M41T81
Serial access real-time clock with alarm
Datasheet - production data
Description
8
1
SO8
8-pin SOIC
Features
For all new designs use M41T81S
Counters for tenths/hundredths of seconds,
seconds, minutes, hours, day, date, month,
year, and century
32 KHz crystal oscillator integrating load
capacitance (12.5 pF) providing exceptional
oscillator stability and high crystal series
resistance operation
Serial interface supports I2C bus (400 kHz
protocol)
Ultra-low battery supply current of 0.6 ΜA
(typ at 3 V)
2.0 to 5.5 V clock operating voltage
Automatic switchover and deselect circuitry (for
3 V application select M41T81S datasheet)
Power-down time stamp (HT bit) allowing
determination of time elapsed in battery
backup
The M41T81 is a low-power serial RTC with a
built-in 32.768 kHz oscillator (external crystal
controlled). Eight bytes of the SRAM are used for
the clock/calendar function and are configured in
binary-coded decimal (BCD) format. An additional
12 bytes of SRAM provide status/control of alarm,
watchdog and square wave functions. Addresses
and data are transferred serially via a two line,
bidirectional I2C interface. The built-in address
register is incremented automatically after each
WRITE or READ data byte.
The M41T81 has a built-in power sense circuit
which detects power failures and automatically
switches to the battery supply when a power
failure occurs. The energy needed to sustain the
SRAM and clock operations can be supplied by a
small lithium button supply when a power failure
occurs.
Functions available to the user include a nonvolatile, time-of-day clock/calendar, alarm
interrupts, watchdog timer and programmable
square wave output. The eight clock address
locations contain the century, year, month, date,
day, hour, minute, second and tenths/hundredths
of a second in 24-hour BCD format. Corrections
for 28, 29 (leap year - valid until year 2100), 30
and 31 day months are made automatically.
The M41T81 is supplied in an 8-pin SOIC.
Programmable alarm and interrupt function
(valid even during battery backup mode)
Accurate programmable watchdog timer (from
62.5 ms to 128 s)
Software clock calibration to compensate
crystal deviation due to temperature
Operating temperature of –40 to 85 °C
ECOPACK® package available
February 2014
This is information on a product in full production.
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www.st.com
Contents
M41T81
Contents
1
Device overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
3
2-wire bus characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1
Bus not busy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.2
Start data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.3
Stop data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4
Data valid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.5
Acknowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2
READ mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3
WRITE mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4
Data retention mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Clock operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1
Power-down time-stamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2
Clock registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
Calibrating the clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.4
Setting alarm clock registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.5
Watchdog timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.6
Square wave output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.7
Century bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.8
Output driver pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.9
Preferred initial power-on default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5
DC and AC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6
Package mechanical information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7
Part numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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M41T81
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Signal names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Clock register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Alarm repeat modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Square wave output frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Preferred default values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Operating and AC measurement conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
DC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Crystal electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Power down/up AC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Power down/up trip points DC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
AC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
SO8 – 8-lead plastic small outline (150 mils body width), package mechanical data. . . . . 26
Ordering information scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Document revision history. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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List of figures
M41T81
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
4/29
Logic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8-pin SOIC connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Serial bus data transfer sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Acknowledgement sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Slave address location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
READ mode sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Alternative READ mode sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
WRITE mode sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Crystal accuracy across temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Clock calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Alarm interrupt reset waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Backup mode alarm waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
AC measurement I/O waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Power down/up mode AC waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Bus timing requirements sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
SO8 – 8-lead plastic small package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
DocID007529 Rev 11
M41T81
1
Device overview
Device overview
Figure 1. Logic diagram
VCC
VBAT
XI
XO
IRQ/FT/OUT/SQW
M41T81
SCL
SDA
VSS
AI04613
Table 1. Signal names
XI
Oscillator input
XO
Oscillator output
IRQ/OUT/FT/SQW
Interrupt / output driver / frequency test / square wave (open drain)
SDA
Serial data input/output
SCL
Serial clock input
VBAT
Battery supply voltage
VCC
Supply voltage
VSS
Ground
Figure 2. 8-pin SOIC connections
XI
XO
VBAT
VSS
1
2
3
4
M41T81
8
7
6
5
VCC
IRQ/FT/OUT/SQW
SCL
SDA
AI04769
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Device overview
M41T81
Figure 3. Block diagram
REAL TIME CLOCK
CALENDAR
32KHz
OSCILLATOR
CRYSTAL
RTC W/ALARM
& CALIBRATION
AFE
IRQ/FT/OUT/SQW(1,2)
WATCHDOG
SDA
I2C
INTERFACE
SQWE
SQUARE WAVE
SCL
WRITE
PROTECT
FREQUENCY TEST FT
OUTPUT DRIVER
OUT
INTERNAL
POWER
VCC
VBAT
(3)
