-divider
Controls the OUT2 pin slew rate
0
Fast OUT2 edge rate
1
Slow OUT2 edge rate
0
Low OUT2 drive (2.5V and 3.3V)
1
High OUT2 drive (1.8V)
rate
Controls the OUT2 pin drive strength
drive
1
1
0
0
VCO PLL feedback divider registers
Description
Software name
Address (0x0)
Access
Default value
reg_n_div
02
Read-write
**** ****
VCO PLL feedback divider (integer portion)
Bit
Bit name
Bit description
Value
Bit settings
[7:0]
n_div[7:0]
Holds the integer portion of the VCO PLL
feedback divider – 67
0x00
Integer division ratio = 67
0x01
Integer division ratio = 68
For correct operation, division ratios that
result in the VCO frequency being
outside the range 2.3 – 2.7 GHz must
not be programmed
:
Reset
:
0xF7
Integer division ratio = 314
0xF8
Integer division ratio = 315
(Do not program values 0xF9– 0xFF)
Description
Bit
[7:0]
Software name
Address (0x0)
Access
Default value
reg_k_div_hi
03
Read-write
**** ****
Bits [23:16] of the VCO PLL feedback divider (fractional part)
Bit name
k_div
[23:16]
Bit description
Value
Bit settings
Reset
Bits [23:16] of the fractional part of the
VCO PLL feedback divider * 224
(Bit 23 corresponds to a value of 0.5)
Description
Bit
[7:0]
Software name
Address (0x0)
Access
Default value
reg_k_div_med
04
Read-write
0000 0000
Bits [15:8] of the VCO PLL feedback divider (fractional part)
Bit name
k_div
[15:8]
Bit description
Value
Bit settings
Reset
Bits [15:8] of the fractional part of the
VCO PLL feedback divider * 224
(Bit 15 corresponds to a value of 2-9)
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
Description
FINAL DATASHEET
Software name
Address (0x0)
Access
Default value
reg_k_div_lo
05
Read-write
0000 0000
Bits [7:0] of the VCO PLL feedback divider (fractional part)
Bit
Bit name
[7:0]
k_div
[7:0]
Bit description
Value
Bit settings
Reset
Bits [7:0] of the fractional part of the
VCO PLL feedback divider * 224
(Bit 0 corresponds to a value of 2-24)
A write to this register forces the VCO
PLL feedback divider to by updated with
the values programmed into the n_div
and k_div registers
Output divider control registers
Description
Software name
Address (0x0)
Access
Default value
reg_o_div
06
Read-write
0000 ****
The O-divider (prescaler) division ratio
Bit
Bit name
[7:4]
-
[3:0]
o_div
[3:0]
Bit description
Value
not used
-
Sets the division ratio of the O-divider,
which is the VCO PLL prescaler shared
between the OUT1 and OUT2 outputs
Revision 1.0 May 2013 © Semtech Corp.
Bit settings
Reset
-
0000
0000
Prescaler divides by 4
****
0001
Prescaler divides by 6
0010
Prescaler divides by 8
0011
Prescaler divides by 10
0100
Prescaler divides by 12
0101
Prescaler divides by 14
0110
Prescaler divides by 16
0111
Prescaler divides by 18
1000
Prescaler divides by 20
1001
Prescaler divides by 22
1010
Prescaler divides by 24
1011
Prescaler divides by 26
1100
Prescaler divides by 28
1101
Prescaler divides by 30
1110
Do not use
1111
Do not use
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
Description
Bit
FINAL DATASHEET
Software name
Address (0x0)
Access
Default value
reg_b_div
07
Read-write
0000 0***
The B-divider (OUT1) division ratio
Bit name
[7:3]
-
[2:0]
b_div
Value
not used
[2:0]
Description
Bit description
-
Sets the division ratio of the B-divider,
which is used to drive the OUT1 output
Bit settings
Reset
-
00000
000
Disable B-divider (OUT1 is held low)
001
Bypass divider
010
Divide by 2
011
Divide by 4
100
Divide by 8
101
Divide by 16
110
Do not use
111
Do not use
***
Software name
Address (0x0)
Access
Default value
reg_t_div
08
Read-write
0000 0000
The T-divider (FBCLK) division ratio
Bit
Bit name
7
fbclk_
Bit description
Value
Bit settings
Reset
0
The T-divider is driven with the OUT1 clock
frequency
0
1
The T-divider is driven with the OUT2 clock
frequency
-
-
000
0000
T-divider is disabled (FBCLK is held low)
0000
0001
Bypass divider
0010
Divide by 2
0011
Divide by 4
0100
Divide by 8
0101
Divide by 16
0110
Divide by 32
0111
Divide by 64
1000
Divide by 128
1001
Divide by 256
1010
Divide by 512
1011
Divide by 1024
1100
Divide by 2048
1101
Divide by 4096
1110
Divide by 8192
1111
Divide by 16384
T-divider clock source select
select
[6:4]
-
[3:0]
t_div
[2:0]
not used
Sets the division ratio of the T-divider,
which divides either the OUT1 or OUT2
output frequency to generate the FBCLK
output
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
Description
Bit
FINAL DATASHEET
Software name
Address (0x0)
Access
Default value
reg_p_div
09
Read-write
000* ****
The P-divider (OUT1) division ratio
Bit name
[7:5]
-
[4:0]
p_div
Bit description
Value
not used
[4:0]
-
Sets the division ratio of the P-divider,
which is used to drive the OUT2 output
Bit settings
-
Reset
000
00000
Disable P-divider (OUT2 is held low)
00001
Divide by 2
00010
Divide by 4
00011
Divide by 5
00100
Divide by 8
00101
Divide by 10
00110
Divide by 16
00111
Divide by 25
01000
Divide by 32
01001
Divide by 64
01010
Divide by 128
01011
Divide by 256
01100
Divide by 512
01101
Divide by 1024
01110
Divide by 2048
01111
Divide by 4096
10000
Divide by 8192
10001
Divide by 16384
10010
Divide by 32768
10011
Divide by 65536
*****
10100
:
Do not use values 10100 - 11111
11111
VCO calibration request register
Description
Address (0x0)
Access
Default value
reg_vco
0A
Read-write
0000 0001
Controls the VCO calibration process
Bit
Bit name
[7:1]
-
0
Software name
vco_
calibrate
Bit description
Value
Bit settings
Reset
not used
-
-
0
Setting this bit forces the VCO to
recalibrate to its optimum operating
point after a large frequency change
0
No action (write) /
1
Calibration complete (read)
1
(Bit is cleared automatically)
Revision 1.0 May 2013 © Semtech Corp.
Force calibration (write) /
Calibration in process (read)
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
Description
Bit
Software name
Address (0x0)
Access
Default value
reg_dither
1B
Read-write
**0* ****
Status register
Bit name
[7:6]
-
5
dither
[4:0]
FINAL DATASHEET
-
Bit description
Value
Bit settings
Reset
Reserved1
-
-
*
Enable fractional divider dithering
0
K-divider value is used as programmed
0
1
K-divider value is extended with one
additional fractional bit forced to 1
-
-
Reserved1
*
For correct operation the current value of these bits must be preserved when changing the dither setting. Therefore, to change
the dither setting software must read this register, and rewrite the value read with bit 5 set or cleared as needed.
1
VCO calibration and lock detection
The VCO can generate any frequency between 2.3 and 2.7 GHz, with an ultra-fine resolution of less than 1 ppb. However,
locking to an arbitrary frequency requires the VCO to be calibrated to select its optimal operating point. The VCO calibration
process is initiated automatically at power-up or reset to set the VCO to its default frequency of 2.5 GHz, and can be retriggered
at any time by writing a ‘1’ to the vco_calibrate bit (bit 0 of reg_vco). This bit will remain set while the calibration process is
executed and will be automatically cleared once it is complete and the VCO PLL is locked.
