XD8870 DIP-18
XL8870 SOP-18
Features
Description
•
Complete DTMF Receiver
•
Low power consumption
•
Internal gain setting amplifier
•
Adjustable guard time
•
Central office quality
The XDXL/8870 is a complete DTMF
receiver integrating both the bandsplit filter and digital
decoder functions. The filter section uses switched
capacitor techniques for high and low group filters;
the decoder uses digital counting techniques to detect
and decode all 16 DTMF tone-pairs into a 4-bit code.
•
Power-down mode
•
Inhibit mode
Applications
•
Paging systems
•
Repeater systems/mobile radio
•
Credit card systems
•
Remote control
•
Personal computers
•
Telephone answering machine
VDD
PWDN
VSS
VRef
Bias
Circuit
VRef
Buffer
Q1
Chip Chip
Power Bias
IN +
High Group
Filter
Dial
Tone
Filter
IN -
INH
Digital
Detection
Algorithm
Code
Converter
and Latch
Q2
Zero Crossing
Detectors
Q3
Low Group
Filter
GS
Q4
to all
Chip
Clocks
OSC1
St
GT
OSC2
St/GT
Steering
Logic
ESt
Figure 1 - Functional Block Diagram
1
STD
TOE
XD8870 DIP-18
XL8870 SOP-18
External component count is minimized by on chip provision of a differential input amplifier, clock oscillator and
latched three-state bus interface.
IN+
INGS
VRef
INH
PWDN
OSC1
OSC2
VSS
18
17
16
15
14
13
12
11
10
1
2
3
4
5
6
7
8
9
VDD
St/GT
ESt
StD
Q4
Q3
Q2
Q1
TOE
DIP/SOP
Figure 2 - Pin Connections
Pin Description
Pin #
Name
Description
18
20
1
1
IN+
Non-Inverting Op-Amp (Input).
2
2
IN-
Inverting Op-Amp (Input).
3
3
GS
Gain Select. Gives access to output of front end differential amplifier for connection of
feedback resistor.
4
4
VRef
Reference Voltage (Output). Nominally VDD/2 is used to bias inputs at mid-rail (see Fig. 6
and Fig. 10).
5
5
INH
Inhibit (Input). Logic high inhibits the detection of tones representing characters A, B, C
and D. This pin input is internally pulled down.
6
6
PWDN
Power Down (Input). Active high. Powers down the device and inhibits the oscillator. This
pin input is internally pulled down.
7
8
OSC1
Clock (Input).
8
9
OSC2
Clock (Output). A 3.579545 MHz crystal connected between pins OSC1 and OSC2
completes the internal oscillator circuit.
9
10
VSS
Ground (Input). 0 V typical.
10
11
TOE
Three State Output Enable (Input). Logic high enables the outputs Q1-Q4. This pin is
pulled up internally.
1114
1215
Q1-Q4
Three State Data (Output). When enabled by TOE, provide the code corresponding to the
last valid tone-pair received (see Table 1). When TOE is logic low, the data outputs are high
impedance.
15
17
StD
Delayed Steering (Output).Presents a logic high when a received tone-pair has been
registered and the output latch updated; returns to logic low when the voltage on St/GT falls
below VTSt.
16
18
ESt
Early Steering (Output). Presents a logic high once the digital algorithm has detected a
valid tone pair (signal condition). Any momentary loss of signal condition will cause ESt to
return to a logic low.
2
XD8870 DIP-18
XL8870 SOP-18
Pin Description
Pin #
Name
Description
19
St/GT
Steering Input/Guard time (Output) Bidirectional. A voltage greater than VTSt detected at
St causes the device to register the detected tone pair and update the output latch. A
voltage less than VTSt frees the device to accept a new tone pair. The GT output acts to
reset the external steering time-constant; its state is a function of ESt and the voltage on St.
20
VDD
Positive power supply (Input). +5 V typical.
7,
16
NC
No Connection.
18
20
17
18
Functional Description
The XDXL/8870 monolithic DTMF receiver of fers small size, low power consumption and high
performance. Its architecture consists of a bandsplit filter section, which separates the high and low group tones,
followed by a digital counting section which verifies the frequency and duration of the received tones before passing
the corresponding code to the output bus.