VSO
COMPARE
AI04616
1. Open drain output
2. Square wave function has the highest priority on IRQ/FT/OUT/SQW output.
3. VSO = VBAT – 0.5 V (typ)
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M41T81
2
Operation
Operation
The M41T81 clock operates as a slave device on the serial bus. Access is obtained by
implementing a start condition followed by the correct slave address (D0h). The 20 bytes
contained in the device can then be accessed sequentially in the following order:
1st byte: tenths/hundredths of a second register
2nd byte: seconds register
3rd byte: minutes register
4th byte: century/hours register
5th byte: day register
6th byte: date register
7th byte: month register
8th byte: year register
9th byte: control register
10th byte: watchdog register
11th - 16th bytes: alarm registers
17th - 19th bytes: reserved
20th byte: square wave register
The M41T81 clock continually monitors VCC for an out-of-tolerance condition. Should VCC
fall below VSO, the device terminates an access in progress and resets the device address
counter. Inputs to the device will not be recognized at this time to prevent erroneous data
from being written to the device from a an out-of-tolerance system. The device also
automatically switches over to the battery and powers down into an ultra low current mode
of operation to conserve battery life. As system power returns and VCC rises above VSO, the
battery is disconnected, and the power supply is switched to external VCC.
For more information on battery storage life refer to application note AN1012.
2.1
2-wire bus characteristics
The bus is intended for communication between different ICs. It consists of two lines: a bidirectional data signal (SDA) and a clock signal (SCL). Both the SDA and SCL lines must be
connected to a positive supply voltage via a pull-up resistor.
The following protocol has been defined:
Data transfer may be initiated only when the bus is not busy.
During data transfer, the data line must remain stable whenever the clock line is high.
Changes in the data line, while the clock line is high, will be interpreted as control
signals.
Accordingly, the following bus conditions have been defined:
2.1.1
Bus not busy
Both data and clock lines remain high.
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Operation
2.1.2
M41T81
Start data transfer
A change in the state of the data line, from high to low, while the clock is high, defines the
START condition.
2.1.3
Stop data transfer
A change in the state of the data line, from low to high, while the clock is high, defines the
STOP condition.
2.1.4
Data valid
The state of the data line represents valid data when after a start condition, the data line is
stable for the duration of the high period of the clock signal. The data on the line may be
changed during the low period of the clock signal. There is one clock pulse per bit of data.
Each data transfer is initiated with a start condition and terminated with a stop condition. The
number of data bytes transferred between the start and stop conditions is not limited. The
information is transmitted byte-wide and each receiver acknowledges with a ninth bit.
By definition a device that gives out a message is called “transmitter,” the receiving device
that gets the message is called “receiver.” The device that controls the message is called
“master.” The devices that are controlled by the master are called “slaves.”
2.1.5
Acknowledge
Each byte of eight bits is followed by one acknowledge bit. This acknowledge bit is a low
level put on the bus by the receiver whereas the master generates an extra acknowledge
related clock pulse. A slave receiver which is addressed is obliged to generate an
acknowledge after the reception of each byte that has been clocked out of the slave
transmitter.
The device that acknowledges has to pull down the SDA line during the acknowledge clock
pulse in such a way that the SDA line is a stable low during the high period of the
acknowledge related clock pulse. Of course, setup and hold times must be taken into
account. A master receiver must signal an end of data to the slave transmitter by not
generating an acknowledge on the last byte that has been clocked out of the slave. In this
case the transmitter must leave the data line high to enable the master to generate the
STOP condition.
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M41T81
Operation
Figure 4. Serial bus data transfer sequence
DATA LINE
STABLE
DATA VALID
CLOCK
DATA
START
CONDITION
CHANGE OF
DATA ALLOWED
STOP
CONDITION
AI00587
Figure 5. Acknowledgement sequence
CLOCK PULSE FOR
ACKNOWLEDGEMENT
START
SCL FROM
MASTER
DATA OUTPUT
BY TRANSMITTER
1
2
MSB
8
9
LSB
DATA OUTPUT
BY RECEIVER
AI00601
2.2
READ mode
In this mode the master reads the M41T81 slave after setting the slave address (see
Figure 7 on page 10). Following the WRITE mode control bit (R/W=0) and the acknowledge
bit, the word address 'An' is written to the on-chip address pointer. Next the START
condition and slave address are repeated followed by the READ mode control bit (R/W=1).
At this point the master transmitter becomes the master receiver. The data byte which was
addressed will be transmitted and the master receiver will send an acknowledge bit to the
slave transmitter. The address pointer is only incremented on reception of an acknowledge
clock. The M41T81 slave transmitter will now place the data byte at address An+1 on the
bus, the master receiver reads and acknowledges the new byte and the address pointer is
incremented to “An+2.”
This cycle of reading consecutive addresses will continue until the master receiver sends a
STOP condition to the slave transmitter.