Once the VCO PLL has been calibrated for a given frequency, the actual frequency can be adjusted by up to 500 ppm either side
of the original frequency without needing to reinitiate a calibration cycle. However, if it is desired to change the output frequency
by more than this amount then a recalibration cycle must be initiated once the new frequency has been programmed.
The active low, open-drain, lock-detect pin, LD, indicates the lock state of the VCO PLL. Whenever an out-of-lock condition is
detected LD is pulled low to indicate that the output frequency cannot be relied on. If the auto-squelch option is enabled then
the corresponding output is automatically held low until lock is established. Initiating a recalibration cycle causes the LD pin to
be driven low immediately.
Fractional divider dithering
By default, the programmed K-divider value is used directly to determine the ratio of the VCO output frequency to the reference
clock frequency (after scaling by the R-divider). This allows the output frequency to be determined precisely, subject to the
resolution of the K-divider. However, for K-divider values in which the fractional portion is 1/2n (0.5, 0.25, 0.125 etc.), the
nature of fractional division can result in excessive deterministic jitter being generated on the output. To avoid this, the dither bit
(bit 5 of reg_dither) can be set to append a fixed ‘1’ bit to the fractional portion of the K-divider, effectively adding 1/2-25 to the
programmed value.
Using this dither feature can significantly reduce the deterministic jitter for certain K-divider settings. However, it will also result
in a fixed error in the output frequency of the order of 0.25 ppb. In applications where the ACS1790T is operated as part of a
closed loop system, such as when being controlled by a ToPSync device with the output frequency monitored through the FBCLK
pin, the control loop will remove this fixed error over time so the average output frequency will be correct. However, when the
ACS1790T is operated in an open-loop mode, either to generate a non-disciplined output frequency or when the reference clock
itself is disciplined, for example when ToPSync is tuning a VCXO, then the fixed error will not be removed and the resultant
output frequency will show a corresponding inaccuracy resulting in an unbounded Maximum Time Interval Error (MTIE)
measurement. Whether this fixed error is acceptable or not will depend on the application, and therefore the overall system
requirements should be considered before enabling the dithering feature in open-loop operation.
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
FINAL DATASHEET
Output enable control and squelching
OUT1 and OUT2
The behavior of the OUT1 and OUT2 outputs is determined by a number of factors which control whether the pins are actively
driven with the corresponding output frequency, held inactive or tri-stated. Table 6 indicates the behavior of the outputs under
various conditions.
The automatic squelch capability shown in Table 6 allows the outputs to be forced inactive at any time the VCO is unlocked, as
indicated by the LD pin being pulled low. This capability is enabled by setting the out1/2_Auto_Squelch bit (bit 5 of reg_output).
Table 6 : OUT1 and OUT2 Output Control
Input Pin Setting
RESETB
OEB
Register Settings
Poweron
Lock State
reset
out1/2_
Auto_
Squelch
Clock Output State
B (OUT1) or
outn_
P (OUT2)
Enable
Divider
Disabled
OUTn
X
X
Yes
Not locked
1
1
X
Hi-impedance
0
0
No
Not locked
1
1
X
Driven Low
0
1
No
Not locked
1
1
X
Hi-impedance
1
1
No
X
X
X
X
Hi-impedance
1
0
No
Not locked
0
0
X
Hi-impedance
Unlocked
1
0
No
Not locked
0
1
No
Active (Toggling)
Operation
1
0
No
Not locked
1
0
X
Hi-impedance
1
0
No
Not locked
1
1
X
Driven Low
1
0
No
Not locked
X
1
Yes
Driven Low
1
0
No
Locked
0
0
X
Hi-impedance
Normal
1
0
No
Locked
0
1
No
Active (Toggling)
Operation
1
0
No
Locked
1
0
X
Hi-impedance
1
0
No
Locked
1
1
No
Active (Toggling)
1
0
No
Locked
X
1
Yes
Driven Low
Reset
Hardware Control
FBCLK
The low-speed feedback clock output pin, FBCLK, is simpler in operation than the OUT1 and OUT2 clocks since it doesn’t
support a tri-state option, and therefore is unaffected by the state of the OEB pin. Table 7 summarizes the behavior of the
FBCLK output under various conditions.
The FBCLK output also features an auto-squelch feature with the same functionality as that of the OUT1 and OUT2 outputs.
However, it is controlled by a separate bit – fbclk_auto_squelch in the reg_output register.
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
FINAL DATASHEET
Table 7 : FBCLK Output Control
Input
Pin
Setting
RESETB
Poweron
Register Settings
Lock State
FBCLK_
reset
Auto_
Squelch
FBCLK_
Enable
Clock Output State
T-Divider
Disabled
FBCLK
X
Yes
Not locked
1
0
X
Driven Low
0
No
Not locked
1
0
X
Driven Low
1
No
Not locked
0
0
X
Driven Low
Unlocked
1
No
Not locked
0
1
No
Active (Toggling)
Operation
1
No
Not locked
1
0
X
Driven Low
1
No
Not locked
1
1
X
Driven Low
1
No
Not locked
X
1
Yes
Driven Low
1
No
Locked
0
0
X
Driven Low
Normal
1
No
Locked
0
1
No
Active (Toggling)
Operation
1
No
Locked
1
0
X
Driven Low
1
No
Locked
1
1
No
Active (Toggling)
1
No
Locked
X
1
Yes
Driven Low
Reset
OUT2 slew-rate and voltage control
The OUT2 CMOS clock output is capable of operating at 1.8V, 2.5V and 3.3V LVCMOS standards. It also supports a slew-rate
limiting capability which can improve signal quality in some circumstances, such as when the output is driving a lengthy PCB
trace, by reducing the rise and fall times of the clock edges.
The OUT2 pin has a separate dedicated power supply pin – VDDD2. This pin must be connected to 1.8V, 2.5V or 3.3V to set the
OUT2 pin signalling level as required. Additionally, when operating at 1.8V it is necessary to set the out2_drive bit in the
reg_output register to increase the drive strength of the output buffer. Failure to do so will result in reduced signal quality on the
OUT2 output.
Table 8 summarises the requirements for each output standard on the OUT2 pin.
Table 8 : OUT2 Drive-Type Setting
OUT2 I/O
Standard
Maximum Voltage
Swing
VDDD2 Voltage
out2_drive Bit
Setting
LVCMOS18
1.8V
1.8V
1
LVCMOS25
2.5V
2.5V
0
LVCMOS33
3.3V
3.3V
0
The slew-rate of the OUT2 pin is controlled by the out2_slew_rate bit in the reg_output register. When this bit is clear the OUT2
signal is driven with fast rise and fall times. Setting the out2_slew_rate bit increases the rise and fall times of the OUT2 signal.
This in turn allows the OUT2 pin to drive considerably longer PCB traces (as long as several inches) without needing any
termination. The out2_slew_rate bit is set by default.
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
FINAL DATASHEET
Electrical Specifications
Maximum Ratings
Important Note: The Absolute Maximum Ratings, in the table below, are stress ratings only, and functional operation of the
device at conditions other than those indicated in the Operating Conditions sections of this specification are not implied.
Exposure to the absolute maximum ratings, where different to the operating conditions, for an extended period may reduce the
reliability or useful lifetime of the product.