Filter Section
Separation of the low-group and high group tones is achieved by applying the DTMF signal to the inputs of two
sixth-order switched capacitor bandpass filters, the bandwidths of which correspond to the low and high group
frequencies. The filter section also incorporates notches at 350 and 440 Hz for exceptional dial tone rejection (see
Figure 3). Each filter output is followed by a single order switched capacitor filter section which smooths the signals
prior to limiting. Limiting is performed by high-gain comparators which are provided with hysteresis to prevent
detection of unwanted low-level signals. The outputs of the comparators provide full rail logic swings at the
frequencies of the incoming DTMF signals.
0
PRECISE
DIAL TONES
10
X=350 Hz
Y=440 Hz
DTMF TONES
20
A=697 Hz
B=770 Hz
C=852 Hz
D=941 Hz
E=1209 Hz
F=1336 Hz
G=1477 Hz
H=1633 Hz
ATTENUATION
(dB)
30
40
50
X
Y
1kHz
A B C D
E
FREQUENCY (Hz)
F
Figure 3 - Filter Response
3
G
H
XD8870 DIP-18
XL8870 SOP-18
Decoder Section
Following the filter section is a decoder employing digital counting techniques to determine the frequencies of the
incoming tones and to verify that they correspond to standard DTMF frequencies. A complex averaging algorithm
protects against tone simulation by extraneous signals such as voice while providing tolerance to small frequency
deviations and variations. This averaging algorithm has been developed to ensure an optimum combination of
immunity to talk-off and tolerance to the presence of interfering frequencies (third tones) and noise. When the
detector recognizes the presence of two valid tones (this is referred to as the “signal condition” in some industry
specifications) the “Early Steering” (ESt) output will go to an active state. Any subsequent loss of signal condition
will cause ESt to assume an inactive state (see “Steering Circuit”).
VDD
C
VDD
St/GT
vc
ESt
R
StD
XDXL/8870
tGTA=(RC)In(VDD/VTSt)
tGTP=(RC)In[VDD/(VDD-VTSt)]
Figure 4 - Basic Steering Circuit
Steering Circuit
Before registration of a decoded tone pair, the receiver checks for a valid signal duration (referred to as character
recognition condition). This check is performed by an external RC time constant driven by ESt. A logic high on ESt
causes vc (see Figure 4) to rise as the capacitor discharges. Provided signal condition is maintained (ESt remains
high) for the validation period (tGTP), vc reaches the threshold (VTSt) of the steering logic to register the tone pair,
latching its corresponding 4-bit code (see Table 1) into the output latch. At this point the GT output is activated and
drives vc to VDD. GT continues to drive high as long as ESt remains high. Finally, after a short delay to allow the
output latch to settle, the delayed steering output flag (StD) goes high, signalling that a received tone pair has been
registered. The contents of the output latch are made available on the 4-bit output bus by raising the three state
control input (TOE) to a logic high. The steering circuit works in reverse to validate the interdigit pause between
signals. Thus, as well as rejecting signals too short to be considered valid, the receiver will tolerate signal
interruptions (dropout) too short to be considered a valid pause. This facility, together with the capability of selecting
the steering time constants externally, allows the designer to tailor performance to meet a wide variety of system
requirements.
Guard Time Adjustment
In many situations not requiring selection of tone duration and interdigital pause, the simple steering circuit shown
in Figure 4 is applicable. Component values are chosen according to the formula:
tREC=tDP+tGTP
tID=tDA+tGTA
4
XD8870 DIP-18
XL8870 SOP-18
The value of tDP is a device parameter (see Figure 11) and tREC is the minimum signal duration to be recognized by
the receiver. A value for C of 0.1 µF is recommended for most applications, leaving R to be selected by the
designer.