The system-to-user transfer of clock data will be halted whenever the address being read is
a clock address (00h to 07h). The update will resume due to a stop condition or when the
pointer increments to any non-clock address (08h-13h).
Note:
This is true both in READ mode and WRITE mode.
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Operation
M41T81
An alternate READ mode may also be implemented whereby the master reads the M41T81
slave without first writing to the (volatile) address pointer. The first address that is read is the
last one stored in the pointer (see Figure 8 on page 10).
Figure 6. Slave address location
R/W
START
A
1
LSB
MSB
SLAVE ADDRESS
1
0
1
0
0
0
AI00602
SLAVE
ADDRESS
DATA n+1
ACK
DATA n
ACK
S
ACK
BUS ACTIVITY:
R/W
START
WORD
ADDRESS (An)
ACK
S
R/W
SDA LINE
ACK
BUS ACTIVITY:
MASTER
START
Figure 7. READ mode sequence
STOP
SLAVE
ADDRESS
DATA n+X
NO ACK
P
AI00899
STOP
SLAVE
ADDRESS
10/29
DATA n+X
ACK
BUS ACTIVITY:
DATA n+1
ACK
DATA n
P
NO ACK
R/W
S
ACK
SDA LINE
ACK
BUS ACTIVITY:
MASTER
START
Figure 8. Alternative READ mode sequence
AI00895
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M41T81
2.3
Operation
WRITE mode
In this mode the master transmitter transmits to the M41T81 slave receiver. Bus protocol is
shown in Figure 9 on page 11. Following the START condition and slave address, a logic '0'
(R/W=0) is placed on the bus and indicates to the addressed device that word address “An”
will follow and is to be written to the on-chip address pointer. The data word to be written to
the memory is strobed in next and the internal address pointer is incremented to the next
address location on the reception of an acknowledge clock. The M41T81 slave receiver will
send an acknowledge clock to the master transmitter after it has received the slave address
see Figure 6 on page 10 and again after it has received the word address and each data
byte.
Data retention mode
With valid VCC applied, the M41T81 can be accessed as described above with READ or
WRITE cycles. Should the supply voltage decay, the power input will be switched from the
VCC pin to the battery when VCC falls below the battery backup switchover voltage (VSO). At
this time the clock registers will be maintained by the attached battery supply. On power-up,
when VCC returns to a nominal value, write protection continues for trec (see Figure 15 on
page 22, Table 11 on page 23).
For a further, more detailed review of lifetime calculations, please see application note
AN1012.
SLAVE
ADDRESS
DATA n+X
P
ACK
DATA n+1
ACK
BUS ACTIVITY:
DATA n
ACK
WORD
ADDRESS (An)
ACK
S
R/W
SDA LINE
ACK
BUS ACTIVITY:
MASTER
STOP
Figure 9. WRITE mode sequence
START
2.4
AI00591
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Clock operation
3
M41T81
Clock operation
The 20-byte register map (see Table 2 on page 13) is used to both set the clock and to read
the date and time from the clock, in a binary coded decimal format. Tenths/hundredths of
seconds, seconds, minutes, and hours are contained within the first four registers.
Note:
The tenths/hundredths of seconds cannot be written to any value other than “00.”
Bits D6 and D7 of clock register 03h (century/hours register) contain the CENTURY
ENABLE bit (CEB) and the CENTURY bit (CB). Setting CEB to a '1' will cause CB to toggle,
either from '0' to '1' or from '1' to '0' at the turn of the century (depending upon its initial
state). If CEB is set to a '0,' CB will not toggle. Bits D0 through D2 of register 04h contain the
day (day of week). Registers 05h, 06h, and 07h contain the date (day of month), month and
years. The ninth clock register is the control register (this is described in the clock calibration
section). Bit D7 of register 01h contains the STOP bit (ST). Setting this bit to a '1' will cause
the oscillator to stop. If the device is expected to spend a significant amount of time on the
shelf, the oscillator may be stopped to reduce current drain. When reset to a '0' the oscillator
restarts within one second.
The eight clock registers may be read one byte at a time, or in a sequential block. Provision
has been made to assure that a clock update does not occur while any of the eight clock
addresses are being read. If a clock address is being read, an update of the clock registers
will be halted. This will prevent a transition of data during the READ.
3.1
Power-down time-stamp
When a power failure occurs, the HT bit will automatically be set to a '1.' This will prevent the
clock from updating the TIMEKEEPER® registers, and will allow the user to read the exact
time of the power-down event. Resetting the HT bit to a '0' will allow the clock to update the
TIMEKEEPER registers with the current time. For more information, see application note
AN1572.
3.2
Clock registers
The M41T81 offers 20 internal registers which contain clock, alarm, watchdog, flag, square
wave and control data. These registers are memory locations which contain external (user
accessible) and internal copies of the data (usually referred to as BiPORT™ cells). The
external copies are independent of internal functions except that they are updated
periodically by the simultaneous transfer of the incremented internal copy. The internal
divider (or clock) chain will be reset upon the completion of a WRITE to any clock address.