Table 9 : Absolute Maximum Ratings
Parameter
Symbol
Minimum
Maximum
Units
Supply Voltage VDDA, VDDD, VDDD2,
VDDF
V DD
-0.5
3.7
V
Input Voltage (non-supply pins)
Vin
-
3.7
V
Operating Junction Temperature
TJCT
-
125
°C
Reflow Temperature
TRE
-
260
°C
Storage Temperature
Tstor
-50
150
°C
ESD (Human Body Model)
ESDHBM
-
2
kV
ESD (Charged Device Model)
ESDCDM
-
1
kV
ILU
-100
100
mA
Latchup
Operating Conditions
Table 10 : Operating Conditions
Parameter
Symbol
Minimum
Typical
Maximum
Units
VDD
3.0
3.3
3.6
V
Power Supply VDDD2 (1.8V operation)
VDDD2
1.71
1.8
1.89
V
Power Supply VDDD2 (2.5V operation)
VDDD2
2.375
2.5
2.625
V
Power Supply VDDD2 (3.3V operation)
VDDD2
3.0
3.3
3.6
V
TA
-40
-
+85
°C
Supply Current Inputs & digital
IDDD
-
5
8
mA
Supply Current Synthesizer only
IDDF
-
52
60
mA
Power Supply
VDDA, VDDD, VDDF
Ambient Temperature Range
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
FINAL DATASHEET
Supply Current HF output1
IDDA
-
19
28
mA
Supply Current LF output2
IDDD2
-
12
15
mA
Total Power Dissipation1,2
PTOT
-
290
400
mW
Package Thermal Resistance – still air
ΘJA
-
-
32.2
c/w
Package Thermal Resistance – 1 m/s
ΘJA
-
-
29.9
c/w
Package Thermal Resistance – 2 m/s
ΘJA
-
-
28.1
c/w
Notes:
1. Assumes 200 MHz OUT1 output into 50 ohm LVPECL load terminated as shown in Figure 21.
2. Assumes 125 MHz OUT2 output driving 10 pF unterminated load.
DC Characteristics
Across all operating conditions, unless otherwise stated
Table 11 : DC Characteristics
DC Input Characteristics: CLK, OEB, A0, A1, OSCFSEL0, OSCFSEL1, MODE, RESETB pins
Parameter
Symbol
Minimum
Typical
Maximum
Units
VIN High
VIH
2.0
-
-
V
VIN Low
VIL
-
-
0.8
V
(A0, A1, MODE, OSCFSEL0,
OSCFSEL1)
RPD
28
36
44
k
Input Low Current (VIN = VSSD)
IIL
-
-
1
A
Input High Current (VIN = VDDD)
IIH
-
-
130
A
Minimum
Typical
Maximum
Pull-down Resistor
DC Input Characteristics: SDA and SCL pins
Parameter
Symbol
Units
VIN High
VIH
0.7 * VDDD
-
-
V
VIN Low
VIL
-
-
0.3 * VDDD
V
Input Low Current (VIN = VSSD)
IIL
-
-
1
A
Input High Current (VIN = VDDD)
IIH
-
-
1
A
Noise margin at low level
VnL
200
mV
Noise margin at high level
VnH
200
mV
Revision 1.0 May 2013 © Semtech Corp.
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ACS1790T
ADVANCED COMMUNICATIONS PRODUCT GROUP
FINAL DATASHEET
DC Output Characteristics: FBCLK pin
Parameter
Symbol
Minimum
Typical
Maximum
Units
Vout Low (lol = 4mA)
VOL
0
-
0.3
V
Vout High (loh = -4mA)
VOH
2.4
-
-
V
Output Low Current
IOL
-
-
4
mA
Output High Current
IOH
-
-
-4
mA
Minimum
Typical
Maximum
DC Output Characteristics: SDA, LD pins
Parameter
Symbol
Units
Vout Low (lol = 4mA)
VOL
0
-
0.4
V
Output Low Current
IOL
-
-
4
Minimum
Typical
Maximum
0
-
0.3
V
1.3
-
-
V
0
-
0.3
V
1.7
-
-
V
0
-
0.3
V
2.4
-
-
V
mA
DC Output Characteristics: OUT2 pin
Parameter
Vout Low (VDDD2 = 1.8V, lol = 1mA)
Symbol
VOL
Vout High (VDDD2 = 1.8V, loh = -1mA) VOH
Vout Low (VDDD2 = 2.5V, lol = 2mA)
VOL
Vout High (VDDD2 = 2.5V, loh = -2mA) VOH
Vout Low (VDDD2 = 3.3V, lol = 4mA)
VOL
Vout High (VDDD2 = 3.3V, loh = -4mA) VOH
Units
Output Low Current
IOL
-
-
4
mA
Output High Current
IOH
-
-
-4
mA
Minimum
Typical
Maximum
DC Characteristics: OUT1P, OUT1N pins
Parameter
Symbol
Units
Vout Low1
VOL
VDD – 2.1
-
VDD – 1.62
V
Vout High1
VOH
VDD – 1.45
-
VDD – 1.0
V
Differential Output Voltage1
VOD
550
-
900
mV
Notes:
1) Driving into a LVPECL 50 load biased to VDD – 2V
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RF Characteristics
Across all operating conditions, unless otherwise stated
Table 12 : RF Characteristics
Symbol
Test Condition
Minimum
Typical
Maximum
Units
FREF
External Clock Source
10
12.8
25
MHz
PFD Update Frequency1
f
f FREF/R
10
-
12.8
MHz
VCO Center Frequency
Range
fCEN
2.3
2.5
2.7
GHz
Parameter
CLK Input Frequency
OUT1 Integrated Jitter2,3
JITT1
155.52 MHz (12 kHz – 20 MHz)
-
0.56
0.8
ps (rms)
OUT1 Integrated Jitter2,3
JITT1
125 MHz (637 kHz – 20 MHz)
-
0.15
0.8
ps (rms)
OUT1 Integrated Jitter2,3
JITT1
125 MHz (1.875 MHz – 20 MHz)
-
0.11
0.8
ps (rms)
OUT1 Integrated Jitter2,3
JITT1
156.25 MHz (4 MHz – 80 MHz)
-
0.29
0.8
ps (rms)
OUT2 Integrated Jitter2,3,4
JITT2
25 MHz (12 kHz to 5 MHz)
-
0.8
1.0
ps (rms)
Loop Bandwidth
BW
Closed Loop
-
160
-
kHz
Harmonic Suppression
H2
Second Harmonic
TBA
-20
-
dBc
Offset = 12.8MHz
-
-67
-
dBc
Output Reference Spurs
Settling Time
TSET
Across entire tuning range to
1 ppm precision
-
250
500
ms
Narrowband Lock Range
TRI
Across entire tuning range
500
-
-
ppm
fSTEP
-
-
1
ppb
Lock Time From Reset5
TLOCKR
-
-
20
ms
Lock Time From Calibration5
TLOCKC
-
-
10
ms
Tuning Step Size
Notes:
1)
2)
3)
4)
5)
Value of f is dependent on the input divider
Measured using Rakon CFPO-DO reference clock (12.8 MHz)
Measured using bandpass filter range and carrier frequency specified in “Test Condition” column
Measured using VDDD2 = 2.5V
As indicated by LD pin state
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Table 13 : I2C Timing Specification
Parameter1
Symbol
SCL Clock Frequency
Conditions
Min
Typ
Max
Unit
fSCL
0
-
400
kHz
SCL low period
tLOW
1.3
-
-
us
SCL high period
tHIGH
0.6
-
-
us
Data setup time
tSU:DAT
100
-
-
ns
Data hold time
tHD:DAT
0
-
-
ns
Repeated start setup time
tSU:STA
0.6
-
-
us
Start condition hold time
tSU:STA
0.6
-
-
us
Stop condition hold time
tSU:STO
0.6
-
-
us
Bus free time between stop and start
tBUF
1.3
-
-
us
Input glitch suppression
tSP
-
50
ns
Notes:
1) Timing specifications refer to voltage levels (VIL, VIH, VOL) defined in DC Characteristics
The interface complies with slave F/S mode as described by NXP: “I2C-bus specification, Rev. 03 – 19 June 2007”.
70%
30%
SDA
70%
SCL
tSU;STA
tHD;STA
tSU;STO
tBUF
Figure 6 : I2C Start and Stop Timing
70%
30%
SDA
SCL
70%
30%
tLOW
tHIGH
tHD;DAT
Figure 7 :
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I2C
tSU;DAT
tSP
Data Timing
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Typical Phase Noise Performance
Figure 8 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 155.52 MHz)
Figure 9 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 125 MHz)
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Figure 10 : OUT1 Phase Noise (fin = 12.8 MHz, fout = 156.25 MHz)
Figure 11 : OUT2 Phase Noise (fin = 12.8 MHz, fout = 25 MHz)
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Applications Information
The ACS1790T can be used either in conjunction with a suitable ToPSync device or standalone.