tGTP=(RPC1)In[VDD/(VDD-VTSt)]
VDD
tGTA=(R1C1)In(VDD/VTSt)
C1
RP=(R1R2)/(R1+R2)
St/GT
R1
R2
ESt
a) decreasing tGTP; (tGTPtGTA)
Figure 5 - Guard Time Adjustment
Digit
TOE
INH
ESt
Q4
Q3
Q2
Q1
ANY
L
X
H
Z
Z
Z
Z
1
H
X
H
0
0
0
1
2
H
X
H
0
0
1
0
3
H
X
H
0
0
1
1
4
H
X
H
0
1
0
0
5
H
X
H
0
1
0
1
6
H
X
H
0
1
1
0
7
H
X
H
0
1
1
1
8
H
X
H
1
0
0
0
9
H
X
H
1
0
0
1
0
H
X
H
1
0
1
0
*
H
X
H
1
0
1
1
#
H
X
H
1
1
0
0
A
H
L
H
1
1
0
1
B
H
L
H
1
1
1
0
C
H
L
H
1
1
1
1
D
H
L
H
0
0
0
0
A
H
H
L
B
H
H
L
C
H
H
L
D
H
H
L
undetected, the output code
will remain the same as the
previous detected code
Table 1 - Functional Decode Table
L=LOGIC LOW, H=LOGIC HIGH, Z=HIGH IMPEDANCE
X = DON‘T CARE
5
XD8870 DIP-18
XL8870 SOP-18
Different steering arrangements may be used to select independently the guard times for tone present (tGTP) and
tone absent (tGTA). This may be necessary to meet system specifications which place both accept and reject limits
on both tone duration and interdigital pause. Guard time adjustment also allows the designer to tailor system
parameters such as talk off and noise immunity. Increasing tREC improves talk-off performance since it reduces the
probability that tones simulated by speech will maintain signal condition long enough to be registered. Alternatively,
a relatively short tREC with a long tDO would be appropriate for extremely noisy environments where fast acquisition
time and immunity to tone drop-outs are required. Design information for guard time adjustment is shown in Figure
5.
Power-down and Inhibit Mode
A logic high applied to pin 6 (PWDN) will power down the device to minimize the power consumption in a standby
mode. It stops the oscillator and the functions of the filters.
Inhibit mode is enabled by a logic high input to the pin 5 (INH). It inhibits the detection of tones representing
characters A, B, C, and D. The output code will remain the same as the previous detected code (see Table 1).
Differential Input Configuration
The input arrangement of the XDXL/8870 provides a differential-input operational amplifier as well as a
bias source (VRef) which is used to bias the inputs at mid-rail. Provision is made for connection of a feedback
resistor to the op-amp output (GS) for adjustment of gain. In a single-ended configuration, the input pins are
connected as shown in Figure 10 with the op-amp connected for unity gain and VRef biasing the input at 1/2VDD.
Figure 6 shows the differential configuration, which permits the adjustment of gain with the feedback resistor R5.
Crystal Oscillator
The internal clock circuit is completed with the addition of an external 3.579545 MHz crystal and is normally
connected as shown in Figure 10 (Single-Ended Input Configuration). However, it is possible to configure several
XDXL/8870 devices employing only a single oscillator crystal. The oscillator output of the first device in
the chain is coupled through a 30 pF capacitor to the oscillator input (OSC1) of the next device. Subsequent
devices are connected in a similar fashion. Refer to Figure 7 for details. The problems associated with unbalanced
loading are not a concern with the arrangement shown, i.e., precision balancing capacitors are not required.
6
XD8870 DIP-18
XL8870 SOP-18
XDXL/8870
C1
R1
IN+
+
IN-
C2
R4
R5
R3
GS
R2
VRef
Differential Input Amplifier
C1=C2=10 nF
R1=R4=R5=100 kΩ
All resistors are ±1% tolerance.
R2=60kΩ, R3=37.5 kΩ
All capacitors are ±5% tolerance.