The system-to-user transfer of clock data will be halted whenever the address being read is
a clock address (00h to 07h). The update will resume either due to a stop condition or when
the pointer increments to any non-clock address (08h-13h).
Clock and alarm registers store data in BCD. Control, watchdog and square wave registers
store data in binary format.
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M41T81
Clock operation
Table 2. Clock register map(1)
Addr
D7
00h
D6
D5
D4
D3
0.1 seconds
D2
D1
D0
Function/range BCD
format
0.01 seconds
Seconds
00-99
01h
ST
10 seconds
Seconds
Seconds
00-59
02h
0
10 minutes
Minutes
Minutes
00-59
03h
CEB
CB
Hours (24 hour format)
Century/
hours
0-1/00-23
04h
0
0
Day
01-7
05h
0
0
Date: day of month
Date
01-31
06h
0
0
Month
Month
01-12
Year
Year
00-99
07h
10 hours
0
0
0
10 date
0
Day of week
10M
10 years
08h
OUT
FT
S
Calibration
09h
0
BMB4
BMB3
BMB2
0Ah
AFE
SQWE
ABE
Al 10M
0Bh
RPT4
RPT5
0Ch
RPT3
HT
0Dh
RPT2
0Eh
RPT1
0Fh
WDF
AF
0
0
0
0
0
0
Flags
10h
0
0
0
0
0
0
0
0
Reserved
11h
0
0
0
0
0
0
0
0
Reserved
12h
0
0
0
0
0
0
0
0
Reserved
13h
RS3
RS2
RS1
RS0
0
0
0
0
SQW
BMB1
BMB0
Control
RB1
RB0
Watchdog
Alarm month
Al month
01-12
AI 10 date
Alarm date
Al date
01-31
AI 10 hour
Alarm hour
Al hour
00-23
Alarm 10 minutes
Alarm minutes
Al min
00-59
Alarm 10 seconds
Alarm seconds
Al sec
00-59
1. Keys:
S = Sign bit
FT = Frequency test bit
ST = Stop bit
0 = Must be set to '0'
BMB0-BMB4 = Watchdog multiplier bits
CEB = Century enable bit
CB = Century bit
OUT = Output level
ABE = Alarm in battery backup mode enable bit
AFE = Alarm flag enable flag
RB0-RB1 = Watchdog resolution bits
RPT1-RPT5 = Alarm repeat mode bits
WDF = Watchdog flag (read only)
AF = Alarm flag (read only)
SQWE = Square wave enable
RS0-RS3 = SQW frequency
HT = Halt update bit
DocID007529 Rev 11
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29
Clock operation
3.3
M41T81
Calibrating the clock
The M41T81 is driven by a quartz controlled oscillator with a nominal frequency of
32,768 Hz. The devices are tested not exceed ±35 ppm (parts per million) oscillator
frequency error at 25oC, which equates to about +1.9 to –1.1 minutes per month (see
Figure 10 on page 15). When the calibration circuit is properly employed, accuracy
improves to better than ±2 ppm at 25°C.
The oscillation rate of crystals changes with temperature. The M41T81 design employs
periodic counter correction. The calibration circuit adds or subtracts counts from the
oscillator divider circuit at the divide by 256 stage, as shown in Figure 11 on page 15. The
number of times pulses which are blanked (subtracted, negative calibration) or split (added,
positive calibration) depends upon the value loaded into the five calibration bits found in the
control register. Adding counts speeds the clock up, subtracting counts slows the clock
down.
The calibration bits occupy the five lower order bits (D4-D0) in the control register 08h.
These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a
sign bit; '1' indicates positive calibration, '0' indicates negative calibration. Calibration occurs
within a 64 minute cycle. The first 62 minutes in the cycle may, once per minute, have one
second either shortened by 128 or lengthened by 256 oscillator cycles. If a binary '1' is
loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a
binary 6 is loaded, the first 12 will be affected, and so on.
Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator
cycles for every 125,829,120 actual oscillator cycles, that is +4.068 or –2.034 ppm of
adjustment per calibration step in the calibration register (see Figure 11 on page 15).
Assuming that the oscillator is running at exactly 32,768 Hz, each of the 31 increments in
the calibration byte would represent +10.7 or –5.35 seconds per month which corresponds
to a total range of +5.5 or –2.75 minutes per month.
Two methods are available for ascertaining how much calibration a given M41T81 may
require.
The first involves setting the clock, letting it run for a month and comparing it to a known
accurate reference and recording deviation over a fixed period of time. Calibration values,
including the number of seconds lost or gained in a given period, can be found in application
note AN934, “TIMEKEEPER® calibration.” This allows the designer to give the end user the
ability to calibrate the clock as the environment requires, even if the final product is
packaged in a non-user serviceable enclosure. The designer could provide a simple utility
that accesses the calibration byte.