Using the ACS1790T with ToPSync
The most common use of the ACS1790T is as a companion to a compatible Semtech ToPSync device as part of an integrated
timing platform solution to allow the generation of ultra-low jitter output clocks locked to a wired or packet reference source.
There are three different modes of operation depending on the ACS1790T and ToPSync local oscillator configuration:
ToPSync and the ACS1790T use separate reference oscillators
ToPSync and the ACS1790T share a common fixed reference oscillator
ToPSync and the ACS1790T share a common voltage-controlled oscillator that is tuned by ToPSync
In all three cases, one or two ACS1790Ts can be supported from a single ToPSync. When only a single ACS1790T is used it
must be assigned the I2C slave address 0xC0 (A1 = 0, A0 = 0). When two ACS1790Ts are used one is designated as the primary
device and must be assigned the I2C slave address 0xC0 (A1 = 0, A0 = 0), while the other one is designated as the secondary
device and is assigned the I2C slave address 0xC2 (A1 = 0, A0 = 1).
Operation with separate oscillators
Figure 12 shows the configuration when ToPSync and the ACS1790T operate from separate reference clocks. In this case, the
ACS1790T FBCLK output is connected to one of ToPSync’s clock inputs and the ToPSync firmware uses this to form a software
PLL control loop, monitoring the ACS1790T output frequency and adjusting it as required by writing to the fractional K-divider
through the I2C interface.
10, 12.8 or 20 MHz
10, 12.8, 20 or 25 MHz
CLK
2
IC
SCL,
SDA
ACS1790 FBCLK
ACS1790
(I2C Address = 0xC0)
ToPSync®
Feedback Clock
CLK
SCL,
SDA
ACS1790 FBCLK
(I2C Address = 0xC2)
Optional
Figure 12 : ToPSync and ACS1790T Operation with Separate Oscillators
In the case where two ACS1790Ts are used in this mode, both must share the same reference oscillator. Only the feedback
clock from the primary device is connected to ToPSync, which can infer the operating frequency of the secondary device, since
both ACS1790Ts share the same reference oscillator, and adjust the output frequency of both by writing to the fractional Kdividers through the I2C interface.
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Operation with a shared fixed oscillator
When the reference oscillator is shared between ToPSync and one or two ACS1790Ts then operation is possible without the
feedback clock connection to ToPSync since the ACS1790T operating frequency can be inferred from ToPSync’s internal
timebase. This configuration is shown in Figure 13.
10, 12.8 or 20 MHz
CLK
2
IC
SCL,
SDA
ACS1790 FBCLK
ACS1790
(I2C Address = 0xC0)
ToPSync®
CLK
SCL,
SDA
ACS1790 FBCLK
(I2C Address = 0xC2)
Optional
Figure 13 : ToPSync and ACS1790T Operation with a Shared Oscillator
Operation with a shared voltage-controlled oscillator
In this mode, both ToPSync and one or two ACS1790Ts share a voltage-controlled oscillator that is tuned by ToPSync, using its
internal digital-to-analogue converter, to align it to the selected reference source. This allows a disciplined clock with the lowest
possible phase noise to be taken directly from the oscillator for applications such as cellular basestations. This configuration is
shown in Figure 14.
Since the reference clock is disciplined directly by ToPSync there is no need to continually tune the ACS1790T feedback dividers
and therefore the I2C connection from ToPSync is used only for initial configuration.
Regardless of the exact configuration, ToPSync is responsible for setting the initial output frequencies from the ACS1790T,
along with options such as output drive strength and slew rate. These parameters are configured through the ToPSync control
API and it is never necessary for the user to interface with the ACS1790T I 2C register set directly when operating in conjunction
with ToPSync. Refer to the ToPSync User Guide, device-specific datasheets and API documentation for more information on
using the ACS1790T in association with a ToPSync.
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10, 12.8 or 20 MHz
VCXO
CLK
I2C
SCL,
SDA
ACS1790 FBCLK
ACS1790
(I2C Address = 0xC0)
DAC
ToPSync®
CLK
SCL,
SDA
ACS1790 FBCLK
(I2C Address = 0xC2)
Optional
Figure 14 : ToPSync and ACS1790T Operation with a Shared VCXO
Using the ACS1790T in a standalone application
The ACS1790T can be used standalone as a simple programmable-frequency clock generator. In this case, a controlling
microprocessor, or other device that can act as an I 2C host, configures the ACS1790T to generate the desired frequency, or
frequencies, and then takes no further role in the device operation. Figure 15 shows a simplified example of this application.
OUT1
10, 12.8, 20
or 25 MHz
CLKIN
ACS1790T
OUT2
FBCLK
SCL
SDA
RESETB
Microprocessor
Figure 15 : Standalone ACS1790T Operation
Using the ACS1790T as part of a phase-locked loop
The ability to ultra-fine tune the VCO frequency allows the ACS1790T to be used as the oscillator component of a phase-locked
loop, for example, locking the output frequency to a Stratum-traceable reference in a Synchronous Ethernet system. An example
of such an application is shown in Figure 16. Typically, the other components of the PLL will be implemented as a combination
of hardware and software using, for example, an FPGA with embedded micro-controller core.
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OUT1
10, 12.8, 20
or 25 MHz
CLKIN
ACS1790T
OUT2
FBCLK
SCL
SDA
RESETB
Reference
Input
VCO
Controller
÷
m
Figure 16 : Using the ACS1790T as Part of a PLL
In this example, the phase-and-frequency detector (PFD) within the external PLL controller compares the feedback clock output,
FBCLK, of the ACS1790T with a scaled version of the reference input. The output of the PFD is low-pass filtered and drives the
VCO controller that is responsible for adjusting the VCO frequency of the ACS1790T to the point where the FBCLK output, and
hence the OUT1 and OUT2 outputs, are phase-locked to the reference input
Configuration for a particular output frequency
This section details the steps necessary to program the ACS1790T to generate a particular output frequency, or frequencies.
When using the ACS1790T as a companion to ToPSync the ToPSync firmware handles the configuration details and therefore
knowledge of the information contained in this section isn’t directly necessary.
Configuring the ACS1790T to generate a particular output frequency, or frequencies, is a three stage process. Firstly, division
ratios for the output divider chain have to be selected such that the VCO will operate within its allowable range of 2.3 – 2.7 GHz.
Secondly, the VCO feedback divider necessary to achieve the correct VCO frequency must be calculated. Finally, if it is desired to
use the FBCLK feedback clock output then the correct source and division ratio for this must be selected. The exact process
depends on whether the OUT1, OUT2 or both outputs are to be used:
Programming for the OUT1 output
Figure 17 shows a simplified diagram of the ACS1790T when using just the OUT1 output.
VCO
fPFD
fVCO
‘O’
DIVIDER
‘B’
DIVIDER
fOUT1
‘N’
INTEGER
DIVIDER
‘K’
FRACTIONAL
DIVIDER
FEEDBACK DIVIDER
Figure 17 : Simplified View of OUT1 Output Path
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The VCO and prescaler O-divider can be programmed to give an output frequency in one of five ranges:
77 – 225 MHz
230 – 270 MHz
288 – 337 MHz
384 – 450 MHz
575 – 675 MHz
Firstly, multiply the desired output frequency by 1, 2, 4, 8 or 16 to obtain a frequency that falls within one of the five ranges.
This multiplier value determines the value to program into the reg_b_div register as shown in Table 14.
Table 14 : OUT1 Output Division Ratios
Division
Ratio
reg_b_div
value
1
01
2
02
4
03
8
04
16
05
Secondly, calculate the prescaler output frequency by multiplying the desired output frequency by the division ratio determined
in the first step and look up the range in which this falls in Table 15 to determine the programming of the reg_o_div register
(from the second column).