R3=
R 2R 5
R2+R5
VOLTAGE GAIN (Av diff)=
R5
R1
INPUT IMPEDANCE
(ZINDIFF) = 2
R 1 2+
1
ωc
2
Figure 6 - Differential Input Configuration
To OSC1 of next
XDXL/8870
C
X-tal
OSC1
OSC2
OSC2
OSC1
C
C=30 pF
X-tal=3.579545 MHz
Figure 7 - Oscillator Connection
7
XD8870 DIP-18
XL8870 SOP-18
Parameter
Unit
Resonator
R1
Ohms
10.752
L1
mH
.432
C1
pF
4.984
C0
pF
37.915
Qm
-
896.37
∆f
%
±0.2%
Table 2 - Recommended Resonator Specifications
Note: Qm=quality factor of RLC model, i.e., 1/2ΠƒR1C1.
Applications
Receiver System for British Telecom Spec POR 1151
The circuit shown in Fig. 9 illustrates the use of XDXL/8870 device in a typical receiver system. BT Spec defines
the input signals less than -34 dBm as the non-operate level. This condition can be attained by choosing a suitable
values of R1 and R2 to provide 3 dB attenuation, such that -34 dBm input signal will correspond to -37 dBm at the
gain setting pin GS of XDXL/8870. As shown in the diagram, the component values of R3 and C2 are the guard
time requirements when the total component tolerance is 6%. For better performance, it is recommended to use the
non-symmetric guard time circuit in Fig. 8.
tGTP=(RPC1)In[VDD/(VDD-VTSt)]
tGTA=(R1C1)In(VDD/VTSt)
RP=(R1R2)/(R1+R2)
VDD
C1
St/GT
R1
R2
ESt
Notes:
R1=368 K Ω ± 1%
R2=2.2 M Ω ± 1%
C1=100 nF ± 5%
Figure 8 - Non-Symmetric Guard Time Circuit
8
XD8870 DIP-18
XL8870 SOP-18
VDD
C1
DTMF
Input
R1
R2
X1
C2
XDXL/8870
IN+
VDD
IN-
St/GT
GS
VRef
ESt
StD
INH
Q4
PWDN
Q3
OSC 1
Q2
OSC 2
VSS
Q1
TOE
R3
NOTES:
R1 = 102 KΩ ± 1%
R2 = 71.5 KΩ ± 1%
R3 = 390 KΩ ±1%
C1,C2 = 100 nF ± 5%
X1 = 3.579545 MHz ± 0.1%
VDD = 5.0 V ± 5%
Figure 9 - Single-Ended Input Configuration for BT or CEPT Spec
9
XD8870 DIP-18
XL8870 SOP-18
Absolute Maximum Ratings†
Parameter
Symbol
1
DC Power Supply Voltage
2
Voltage on any pin
VI
3
Current at any pin (other than supply)
II
4
Storage temperature
5
Package power dissipation
Min.
Max.
Units
7
V
VDD+0.3
V
10
mA
+150
°C
500
mW
VDD
VSS-0.3
TSTG
-65
PD
† Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.
Derate above 75°C at 16 mW / °C. All leads soldered to board.
Recommended Operating Conditions - Voltages are with respect to ground (VSS) unless otherwise stated.
Parameter
Sym.
Min.
Typ.‡
Max.
Units
5.0
5.25
V
+85
°C
1
DC Power Supply Voltage
VDD
4.75
2
Operating Temperature
TO
-40
3
Crystal/Clock Frequency
fc
3.579545
MHz
4
Crystal/Clock Freq.Tolerance
∆fc
±0.1
%
Test Conditions
‡ Typical figures are at 25°C and are for design aid only: not guaranteed and not subject to production testing.
DC Electrical Characteristics - VDD=5.0V± 5%, VSS=0V, -40°C ≤ TO ≤ +85°C, unless otherwise stated.
Typ.‡
Max.
Units
IDDQ
10
25
µA
Operating supply current
IDD
3.0
9.0
mA
Power consumption
PO
15
4
High level input
VIH
5
Low level input voltage
VIL
Characteristics
1
2
3
S
U
P
P
L
Y
6
Standby supply current
Min.
mW
3.5
1.5
Test Conditions
PWDN=VDD
fc=3.579545 MHz
V
VDD=5.0 V
V
VDD=5.0 V
µA
VIN=VSS or VDD
IIH/IIL
0.1
Pull up (source) current
ISO
7.5
20
µA
TOE (pin 10)=0,
VDD=5.0 V
Pull down (sink) current
ISI
15
45
µA
INH=5.0 V,
PWDN=5.0 V,
VDD=5.0 V
9
Input impedance (IN+, IN-)
RIN
10
MΩ
@ 1 kHz
10
Steering threshold voltage
VTSt
7
8
Input leakage current
Sym.