The second approach is better suited to a manufacturing environment, and involves the use
of the IRQ/FT/OUT/SQW pin. The pin will toggle at 512Hz, when the stop bit (ST, D7 of 01h)
is '0,' the frequency test bit (FT, D6 of 08h) is '1,' the alarm flag enable bit (AFE, D7 of 0Ah)
is '0,' and the square wave enable bit (SQWE, D6 of 0Ah) is '0' and the watchdog register
(09h = 0) is reset.
Any deviation from 512 Hz indicates the degree and direction of oscillator frequency shift at
the test temperature. For example, a reading of 512.010124 Hz would indicate a +20 ppm
oscillator frequency error, requiring a –10 (XX001010) to be loaded into the calibration byte
for correction. Note that setting or changing the calibration byte does not affect the
frequency test output frequency.
14/29
DocID007529 Rev 11
M41T81
Clock operation
The IRQ/FT/OUT/SQW pin is an open drain output which requires a pull-up resistor to VCC
for proper operation. A 500-10 k resistor is recommended in order to control the rise time.
The FT bit is cleared on power-down.
Figure 10. Crystal accuracy across temperature
Frequency (ppm)
20
0
–20
–40
–60
ΔF = K x (T – T )2
O
F
–80
2
2
K = –0.036 ppm/°C ± 0.006 ppm/°C
–100
TO = 25°C ± 5°C
–120
–140
–160
–40
–30
–20
–10
0
10
20
30
40
50
60
70
80
Temperature °C
AI07888
Figure 11. Clock calibration
NORMAL
POSITIVE
CALIBRATION
NEGATIVE
CALIBRATION
AI00594B
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29
Clock operation
3.4
M41T81
Setting alarm clock registers
Address locations 0Ah-0Eh contain the alarm settings. The alarm can be configured to go
off at a prescribed time on a specific month, date, hour, minute, or second or repeat every
year, month, day, hour, minute, or second. It can also be programmed to go off while the
M41T81 is in the battery backup mode to serve as a system wake-up call.
Bits RPT5-RPT1 put the alarm in the repeat mode of operation. Table 3 on page 17 shows
the possible configurations. Codes not listed in the table default to the once per second
mode to quickly alert the user of an incorrect alarm setting.
When the clock information matches the alarm clock settings based on the match criteria
defined by RPT5-RPT1, the AF (alarm flag) is set. If AFE (alarm flag enable) is also set (and
SQWE is '0.'), the alarm condition activates the IRQ/FT/OUT/SQW pin.
Note:
If the address pointer is allowed to increment to the flag register address, an alarm condition
will not cause the interrupt/flag to occur until the address pointer is moved to a different
address. It should also be noted that if the last address written is the “alarm seconds,” the
address pointer will increment to the flag address, causing this situation to occur.
The IRQ/FT/OUT/SQW output is cleared by a READ to the flags register as shown in
Figure 12. A subsequent READ of the flags register is necessary to see that the value of the
alarm flag has been reset to '0.'
The IRQ/FT/OUT/SQW pin can also be activated in the battery backup mode. The
IRQ/FT/OUT/SQW will go low if an alarm occurs and both ABE (alarm in battery backup
mode enable) and AFE are set. Figure 13 illustrates the backup mode alarm timing.
Figure 12. Alarm interrupt reset waveform
0Eh
0Fh
10h
ACTIVE FLAG
HIGH-Z
IRQ/FT/OUT/SQW
AI04617
16/29
DocID007529 Rev 11
M41T81
Clock operation
Figure 13. Backup mode alarm waveform
VCC
VSO
trec
ABE and AFE Bits
AF Bit in Flags
Register
IRQ/FT/OUT/SQW
HIGH-Z
AI05663
Table 3. Alarm repeat modes
3.5
RPT5
RPT4
RPT3
RPT2
RPT1
Alarm setting
1
1
1
1
1
Once per second
1
1
1
1
0
Once per minute
1
1
1
0
0
Once per hour
1
1
0
0
0
Once per day
1
0
0
0
0
Once per month
0
0
0
0
0
Once per year
Watchdog timer
The watchdog timer can be used to detect an out-of-control microprocessor. The user
programs the watchdog timer by setting the desired amount of time-out into the watchdog
register, address 09h. Bits BMB4-BMB0 store a binary multiplier and the two lower order bits
RB1-RB0 select the resolution, where 00 = 1/16 second, 01 = 1/4 second, 10 = 1 second,
and 11 = 4 seconds. The amount of time-out is then determined to be the multiplication of
the five-bit multiplier value with the resolution. (For example: writing 00001110 in the
watchdog register = 3*1, or 3 seconds). If the processor does not reset the timer within the
specified period, the M41T81 sets the WDF (watchdog flag) and generates a watchdog
interrupt.