Table 15 : VCO PLL Prescaler Division Ratios
Prescaler Output
Frequency Range
reg_o_div
O-Divider
value
ratio
(0x0)
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77 – 86 MHz
0D
30
86 – 93 MHz
0C
28
93 – 100 MHz
0B
26
100 – 109 MHz
0A
24
109 – 119 MHz
09
22
119 – 131 MHz
08
20
131 – 147 MHz
07
18
147 – 166 MHz
06
16
166 – 192 MHz
05
14
192 – 225 MHz
04
12
230 – 270 MHz
03
10
288 – 337 MHz
02
8
384 – 450 MHz
01
6
575 – 675 MHz
00
4
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Thirdly, the VCO feedback divider value must be calculated. This is given by the equation:
feedback_divider_ratio = fVCO / fPFD
fVCO is the VCO operating frequency and is given by multiplying the output frequency by the B-divider value, as in the previous
step, and then multiplying this by the O-divider value from the third column of Table 15 corresponding to the reg_o_div register
setting determined in the previous step. fPFD is the phase and frequency detector input clock and is directly related to the
ACS1790T input clock as shown in table Table 16.
Table 16 : VCO PLL Reference Frequencies
ACS1790T CLK
fPFD
Frequency
10 MHz
10 MHz
12.8 MHz
12.8 MHz
20 MHz
10 MHz
25 MHz
12.5 MHz
The calculated value of feedback_divider_ratio will be in the range 179.6875 to 270. The reg_n_div register is determined by
the equation:
reg_n_div = INT(feedback_divider_ratio) – 67
where ‘INT’ means take the integer part
For example, if feedback_divider_ratio is 200.457 then the reg_n_div register must be set to 200 – 67 = 133 (0x85). The
fractional portion is given by:
reg_k_div = ROUND(FRAC(feedback_divider_ratio) * 224)
where ‘FRAC’ means take the fractional part only, and ‘ROUND’ means round to the closest integer.
The value of reg_k_div must be programmed into three separate registers:
reg_k_div_hi = reg_k_div[23:16],
reg_k_div_med = reg_k_div[15:8], and
reg_k_div_lo = reg_k_div[7:0]
For example, if feedback_divider_ratio is 200.457 then reg_k_div is 7667187 (0x74FDF3) and the values programmed into the
corresponding three registers are:
reg_k_div_hi = 0x74
reg_k_div_med = 0xFD
reg_k_div_lo = 0xF3
In general, the finite resolution of the fractional divider setting means that it will not be possible to precisely set the required
feedback division ratio and hence the output frequency will not exactly match the desired value. However, the twenty-three bit
resolution of the fractional portion means that the output frequency will always be within 1 ppb of the desired value.
It is important to note that when updating the feedback divider registers, the actual value used by the feedback divider is not
updated until the reg_k_div_lo register is written. This prevents transient changes in frequency as individual registers are written
while still allowing the output frequency to be fine-tuned without requiring all the associated registers to be rewritten.
Additionally, when using just the OUT1 output, the reg_p_div register, which controls the output divider for the OUT2 output,
should be set to zero to disable the divider, reducing power consumption and output noise.
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Example:
Assume that it is desired to generate a frequency on the OUT1 output of 155.52 MHz for driving a Sonet framer, and that the
ACS1790T is driven by a 25 MHz reference clock. The steps in programming the ACS1790T to generate this frequency are:
1) 155.52 MHz is within one of the allowable ranges for the prescaler output. Therefore, no additional division in the Bdivider is needed and from the first row of Table 14, the required value for the reg_b_div register is 0x01.
2) From the eighth row of Table 15, the required value for the reg_o_div register is 0x06, with a corresponding O-divider
ratio of 16.
3)
The VCO output frequency is the product of the OUT1 frequency with the B-divider and O-divider ratios. In this case,
fVCO = 155.52 MHz * 1 * 16 = 2488.32 MHz
Using the equations from the previous section, noting from Table 16 that fPFD is 12.5 MHz, gives the following:
feedback_divider_ratio = 2488.32 / 12.5 = 199.0656
reg_n_div = INT(199.0656) – 67 = 132 (0x84)
reg_k_div = ROUND (FRAC(199.0656) * 224) = 1100585 (0x10CB29)
reg_k_div_hi = 16 (0x10)
reg_k_div_med = 203 (0xCB)
reg_k_div_lo = 41 (0x29)
As a check, the rounded feedback divider ratio is 199 + 1100585 / 224, which gives a VCO output frequency that is
less than 0.12 ppb from the desired value. This corresponds to an error in the OUT1 frequency of less than 18 mHz.
4) Finally, reg_p_div should be set to zero since the OUT2 output is not being used and the vco_calibrate bit of the
reg_vco register should be set to force a recalibration of the VCO PLL. Once the vco_calibrate bit has returned to
zero then the VCO is calibrated and the OUT1 output will be at the desired 155.52 MHz frequency.
Table 17 summarizes the divider-related register settings for this example.
Table 17 : Register Settings for 155.52 MHz Output
Register name
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Address
Value
(0x0)
(0x0)
reg_n_div
02
84
reg_k_div_hi
03
10
reg_k_div_med
04
CB
reg_k_div_lo
05
29
reg_o_div
06
06
reg_b_div
07
01
reg_p_div
09
00
reg_vco
0A
01
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Programming for the OUT2 output
Figure 18 shows a simplified diagram of the ACS1790T when using just the OUT2 output.
VCO
fPFD
fVCO
‘O’
DIVIDER
‘P’
DIVIDER
fOUT2
‘N’
INTEGER
DIVIDER
‘K’
FRACTIONAL
DIVIDER
FEEDBACK DIVIDER
Figure 18 : Simplified View of OUT2 Output Path
The architecture, and hence programming, in this case is very similar to that for the OUT1 output, the only difference being that
the B-divider is replaced by the P-divider, which provides a wider range of division ratios. Table 18 lists the allowable division
ratios for the P-divider.
Table 18 : OUT2 Output Division Ratios
P-Divider Division
reg_p_div Register
Ratio
Setting
Output Frequency
(0x0)
Limited to 250 MHz?
2
01
No
4
02
Yes
5
03
No
8
04
Yes
10
05
No
16
06
Yes
25
07
No
32
08
Yes
64
09
Yes
128
0A
Yes
256
0B
Yes
512
0C
Yes
1024
0D
Yes
2048
0E
Yes
4096
0F
Yes
8192
10
Yes
16384
11
Yes
32768
12
Yes
65536
13
Yes
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The first step in the programming is to select a division ratio from Table 18 that when multiplied by the desired output frequency
gives a value within one of the five O-divider output ranges specified in the previous section. The corresponding entry in the
second column of Table 18 then gives the value to program into the reg_p_div register.
Due to the design of the P-divider, there is a restriction on the allowable O-divider output frequency in some cases when using
the OUT2 output. A ‘Yes’ in the third column of Table 18 indicates that, for the corresponding P-divider setting, the O-divider
output frequency must not exceed 250 MHz, which can be ensured by selecting an O-divider ratio of 12 or higher. In practice
this does not restrict the available output frequencies, but does restrict the combination of divider ratios used to achieve a given
frequency.
Having defined the P-divider ratio, the remaining steps are the same as for the OUT1 output as documented in the previous
section. Since the OUT1 output is not being used, the reg_b_div register should be programmed to zero to disable the OUT1 Bdivider.
Example:
Assume that it is desired to generate a frequency on the OUT2 output of 10.24 MHz for a DOCSIS 3.0 DTI application, and that
the ACS1790T is driven by a 12.8 MHz reference clock. The steps in programming the ACS1790T to generate this frequency
are:
1) Note that 81.92 MHz (10.24 MHz * 8) is within one of the allowable O-divider output ranges and therefore 8 is an
acceptable division ratio for the P-divider and, from Table 18, the reg_p_div register should be programmed to 0x04.
2) From the first row of Table 15, the required value for the reg_o_div register is 0x0D, with a corresponding O-divider
ratio of 30.