I
N
P
U
T
S
2.2
10
2.4
2.5
V
VDD = 5.0 V
XD8870 DIP-18
XL8870 SOP-18
DC Electrical Characteristics - VDD=5.0V± 5%, VSS=0V, -40°C ≤ TO ≤ +85°C, unless otherwise stated.
Characteristics
11
12
13
14
15
O
U
T
P
U
T
S
Sym.
Typ.‡
Min.
Max.
Units
Test Conditions
VSS+0.0
3
V
No load
V
No load
Low level output voltage
VOL
High level output voltage
VOH
VDD0.03
Output low (sink) current
IOL
1.0
2.5
mA
VOUT=0.4 V
Output high (source) current
IOH
0.4
0.8
mA
VOUT=4.6 V
VRef output voltage
VRef
2.3
2.5
2.7
V
No load, VDD = 5.0V
16
VRef output resistance
ROR
1
kΩ
‡ Typical figures are at 25°C and are for design aid only: not guaranteed and not subject to production testing.
Operating Characteristics - VDD=5.0V±5%, VSS=0V, -40°C ≤ TO ≤ +85°C,unless otherwise stated.
Gain Setting Amplifier
Characteristics
Sym.
Min.
Typ.‡
Max.
Units
100
nA
Test Conditions
VSS ≤ VIN ≤ VDD
1
Input leakage current
IIN
2
Input resistance
RIN
3
Input offset voltage
VOS
4
Power supply rejection
PSRR
50
dB
1 kHz
5
Common mode rejection
CMRR
40
dB
0.75 V ≤ VIN ≤ 4.25 V biased
at VRef =2.5 V
6
DC open loop voltage gain
AVOL
32
dB
7
Unity gain bandwidth
fC
0.30
MHz
8
Output voltage swing
VO
4.0
Vpp
9
Maximum capacitive load
(GS)
CL
100
pF
10
Resistive load (GS)
RL
50
kΩ
11
Common mode range
VCM
10
MΩ
25
2.5
mV
Vpp
11
Load ≥ 100 kΩ to VSS @ GS
No Load
XD8870 DIP-18
XL8870 SOP-18
MT8870D AC Electrical Characteristics -VDD=5.0V ±5%, VSS=0V, -40°C ≤ TO ≤ +85°C, using Test Circuit shown in Figure 10.
Characteristics
Sym.
Min.
Typ.‡
Max.
Units
Notes*
-29
+1
dBm
1,2,3,5,6,9
27.5
869
mVRMS
1,2,3,5,6,9
1
Valid input signal levels (each
tone of composite signal)
2
Negative twist accept
8
dB
2,3,6,9,12
3
Positive twist accept
8
dB
2,3,6,9,12
4
Frequency deviation accept
±1.5% ± 2 Hz
2,3,5,9
5
Frequency deviation reject
±3.5%
2,3,5,9
6
Third tone tolerance
-16
dB
2,3,4,5,9,10
7
Noise tolerance
-12
dB
2,3,4,5,7,9,10
8
Dial tone tolerance
+22
dB
2,3,4,5,8,9,11
‡ Typical figures are at 25°C and are for design aid only: not guaranteed and not subject to production testing.
*NOTES
1. dBm= decibels above or below a reference power of 1 mW into a 600 ohm load.
2. Digit sequence consists of all DTMF tones.
3. Tone duration= 40 ms, tone pause= 40 ms.
4. Signal condition consists of nominal DTMF frequencies.
5. Both tones in composite signal have an equal amplitude.
6. Tone pair is deviated by ±1.5% ± 2 Hz.
7. Bandwidth limited (3 kHz) Gaussian noise.
8. The precise dial tone frequencies are (350 Hz and 440 Hz) ± 2%.
9. For an error rate of better than 1 in 10,000.
10. Referenced to lowest level frequency component in DTMF signal.
11. Referenced to the minimum valid accept level.
12. Guaranteed by design and characterization.
12
XD8870 DIP-18
XL8870 SOP-18
XDXL/8870 AC Electrical Characteristics -VDD=5.0V±5%, VSS=0V, -40°C ≤ TO ≤ +85°C, using Test Circuit shown in Figure 10.