The watchdog timer can be reset by having the microprocessor perform a WRITE of the
watchdog register. The time-out period then starts over.
Should the watchdog timer time-out, a value of 00h needs to be written to the watchdog
register in order to clear the IRQ/FT/OUT/SQW pin. This will also disable the watchdog
function until it is again programmed correctly. A READ of the flags register will reset the
watchdog flag (bit D7; register 0Fh).
The watchdog function is automatically disabled upon power-up and the watchdog register
is cleared. If the watchdog function is set, the frequency test function is activated, and the
SQWE bit is '0,' the watchdog function prevails and the frequency test function is denied.
DocID007529 Rev 11
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29
Clock operation
3.6
M41T81
Square wave output
The M41T81 offers the user a programmable square wave function which is output on the
SQW pin. RS3-RS0 bits located in 13h establish the square wave output frequency. These
frequencies are listed in Table 4. Once the selection of the SQW frequency has been
completed, the IRQ/FT/OUT/SQW pin can be turned on and off under software control with
the square wave enable bit (SQWE) located in register 0Ah.
Table 4. Square wave output frequency
Square wave bits
3.7
Square wave
RS3
RS2
RS1
RS0
Frequency
Units
0
0
0
0
None
-
0
0
0
1
32.768
kHz
0
0
1
0
8.192
kHz
0
0
1
1
4.096
kHz
0
1
0
0
2.048
kHz
0
1
0
1
1.024
kHz
0
1
1
0
512
Hz
0
1
1
1
256
Hz
1
0
0
0
128
Hz
1
0
0
1
64
Hz
1
0
1
0
32
Hz
1
0
1
1
16
Hz
1
1
0
0
8
Hz
1
1
0
1
4
Hz
1
1
1
0
2
Hz
1
1
1
1
1
Hz
Century bit
Bits D7 and D6 of clock register 03h contain the CENTURY ENABLE bit (CEB) and the
CENTURY bit (CB). Setting CEB to a '1' will cause CB to toggle, either from a '0' to '1' or
from '1' to '0' at the turn of the century (depending upon its initial state). If CEB is set to a '0,'
CB will not toggle.
18/29
DocID007529 Rev 11
M41T81
3.8
Clock operation
Output driver pin
When the FT bit, AFE bit, SQWE bit, and watchdog register are not set, the
IRQ/FT/OUT/SQW pin becomes an output driver that reflects the contents of D7 of the
control register. In other words, when D7 (OUT bit) and D6 (FT bit) of address location 08h
are a '0,' then the IRQ/FT/OUT/SQW pin will be driven low.
Note:
The IRQ/FT/OUT/SQW pin is an open drain which requires an external pull-up resistor.
3.9
Preferred initial power-on default
Upon initial application of power to the device, the following register bits are set to a '0' state:
watchdog register; AFE; ABE; SQWE; and FT. The following bits are set to a '1' state: ST;
OUT; and HT (see Table 5).
Table 5. Preferred default values
Condition
Initial power-up(2)
Subsequent power-up (with
battery backup)(3)
ST
HT
OUT
FT
AFE
SQWE
ABE
Watchdog
register(1)
1
1
1
0
0
0
0
0
UC
1
UC
0
UC
UC
UC
0
1. BMB0-BMB4, RB0, RB1.
2. State of other control bits undefined.
3. UC = unchanged
DocID007529 Rev 11
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29
Maximum ratings
4
M41T81
Maximum ratings
Stressing the device above the rating listed in the absolute maximum ratings table may
cause permanent damage to the device. These are stress ratings only and operation of the
device at these or any other conditions above those indicated in the operating sections of
this specification is not implied. Exposure to absolute maximum rating conditions for
extended periods may affect device reliability.
Table 6. Absolute maximum ratings
Sym
TSTG
VCC
TSLD(1)
VIO
Parameter
Storage temperature (VCC off, oscillator off)
Supply voltage
Lead solder temperature for 10 seconds
Input or output voltages
Value
Unit
–55 to 125
°C
–0.3 to 7
V
260
°C
–0.3 to VCC+0.3
V
IO
Output current
20
mA
PD
Power dissipation
1
W
1. Reflow at peak temperature of 260°C. The time above 255 °C must not exceed 30 seconds.
Caution:
20/29
Negative undershoots below –0.3 volts are not allowed on any pin while in the battery
backup mode
DocID007529 Rev 11
M41T81
5
DC and AC parameters
DC and AC parameters
This section summarizes the operating and measurement conditions, as well as the DC and
AC characteristics of the device. The parameters in the following DC and AC characteristic
tables are derived from tests performed under the measurement conditions listed in the
relevant tables. Designers should check that the operating conditions in their projects match
the measurement conditions when using the quoted parameters.