3)
The VCO output frequency is the product of the OUT1 frequency with the P-divider and O-divider ratios. In this case,
fVCO = 10.24 MHz * 8 * 30 = 2457.6 MHz
Using the equations from the previous section, noting from Table 16 that fPFD is 12.8 MHz, gives the following:
feedback_divider_ratio = 2457.6 / 12.8 = 192
reg_n_div = INT(192) – 67 = 125 (0x7D)
reg_k_div = ROUND (FRAC(192) * 224) = 0 (0x00)
reg_k_div_hi = 0 (0x00)
reg_k_div_med = 0 (0x00)
reg_k_div_lo = 0 (0x00)
Since the VCO feedback division ratio is an integer the output frequency will be exactly the desired 10.24 MHz.
Note that although the three K-divider registers are set to their default value of zero, it is still necessary to perform a
write to the reg_k_div_lo register since this is required to force an update of the feedback divider ratio.
4) Finally, reg_b_div should be set to zero since the OUT1 output is not being used and the vco_calibrate bit of the
reg_vco register should be set to force a recalibration of the VCO PLL. Once the vco_calibrate bit has returned to
zero then the VCO is calibrated and the OUT2 output will be at the desired 10.24 MHz frequency.
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Table 19 summarizes the divider-related register settings for this example.
Table 19 : Register Settings for 10.24 MHz Output
Register name
Address
Value
(0x0)
(0x0)
reg_n_div
02
7D
reg_k_div_hi
03
00
reg_k_div_med
04
00
reg_k_div_lo
05
00
reg_o_div
06
0D
reg_b_div
07
00
reg_p_div
09
04
reg_vco
0A
01
Programming to use both outputs
Figure 19 shows a simplified diagram of the ACS1790T when using both the OUT1 and OUT2 outputs.
VCO
fPFD
fVCO
‘O’
DIVIDER
‘N’
INTEGER
DIVIDER
‘B’
DIVIDER
fOUT1
‘P’
DIVIDER
fOUT2
‘K’
FRACTIONAL
DIVIDER
FEEDBACK DIVIDER
Figure 19 : Simplified View of Both Output Paths
It can be seen that the VCO and O-divider is shared between both outputs. Therefore, it is only possible to use both outputs
simultaneously if a B-divider division ratio for the OUT1 frequency and a P-divider division ratio for the OUT2 frequency can be
found that both give a common frequency that is within one of the five allowable ranges for the O-divider output. If a suitable
common value can be found then the programming steps are just the combination of those in the previous two sections. If no
suitable common value can be found then it will be necessary to adopt an alternative approach, such as using two ACS1790Ts.
Table 20 gives the appropriate register values for OUT1 = 125 MHz, OUT2 = 25 MHz and OUT1 = 156.25 MHz, OUT2 = 25 MHz,
when using a 25 MHz reference clock. These output frequencies are typical of those required for 1G and 10G Ethernet systems
and are the two default output frequency sets of the ACS1790T, as selected at reset by the MODE pin. (Note that during reset
the ACS1790T programs the VCO PLL feedback divider depending on the input clock frequency so that the default output
frequencies are correct regardless of the input frequency being used.)
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Table 20 : Register Settings for Ethernet (25MHz input)
Register name
1G Ethernet
10G Ethernet
Address
OUT1 = 125 MHz,
OUT1 = 156.25 MHz,
(0x0)
OUT2 = 25 MHz
OUT2 = 25 MHz
Value (0x0)
Value (0x0)
reg_n_div
02
85
85
reg_k_div_hi
03
00
00
reg_k_div_med
04
00
00
reg_k_div_lo
05
00
00
reg_o_div
06
03
00
reg_b_div
07
02
03
reg_p_div
09
05
07
reg_vco
0A
01
01
fVCO = 2.5 GHz for both cases
Programming the feedback clock output
The feedback clock output FBCLK is designed to be used as the feedback output to an external phase-and-frequency detector
when the ACS1790T is used as part of a PLL application. The FBCLK output is a divided down version of either the OUT1 or
OUT2 outputs as shown in Figure 20.
‘P’
DIVIDER
fOUT2
‘B’
DIVIDER
fOUT1
‘T’
DIVIDER
fFBCLK
fbclk_
select
Figure 20 : FBCLK Output Path
The division ratio between the OUT1 or OUT2 and the FBCLK output is determined by the T-divider, which is programmed
through the reg_t_div register. Table 21 gives the allowable division ratios and corresponding reg_t_div register values. The
source of the T-divider is controlled by the fbclk_select bit in the reg_t_div register. When fbclk_select is zero the T-divider is
driven from the OUT1 clock frequency (the B-divider output) whereas when fbclk_select is one the T-divider is driven from the
OUT1 clock frequency (the P-divider output).
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The OUT1 and OUT2 outputs can be used to drive the T-divider even if the actual output pin is disabled through either the OEB
pin or the corresponding bits in the reg_output register. However, if either the B- or T-dividers are disabled (reg_b_div = 0, or
reg_t_div = 0) then that divider cannot be used as the input source for the T-divider.
Table 21 : Feedback Clock Output Division Ratios
T-Divider Division
reg_t_div Register
reg_t_div Register
Ratio
Setting
(source is OUT1)
Setting
(source is OUT2)
(0x0)
(0x0)
T-divider disabled
00
80
1
01
81
2
02
82
4
03
83
8
04
84
16
05
85
32
06
86
64
07
87
128
08
88
256
09
89
512
0A
8A
1024
0B
8B
2048
0C
8C
4096
0D
8D
8192
0E
8E
16384
0F
8F
Example:
Assume that the ACS1790T in the previous DOCSIS 3.0 DTI application is to be used as part of an FPGA-based phase locked
loop that has a phase-and-frequency detector that can accept a maximum input frequency of 1 MHz. Selecting a T-division ratio
of 16 will give a frequency at the FBCLK output of 10.24 / 16 = 650 kHz. From Table 21, noting that the T-divider needs to be
driven from the OUT2 frequency, the value to program into the reg_t_div register is 0x85. While a bigger division ratio could
have been chosen, too low a frequency is likely to impact the locking time of the external PLL and therefore it is recommended
to chose the highest acceptable FBCLK output frequency. Of course, the VCO controller in the external PLL needs to allow for the
selected T-divider division ratio in its loop gain calculation when setting the VCO frequency.
When using the FBCLK output, it must be explicitly enabled by setting the fbclk_enable bit in the reg_output register.
Configuration for common output frequencies
Table 22, Table 23 and Table 24 show the register settings for various common ACS1790T clock generator applications for
each possible input frequency. In each case, the value given for the reg_output register assumes that all three outputs (OUT1,
OUT2 and FBCLK) are enabled with the auto-squelch option on for each output, and with OUT2 using low drive strength with
slew-rate limiting. In some cases, the values in these tables may differ from those calculated using the information contained
elsewhere in this section. In such cases, the values from the tables should be used in preference to the calculated values.