Characteristics
Sym.
Min.
Max.
Units
-31
+1
dBm
21.8
869
mVRMS
Notes*
Tested at
VDD=5.0 V
1,2,3,5,6,9
1
Valid input signal levels (each
tone of composite signal)
2
Input Signal Level Reject
3
Negative twist accept
4
Positive twist accept
5
Frequency deviation accept
±1.5%± 2 Hz
2,3,5,9
6
Frequency deviation reject
±3.5%
2,3,5,9
7
Third zone tolerance
8
Noise tolerance
-37
dBm
10.9
mVRMS
Tested at
VDD=5.0 V
1,2,3,5,6,9
8
dB
2,3,6,9,13
8
dB
2,3,6,9,13
9
‡
Typ.‡
-18.5
dB
2,3,4,5,9,12
-12
dB
2,3,4,5,7,9,10
Dial tone tolerance
+22
dB
Typical figures are at 25 °C and are for design aid only: not guaranteed and not subject to production testing.
2,3,4,5,8,9,11
*NOTES
1. dBm= decibels above or below a reference power of 1 mW into a 600 ohm load.
2. Digit sequence consists of all DTMF tones.
3. Tone duration= 40 ms, tone pause= 40 ms.
4. Signal condition consists of nominal DTMF frequencies.
5. Both tones in composite signal have an equal amplitude.
6. Tone pair is deviated by ±1.5%± 2 Hz.
7. Bandwidth limited (3 kHz) Gaussian noise.
8. The precise dial tone frequencies are (350 Hz and 440 Hz) ± 2%.
9. For an error rate of better than 1 in 10,000.
10. Referenced to lowest level frequency component in DTMF signal.
11. Referenced to the minimum valid accept level.
12. Referenced to Fig. 10 input DTMF tone level at -25dBm (-28dBm at GS Pin) interference frequency range between 480-3400Hz.
13. Guaranteed by design and characterization.
13
XD8870 DIP-18
XL8870 SOP-18
AC Electrical Characteristics - VDD=5.0V±5%, VSS=0V, -40°C ≤ To ≤ +85°C, using Test Circuit shown in Figure 10.
Sym.
Min.
Typ.‡
Max.
Units
Tone present detect time
tDP
5
11
14
ms
Note 1
Tone absent detect time
tDA
0.5
4
8.5
ms
Note 1
Tone duration accept
tREC
40
ms
Note 2
Tone duration reject
tREC
ms
Note 2
ms
Note 2
ms
Note 2
Characteristics
1
2
3
4
5
T
I
M
I
N
G
20
Interdigit pause accept
tID
6
Interdigit pause reject
tDO
7
Propagation delay (St to Q)
tPQ
8
11
µs
TOE=VDD
Propagation delay (St to StD)
tPStD
12
16
µs
TOE=VDD
Output data set up (Q to StD)
tQStD
3.4
µs
TOE=VDD
Propagation delay (TOE to Q
ENABLE)
tPTE
50
ns
load of 10 kΩ,
50 pF
Propagation delay (TOE to Q
DISABLE)
tPTD
300
ns
load of 10 kΩ,
50 pF
Power-up time
tPU
30
ms
Note 3
Power-down time
tPD
20
ms
Crystal/clock frequency
fC
8
9
10
11
12
13
O
U
T
P
U
T
S
P
D
W
N
14
15
16
17
18
C
L
O
C
K
40
Conditions
20
3.575
9
3.579
5
3.583
1
MHz
Clock input rise time
tLHCL
110
ns
Ext. clock
Clock input fall time
tHLCL
110
ns
Ext. clock
Clock input duty cycle
DCCL
60
%
Ext. clock
30
pF
Capacitive load (OSC2)
CLO
40
50
‡ Typical figures are at 25°C and are for design aid only: not guaranteed and not subject to production testing.