Table 7. Operating and AC measurement conditions
Parameter
Note:
M41T81
Supply voltage (VCC)
2.0 to 5.5 V
Ambient operating temperature (TA)
–40 to 85°C
Load capacitance (CL)
100 pF
Input rise and fall times
50 ns
Input pulse voltages
0.2VCC to 0.8VCC
Input and output timing ref. voltages
0.3VCC to 0.7VCC
Output Hi-Z is defined as the point where data is no longer driven.
Figure 14. AC measurement I/O waveform
0.8VCC
0.7VCC
0.3VCC
0.2VCC
AI02568
Table 8. Capacitance
Parameter(1)(2)
Symbol
CIN
COUT(3)
tLP
Min
Max
Unit
Input capacitance
-
7
pF
Output capacitance
-
10
pF
Low-pass filter input time constant (SDA and SCL)
-
50
ns
1. Effective capacitance measured with power supply at 5 V; sampled only, not 100% tested.
2. At 25°C, f = 1 MHz.
3. Outputs deselected.
DocID007529 Rev 11
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29
DC and AC parameters
M41T81
Table 9. DC characteristics
Sym
Test condition(1)
Parameter
Min
Typ
Max
Unit
0V VIN VCC
±1
μA
0V VOUT VCC
±1
μA
Switch freq = 400 kHz
400
μA
SCL,SDA = VCC – 0.3V
100
μA
ILI
Input leakage current
ILO
Output leakage current
ICC1
Supply current
ICC2
Supply current (standby)
VIL
Input low voltage
–0.3
0.3VCC
V
VIH
Input high voltage
0.7VCC
VCC + 0.3
V
VOL
Output low voltage
IOL = 3.0 mA
0.4
V
Output low voltage (open
drain)(2)
IOL = 10 mA
0.4
V
IRQ/OUT/FT/SQW
5.5
V
3
3.5(5)
V
0.6
1
μA
Pull-up supply voltage
(open drain)
VBAT(3) Battery supply voltage
IBAT
2.5(4)
TA = 25 °C, VCC = 0 V
oscillator on, VBAT = 3 V
Battery supply current
1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.0 to 5.5 V (except where noted).
2. For IRQ/FT/OUT/SQW pin (open drain)
3. STMicroelectronics recommends the RAYOVAC BR1225 or BR1632 (or equivalent) as the battery supply.
4. After switchover (VSO), VBAT (min) can be 2.0 V for crystal with RS = 40 k.
5. For rechargeable backup, VBAT (max) may be considered VCC.
Table 10. Crystal electrical characteristics
Parameter(1)(2)
Sym
Min
Typ
32.768
fO
Resonant frequency
-
RS
Series resistance
-
CL
Load capacitance
-
Max
Units
kHz
60
k
12.5
pF
1. Externally supplied. STMicroelectronics recommends the KDS DT-38: 1TA/1TC252E127, Tuning Fork
Type (thru-hole) or the DMX-26S: 1TJS125FH2A212, (SMD) quartz crystal for industrial temperature
operations.
2. Load capacitors are integrated within the M41T81. Circuit board layout considerations for the 32.768 kHz
crystal of minimum trace lengths and isolation from RF generating signals should be taken into account.
Figure 15. Power down/up mode AC waveforms
VCC
VSO
tPD
SDA
SCL
trec
DON'T CARE
AI00596
22/29
DocID007529 Rev 11
M41T81
DC and AC parameters
Table 11. Power down/up AC characteristics
Parameter(1)(2)
Symbol
Min
Typ
Max
Unit
tPD
SCL and SDA at VIH before power-down
0
-
-
nS
trec
SCL and SDA at VIH after power-up
10
-
-
μS
1. VCC fall time should not exceed 5 mV/μs.
2. Valid for ambient operating temperature: TA = –40 to 85 °C; VCC = 2.0 to 5.5 V (except where noted).
Table 12. Power down/up trip points DC characteristics
Parameter(1)(2)
Sym
VSO
Min
Battery backup switchover voltage
Typ
Max
Unit
VBAT – 0.80 VBAT – 0.50 VBAT – 0.30
V
1. All voltages referenced to VSS.
2. Valid for ambient operating temperature: TA = –40 to 85 °C; VCC = 2.0 to 5.5 V (except where noted).
Figure 16. Bus timing requirements sequence
SDA
tBUF
tHD:STA
tR
tHD:STA
tF
SCL
tHIGH
P
S
tLOW
tSU:DAT
tHD:DAT
tSU:STA
SR
tSU:STO
P
AI00589
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29
DC and AC parameters
M41T81
Table 13. AC characteristics
Parameter(1)
Sym
Min
Typ
Max
Units
0
-
400
kHz
fSCL
SCL clock frequency
tLOW
Clock low period
1.3
-
μs
tHIGH
Clock high period
600
-
ns
tR
SDA and SCL rise time
-
300
ns
tF
SDA and SCL fall time
-
300
ns
tHD:STA
START condition hold time
(after this period the first clock pulse is
generated)
600
-
ns
tSU:STA
START condition setup time
(only relevant for a repeated start condition)
600
-
ns
tSU:DAT
Data setup time
100
-
ns
Data hold time
0
-
μs
STOP condition setup time
600
-
ns
Time the bus must be free before a new
transmission can start
1.3
-
μs
tHD:DAT
(2)
tSU:STO
tBUF
1. Valid for ambient operating temperature: TA = –40 to 85 °C; VCC = 2.0 to 5.5 V (except where noted).
2. Transmitter must internally provide a hold time to bridge the undefined region (300 ns max) of the falling
edge of SCL.