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Table 22 : Register Settings for Common Frequencies (Input clock 10 or 20 MHz)
Output
Register Settings
Output
Frequency
reg_output
reg_n_div
reg_k_div_hi
reg_k_div_med
reg_k_div_lo
reg_o_div
reg_b_div
reg_p_div
(MHz)
(0x01)
(0x02)
(0x03)
(0x04)
(0x05)
(0x06)
(0x07)
(0x09)
Used
161.1328
68
BE
D0
0
0
06
01
00
1
156.25+25
6E
B7
0
0
0
00
03
07
1&2
155.52
68
B5
D4
FD
F4
06
01
00
1
149.797
68
AC
AC
AC
F8
06
01
00
1
125 + 25
6E
B7
0
0
0
03
02
05
1&2
77.76
68
B5
D4
FD
F4
06
02
00
1
51.84
68
B5
D4
FD
F4
04
03
00
1
49.152
68
BC
97
24
74
0B
02
00
1
44.736
68
B7
85
87
94
05
03
00
1
34.368
68
B4
73
18
FC
07
03
00
1
38.88
68
B5
D4
FD
F4
06
03
00
1
25.92
68
CA
91
68
72
0B
03
00
1
25
68
C1
0
0
0
0B
03
00
1
24.576
68
BC
97
24
74
0B
03
00
1
19.44
68
B5
D4
FD
F4
06
04
00
1
12.288
68
BC
97
24
74
0B
04
00
1
10
68
BD
0
0
0
06
05
00
1
49.152
66
BC
97
24
74
0B
00
01
2
44.736
66
B7
85
87
94
0C
00
01
2
34.368
66
B4
73
18
FC
07
00
02
2
38.88
66
B5
D4
FD
F4
06
00
02
2
25.92
66
CA
91
68
72
0B
00
02
2
25
66
C1
0
0
0
0B
00
02
2
24.576
66
BC
97
24
74
0B
00
02
2
19.44
66
B5
D4
FD
F4
06
00
04
2
12.288
66
BC
97
24
74
0B
00
04
2
10.24
66
B2
C2
8F
5C
0D
00
04
2
10
66
BD
0
0
0
06
00
06
2
6.48
66
B5
D4
FD
F4
0A
00
06
2
6.144
66
A8
ED
FA
44
0A
00
06
2
2.048
66
A8
ED
FA
44
07
00
09
2
1.544
66
BD
EB
ED
FA
0B
00
09
2
1.536
66
BC
97
24
74
0B
00
09
2
8 kHz
66
C3
24
DD
30
08
00
11
2
2 kHz
66
C3
24
DD
30
08
00
13
2
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Table 23 : Register Settings for Common Frequencies (Input clock 12.8 MHz)
Output
Register Settings
Output
Frequency
reg_output
reg_n_div
reg_k_div_hi
reg_k_div_med
reg_k_div_lo
reg_o_div
reg_b_div
reg_p_div
(MHz)
(0x01)
(0x02)
(0x03)
(0x04)
(0x05)
(0x06)
(0x07)
(0x09)
161.1328
68
86
6A
80
0
06
01
00
1
156.25+25
6E
80
50
0
0
00
03
07
1&2
155.52
68
7F
66
66
66
06
01
00
1
149.797
68
78
3E
E7
22
06
01
00
1
125 + 25
6E
80
50
0
0
03
02
05
1&2
77.76
68
7F
66
66
66
06
02
00
1
51.84
68
7F
66
66
66
04
03
00
1
49.152
68
84
AE
14
7A
0B
02
00
1
44.736
68
80
B8
51
EC
05
03
00
1
34.368
68
7E
51
EB
86
07
03
00
1
38.88
68
7F
66
66
66
06
03
00
1
25.92
68
8F
99
99
9A
0B
03
00
1
25
68
88
20
0
0
0B
03
00
1
24.576
68
84
AE
14
7A
0B
03
00
1
19.44
68
7F
66
66
66
06
04
00
1
12.288
68
84
AE
14
7A
0B
04
00
1
10
68
85
0
0
0
06
05
00
1
49.152
66
84
AE
14
7A
0B
00
01
2
44.736
66
80
B8
51
EC
0C
00
01
2
34.368
66
7E
51
EB
86
07
00
02
2
38.88
66
7F
66
66
66
06
00
02
2
25.92
66
8F
99
99
9A
0B
00
02
2
25
66
88
20
0
0
0B
00
02
2
24.576
66
84
AE
14
7A
0B
00
02
2
19.44
66
7F
66
66
66
06
00
04
2
12.288
66
84
AE
14
7A
0B
00
04
2
10.24
66
7D
0
0
0
0D
00
04
2
10
66
85
0
0
0
06
00
06
2
6.48
66
7F
66
66
66
0A
00
06
2
6.144
66
75
51
EB
86
0A
00
06
2
2.048
66
75
51
EB
86
07
00
09
2
1.544
66
85
B8
51
EC
0B
00
09
2
1.536
66
84
AE
14
7A
0B
00
09
2
8 kHz
66
89
CC
CC
CC
08
00
11
2
2 kHz
66
89
CC
CC
CC
08
00
13
2
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Table 24 : Register Settings for Common Frequencies (Input clock 25 MHz)
Output
Register Settings
Output
Frequency
reg_output
reg_n_div
reg_k_div_hi
reg_k_div_med
reg_k_div_lo
reg_o_div
reg_b_div
reg_p_div
(MHz)
(0x01)
(0x02)
(0x03)
(0x04)
(0x05)
(0x06)
(0x07)
(0x09)
161.1328
68
8B
40
0
0
06
01
00
1
156.25+25
6E
85
0
0
0
00
03
07
1&2
155.52
68
84
10
CB
2A
06
01
00
1
149.797
68
7C
BD
57
2C
06
01
00
1
125 + 25
6E
85
0
0
0
03
02
05
1&2
77.76
68
84
10
CB
2A
06
02
00
1
51.84
68
84
10
CB
2A
04
03
00
1
49.152
68
89
78
E9
F6
0B
02
00
1
44.736
68
85
6A
D2
DC
05
03
00
1
34.368
68
82
F5
AD
96
07
03
00
1
38.88
68
84
10
CB
2A
06
03
00
1
25.92
68
94
A7
86
C2
0B
03
00
1
25
68
8D
0
0
0
0B
03
00
1
24.576
68
89
78
E9
F6
0B
03
00
1
19.44
68
84
10
CB
2A
06
04
00
1
12.288
68
89
78
E9
F6
0B
04
00
1
10
68
89
CC
CC
CC
06
05
00
1
49.152
66
89
78
E9
F6
0B
00
01
2
44.736
66
85
6A
D2
DC
0C
00
01
2
34.368
66
82
F5
AD
96
07
00
02
2
38.88
66
84
10
CB
2A
06
00
02
2
25.92
66
94
A7
86
C2
0B
00
02
2
25
66
8D
0
0
0
0B
00
02
2
24.576
66
89
78
E9
F6
0B
00
02
2
19.44
66
84
10
CB
2A
06
00
04
2
12.288
66
89
78
E9
F6
0B
00
04
2
10.24
66
81
9B
A5
E4
0D
00
04
2
10
66
89
CC
CC
CC
06
00
06
2
6.48
66
84
10
CB
2A
0A
00
06
2
6.144
66
79
BE
61
D0
0A
00
06
2
2.048
66
79
BE
61
D0
07
00
09
2
1.544
66
8A
89
8B
2E
0B
00
09
2
1.536
66
89
78
E9
F6
0B
00
09
2
8 kHz
66
8E
B7
17
58
08
00
11
2
2 kHz
66
8E
B7
17
58
08
00
13
2
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Design considerations
As with all high-performance devices, correct circuit design and PCB layout is critical to achieve optimum performance from the
ACS1790T.
PCB placement
The ACS1790T should be placed on the PCB as close to the clock load as possible to minimize the length of the output clock
traces. As the OUT1 clock uses differential signaling it is important to route the two halves of the differential pair with as near
constant spacing between the traces as possible and with minimum difference in trace lengths between the positive and
negative signals.
Output clock termination
The OUT1 clock uses LVPECL signaling and must be properly terminated for correct operation. Figure 21 shows an example
termination scheme for use when the OUT1 clock drives an LVPECL input buffer.
+3.3V
130Ω
LVPECL
RECEIVER
130Ω
Z0 = 50Ω
ACS1790T OUT1
Z0 = 50Ω
82Ω
82Ω
Figure 21 : Recommended LVPECL DC Termination
In certain cases it may be desirable to implement an AC-coupling scheme between the OUT1 clock and its load. One example of
where this would be necessary is if there may be a difference in ground potential between the ACS1790T and its load, as may
happen if the two are on separate blades connected through a backplane. If DC-coupling was used in such a case then the
common-mode voltage specification of the receiver may be violated. Figure 22 shows an example termination circuit for using
the ACS1790T with AC coupling.