*NOTES:
1.
Used for guard-time calculation purposes only.
2.
These, user adjustable parameters, are not device specifications. The adjustable settings of these minimums and maximums
are recommendations based upon network requirements.
3.
With valid tone present at input, t PU equals time from PDWN going low until ESt going high.
14
XD8870 DIP-18
XL8870 SOP-18
VDD
C1
DTMF
Input
R1
R2
X-tal
C2
XDXL/8870
IN+
VDD
IN-
St/GT
GS
VRef
StD
INH
Q4
PDWN
Q3
OSC 1
Q2
OSC 2
VSS
Q1
TOE
R3
ESt
NOTES:
R1,R2=100 KΩ ± 1%
R3=300 KΩ ± 1%
C1,C2=100 nF ± 5%
X-tal=3.579545 MHz ± 0.1%
Figure 10 - Single-Ended Input Configuration
15
XD8870 DIP-18
XL8870 SOP-18
D
A
EVENTS
B
C
tREC
tREC
E
TONE
#n + 1
tDP
G
tDO
tID
TONE #n
Vin
F
TONE
#n + 1
tDA
ESt
tGTA
tGTP
VTSt
St/GT
tPQ
Q1-Q4
tQStD
#n
DECODED TONE # (n-1)
HIGH IMPEDANCE
# (n + 1)
tPSrD
StD
tPTD
TOE
tPTE
EXPLANATION OF EVENTS
A)
TONE BURSTS DETECTED, TONE DURATION INVALID, OUTPUTS NOT UPDATED.
B)
TONE #n DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN OUTPUTS
C)
END OF TONE #n DETECTED, TONE ABSENT DURATION VALID, OUTPUTS REMIAN LATCHED UNTIL NEXT VALID
TONE.
D)
OUTPUTS SWITCHED TO HIGH IMPEDANCE STATE.
E)
TONE #n + 1 DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN OUTPUTS (CURRENTLY HIGH
IMPEDANCE).
F)
ACCEPTABLE DROPOUT OF TONE #n + 1, TONE ABSENT DURATION INVALID, OUTPUTS REMAIN LATCHED.
G)
END OF TONE #n + 1 DETECTED, TONE ABSENT DURATION VALID, OUTPUTS REMAIN LATCHED UNTIL NEXT VALID
TONE.
EXPLANATION OF SYMBOLS
Vin
DTMF COMPOSITE INPUT SIGNAL.
ESt
EARLY STEERING OUTPUT. INDICATES DETECTION OF VALID TONE FREQUENCIES.
St/GT
STEERING INPUT/GUARD TIME OUTPUT. DRIVES EXTERNAL RC TIMING CIRCUIT.
Q1-Q4
4-BIT DECODED TONE OUTPUT.
StD
DELAYED STEERING OUTPUT. INDICATES THAT VALID FREQUENCIES HAVE BEEN PRESENT/ABSENT FOR THE
REQUIRED GUARD TIME THUS CONSTITUTING A VALID SIGNAL.
TOE
TONE OUTPUT ENABLE (INPUT). A LOW LEVEL SHIFTS Q1-Q4 TO ITS HIGH IMPEDANCE STATE.
tREC
MAXIMUM DTMF SIGNAL DURATION NOT DETECED AS VALID
tREC
MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION
tID
MAXIMUM TIME BETWEEN VALID DTMF SIGNALS.
tDO
MAXIMUM ALLOWABLE DROP OUT DURING VALID DTMF SIGNAL.
tDP
TIME TO DETECT THE PRESENCE OF VALID DTMF SIGNALS.
tDA
TIME TO DETECT THE ABSENCE OF VALID DTMF SIGNALS.
tGTP
GUARD TIME, TONE PRESENT.
tGTA
GUARD TIME, TONE ABSENT.
Figure 11 - Timing Diagram
16
XD8870 DIP-18
XL8870 SOP-18
DIP18
17
XD8870 DIP-18
XL8870 SOP-18
SOP-18
18
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