24/29
DocID007529 Rev 11
M41T81
6
Package mechanical information
Package mechanical information
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK®
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK® is an ST trademark.
DocID007529 Rev 11
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29
Package mechanical information
M41T81
Figure 17. SO8 – 8-lead plastic small package outline
h x 45°
A2
A
c
ccc
b
e
0.25 mm
GAUGE PLANE
D
k
8
E1
E
1
A1
L
L1
SO-A
Note:
Drawing is not to scale.
Table 14. SO8 – 8-lead plastic small outline (150 mils body width), package
mechanical data
Millimeters
inches
Symbol
Typ
Min
A
Typ
Min
1.75
Max
0.069
A1
0.10
A2
1.25
b
0.28
0.48
0.011
0.019
c
0.17
0.23
0.007
0.009
ccc
0.25
0.004
0.010
0.049
0.10
0.004
D
4.90
4.80
5.00
0.193
0.189
0.197
E
6.00
5.80
6.20
0.236
0.228
0.244
E1
3.90
3.80
4.00
0.154
0.150
0.157
e
1.27
–
–
0.050
–
–
h
0.25
0.50
0.010
0.020
k
0°
8°
0°
8°
L
0.40
1.27
0.016
0.050
L1
26/29
Max
1.04
0.041
DocID007529 Rev 11
M41T81
7
Part numbering
Part numbering
Table 15. Ordering information scheme
Example:
M41T
81
M
6
F
Device type
M41T
Supply voltage and write protect voltage
81 = VCC = 2.0 to 5.5 V
Package
M = SO8
Temperature range
6 = –40°C to 85°C
Shipping method
F = ECOPACK® package, tape & reel
For other options or for more information on any aspect of this device, please contact the ST
sales office nearest you.
DocID007529 Rev 11
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29
Revision history
8
M41T81
Revision history
Table 16. Document revision history
Date
Revision
Dec-2001
1
21-Jan-2002
1.1
Fix table footnotes (Table 9, Table 10)
01-May-2002
1.2
Modify reflow time and temperature footnote (Table 6)
05-Jun-2002
1.3
Modify data retention text, trip points (Table 12)
10-Jun-2002
1.4
Corrected supply voltage values (Table 6, Table 7)
03-Jul-2002
1.5
Modify DC characteristics, crystal electrical table footnotes, preferred
default values (Table 9, Table 10, Table 5)
11-Oct-2002
1.6
Add marketing status (Figure 2; Table 15); adjust footnotes (Figure 2;
Table 9)
21-Jan-2003
1.7
Add embedded crystal package option (Figure 1, 3, 23; Table 16);
modified pre-existing mechanical drawing (Figure 17; Table 14).
05-Mar-2003
1.8
Correct dimensions (Table 16); remove SNAPHAT® package option
12-Sep-2003
2
Updated disclaimer, v2.2 template; add SOX18 package (Figure 2, 4;
Table 15)
27-Apr-2004
3
Reformatted; update characteristics (Figure 4, 3, Figure 3, Figure 10,
Figure 13, Table 1, Table 6, Table 9, Table 12, Table 15)
17-Jun-2004
4
Reformatted; add lead-free information; add dual footprint connections
(Figure 5;Table 6, Table 15)
7-Sep-2004
5
Update footprint and maximum ratings (Figure 5; Table 6)
13-Sep-2004
6
Update max ratings (Table 6)
03-Jun-2005
7
Remove SOX18 and SOX28 references (Features summary, Figure 1;
Table 1, Table 6, Table 10, Table 15)
22-Aug-2006
8
Changed document to new template; Updated package mechanical
data in Section 6: Package mechanical information; small text changes
for entire document; Ecopack compliant.
28-May-2008
9
Datasheet status updated to “not for new design” (updated cover page);
updated Table 1, 6, 15.
11-Jun-2010
10
Updated Features, Section 4, text in Section 6; minor textual changes;
reformatted document.
11
Datasheet status changed to “Datasheet - production data”
Updated Features and footnote 1 in Table 10
Removed tubes from shipping method in Table 15: Ordering
information scheme
10-Feb-2014
28/29
Changes
First issue
DocID007529 Rev 11
M41T81
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