+3.3V
Z0 = 50Ω
0.01μF
Z0 = 50Ω
Z0 = 50Ω
0.01μF
Z0 = 50Ω
130Ω
130Ω
82Ω
82Ω
LVPECL
RECEIVER
ACS1790T OUT1
195Ω
195Ω
Figure 22 : Recommended LVPECL AC Termination
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The OUT2 clock uses LVCMOS signaling and should be properly terminated unless the distance between the ACS1790T and the
clock load is very short (less than 10 mm). In most instances a simple series termination resistor of between 25 and 35 ohms
placed close to the ACS1790T OUT2 pin will be sufficient. The FBCLK output should be terminated in the same way as the OUT2
clock.
In all cases, it is recommended that the behavior of the clock outputs is simulated using the Semtech supplied IBIS model for
the ACS1790T along with the appropriate vendor IBIS models for the clock receivers.
Power supplies and filtering
A multi-layer PCB design with solid power and ground planes is vital. If the power and ground planes are perforated by excessive
vias or split in any way then the design should be simulated as fully as possible to ensure that the output clock quality meets the
desired level.
Power supply filtering is also critical since any noise on the power supplies can adversely affect the VCO and PFD operation and
can result in increased output phase noise. The ACS1790T features separate power supply pins for the VCO (VDDF), high-speed
analogue circuitry (VDDA), digital circuitry (VDDD) and the OUT2 CMOS output (VDDD2). The VDDF, VDDA and VDDD supplies all
operate at 3.3V whereas the VDDD2 supply may be connected to 1.8V, 2.5V or 3.3V depending on the signaling level required
for the OUT2 output. Figure 23 shows the recommended power supply filtering for all the supplies. All filter components should
be placed as close to the corresponding device pins as possible. The filter capacitors should be selected to minimize series
inductance which would otherwise negate their filtering capability. Suitable parts include multi-layer ceramic capacitors in 0603
or smaller surface-mount packages. The stub lengths between PCB vias and the component pads should also be minimized and
sharing of vias for multiple connections on the same side of the PCB should be avoided unless it significantly reduces the
achievable stub length.
The center pad of the QFN-24 package, GND, should be connected to ground on the PCB for additional grounding and to provide
thermal coupling between the ACS1790T die and the PCB.
+3.3V
1Ω
VDDF
1nF
0.1μF
VSSF
1Ω
VDDA
1nF
0.1μF
VSSA
ACS1790T
+1.8/2.5/3.3V
VDDD2
+3.3V
0.1μF
VDDD
1nF
0.1μF
10μF
VSSD
VSSD2
GND
Figure 23 : Recommended Power Supply Filtering
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Application Example
Figure 24 shows an example application using the ACS1790T to provide the transmit clocks to a Synchronous Ethernet switch
and associated phy to support both 1G and 10G Ethernet.
In this example, the ACS1790T is configured to provide 25 MHz and 156.25 MHz output frequencies by default (MODE = 1). The
25 MHz output from OUT2 drives the Ethernet switch, which uses it as the timing reference for its 1G SGMII outputs. The
156.25 MHz output from OUT1 drives a 10G Ethernet phy that converts a 10G XAUI port from the switch into an SFI or XFI port.
This configuration allows for Synchronous Ethernet support on both the 1G and 10G external Ethernet links. The example
schematic shows the ACS1790T being used with a 12.8 MHz input clock (OSCFSLE1 = 1, OSCFSEL0 = 0) and configured to
appear at I2C address 0xC0 (A0, A1 = 0).
+3.3V
12.8 MHz
OCXO
4.7k
LOCK
DETECT
FEEDBACK
CLOCK
4.7k
LD
+3.3V
+3.3V
MODE
NC
CLK
33Ω
FBCLK
2.7k
25MHz
OUT2
2.7k
SCL
SDA
I2C
RESET
4.7k
4.7k
XAUI
130Ω
OSCFSEL1
156.25MHz
A0 A1
OUT1N
82Ω
4.7k
130Ω
OUT1P
OSCFSEL0
OEB
SGMII
+3.3V
ACS1790T
RESETB
+3.3V
4.7k
1/10G
Ethernet
Switch
10G
Ethernet
Phy
SFI/XFI
82Ω
1k
Figure 24 : Example Synchronous Ethernet Application
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Packaging Information
A
D
B
DIM
PIN 1
INDICATOR
(LASER MARK)
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
E
A2
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.031 .035 .039
.000 .001 .002
- (.008) .007 .010 .012
.152 .157 .163
.100 .106 .110
.152 .157 .163
.100 .106 .110
.020 BSC
.012 .016 .020
24
.004
.004
0.80 0.90 1.00
0.00 0.02 0.05
- (0.20) 0.18 0.25 0.30
3.85 4.00 4.15
2.55 2.70 2.80
3.85 4.00 4.15
2.55 2.70 2.80
0.50 BSC
0.30 0.40 0.50
24
0.10
0.10
A
SEATING
PLANE
aaa C
A1
C
D1
LxN
E/2
E1
2
1
N
bxN
bbb
e
C A B
D/2
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
Figure 25 : QFN-24 Physical Package Detail
K
DIMENSIONS
(C)
G
H
Y
Z
DIM
C
G
H
K
P
X
Y
Z
INCHES
(.156)
.122
.106
.106
.020
.010
.033
.189
MILLIMETERS
(3.95)
3.10
2.70
2.70
0.50
0.25
0.85
4.80
X
P
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
Figure 26 : Recommended PCB Land Pattern
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Revision Status/ History
PRELIMINARY status when initial prototype devices are
physically available, and the datasheet content more
accurately represents the realization of the design. The
datasheet is only raised to FINAL status after the device
has been fully characterized, and the datasheet content
updated with measured, rather than simulated
parameter values.
The Revision Status, as shown in top right corner of the
datasheet, may be TARGET, PRELIMINARY, or FINAL, and
refers to the status of the Device (not the datasheet),
with the design cycle. TARGET status is used when the
design is being realized but is not yet physically
available, and the datasheet content reflects the
intention of the design. The datasheet is raised to
Revision History
Revision
1.0
Date
May 2013
Revision 1.0 May 2013 © Semtech Corp.
Description of Changes
First release of final datasheet
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Ordering information
Table 25 : Ordering Information
Part Number
Description
Ultra Low Noise Frequency Synthesizer 2 kHz- 200MHz band – Pb and Halogen free, RoHS compliant
ACS1790IMLTRT Tape and reel packing – 3000 devices per reel
ACS1790IMLT
Ultra Low Noise Frequency Synthesizer 2 kHz- 200MHz band – Pb and Halogen free, RoHS compliant
Tray packing – 490 devices per tray
Disclaimers
Life support- This product is not designed or intended for use in life support equipment, devices or systems, or other critical
applications. This product is not authorized or warranted by Semtech for such use.
Right to change- Semtech Corporation reserves the right to make changes, without notice, to this product. Customers are
advised to obtain the latest version of the relevant information before placing orders.
Compliance to relevant standards- Operation of this device is subject to the User's implementation, and design practices. It is
the responsibility of the user to ensure equipment using this device is compliant to any relevant standards.
Notice: All referenced brands, product names, service names and trademarks are the property of their respective owners.
Contacts
For Additional Information, contact the following:
Semtech Corporation Advanced Communications Product Group
E-mail:
sales@semtech.com
Support_TOP@semtech.com
Internet:
http://www.semtech.com
USA:
Mailing Address: P.O. Box 6097, Camarillo, CA 93011-6097
Street Address: 200 Flynn Road, Camarillo, CA 93012-8790
Tel: +1 805 498 2111, Fax: +1 805 498 3804
FAR EAST:
11F, No. 46, Lane 11, Kuang Fu North Road, Taipei, R.O.C.
Tel: +886 2 2748 3380 Fax: +886 2 2748 3390
EUROPE:
Semtech Ltd., Units 2 and 3, Park Court, Premier Way,
Abbey Park Industrial Estate, Romsey, Hampshire, SO51 9DN
Tel: +44 (0)1794 527 600 Fax: +44 (0)1794 527 601
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CERTIFIED
Revision 1.0 May 2013 © Semtech Corp.
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