October 2011 Doc ID 7531 Rev 11 1/43
1
M41ST85W
3.0/3.3 V I2C combination serial RTC, NVRAM
supervisor and microprocessor supervisor
Features
■Automatic battery switchover and WRITE
protect for:
– Internal serial RTC and
– External low power SRAM (LPSRAM)
■400 kHz I2C serial interface
■3.0/3.3 V operating voltage
–V
CC = 2.7 to 3.6 V
■Ultralow battery supply current of 500 nA (max)
■RoHS compliant
– Lead-free second level interconnect
Serial RTC features
■400 kHz I2C
■44 bytes of general purpose NVRAM
■Counters for:
– Seconds, minutes, hours, day, date, month,
and year
–Century
– Tenths/hundredths of seconds
– Clock calibration register allows
compensation for crystal variations over
temperature
■Programmable alarm with repeat modes
– Functions in battery back-up mode
■Power-down timestamp (HT bit)
■2.5 to 5.5 V oscillator operating voltage
Microprocessor supervisor features
■Programmable watchdog
– 62.5 ms to 128 s time-out period
■Early power-fail warning circuit (PFI/PFO) with
1.25 V precision reference
■Power-on reset/low voltage detect
– Open drain reset output
– Reset voltage, VPFD = 2.60 V (nom)
– Two reset input pins
– Watchdog can be steered to reset output
NVRAM supervisor features
■Non-volatizes external LPSRAM
– Automatically switches to back-up battery
and deselects (write-protects) external
LPSRAM via chip-enable gate
– Power-fail deselect (write protect) voltage,
VPFD = 2.60 V (nom)
– Switchover, VSO = 2.50 V (nom)
■Battery monitor (battery low flag)
Other features
■Programmable squarewave generator (1 Hz to
32 KHz)
■–40°C to +85°C operation
■Package options:
– 28-lead SNAPHAT® IC (SOH28) SNAPHAT
battery/crystal top to be ordered separately
– 28-lead embedded crystal SOIC (SOX28)
28
1
SNAPHAT battery & crystal
SOH28
Embedded crystal
SOX28
www.st.com
Contents M41ST85W
2/43 Doc ID 7531
Contents
1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 2-wire bus characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Bus not busy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 Start data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.3 Stop data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.4 Data valid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.5 Acknowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Read mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Write mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Data retention mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3 Clock operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1 Power-down time-stamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 TIMEKEEPER® registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3 Calibrating the clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.4 Setting alarm clock registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.5 Watchdog timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.6 Square wave output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.7 Power-on reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.8 Reset inputs (RSTIN1 & RSTIN2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.9 Power-fail input/output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.10 Century bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.11 Output driver pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.12 Battery low warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.13 trec bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.14 Initial power-on defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5 DC and AC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
M41ST85W Contents
Doc ID 7531 3/43
6 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7 Part numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
8 Environmental information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
List of tables M41ST85W
4/43 Doc ID 7531
List of tables
Table 1. Signal names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 2. TIMEKEEPER® register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 3. Alarm repeat modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Table 4. Square wave output frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 5. Reset AC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Table 6. trec definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Table 7. Default values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Table 8. Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Table 9. DC and AC measurement conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Table 10. Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Table 11. DC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Table 12. AC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Table 13. Power down/up AC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Table 14. SOH28 – 28-lead plastic small outline, battery SNAPHAT®, package mechanical data . . 34
Table 15. 4-pin SNAPHAT® housing for 48 mAh battery & crystal, mechanical data. . . . . . . . . . . . . 35
Table 16. 4-pin SNAPHAT® housing for 120 mAh battery & crystal, mechanical data. . . . . . . . . . . . 36
Table 17. SOX28 – 28-lead plastic small outline, 300 mils, embedded crystal, mechanical data . . . 37
Table 18. Carrier tape dimensions for SOH28 and SOX28 packages . . . . . . . . . . . . . . . . . . . . . . . . 38
Table 19. Reel dimensions for 24 mm carrier tape (SOH28 and SOX28 packages) . . . . . . . . . . . . . 39
Table 20. Ordering information scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Table 21. SNAPHAT® battery/crystal table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Table 22. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
M41ST85W List of figures
Doc ID 7531 5/43
List of figures
Figure 1. Logic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 2. 28-pin SOIC connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 3. 28-pin, 300 mil SOIC connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 4. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 5. Hardware hookup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 6. Serial bus data transfer sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 7. Acknowledgement sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 8. Write cycle timing: RTC & external SRAM control signals . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 9. Slave address location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 10. Read mode sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 11. Alternate read mode sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 12. Write mode sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 13. Crystal accuracy across temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 14. Calibration waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 15. Alarm interrupt reset waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 16. Backup mode alarm waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Figure 17. RSTIN1 & RSTIN2 timing waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 18. AC testing input/output waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Figure 19. Bus timing requirements sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Figure 20. Power down/up mode AC waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Figure 21. SOH28 – 28-lead plastic small outline, battery SNAPHAT®, package outline . . . . . . . . . . 34
Figure 22. 4-pin SNAPHAT® housing for 48 mAh battery & crystal, package outline . . . . . . . . . . . . . 35
Figure 23. 4-pin SNAPHAT® housing for 120 mAh battery & crystal, package outline . . . . . . . . . . . . 36
Figure 24. SOX28 – 28-lead plastic small outline, 300 mils, embedded crystal, package outline . . . . 37
Figure 25. Carrier tape for SOH28 and SOX28 package. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Figure 26. Reel schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Figure 27. Recycling symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Description M41ST85W
6/43 Doc ID 7531 Rev 11
1 Description
The M41ST85W is a combination serial real-time clock, microprocessor supervisor, and
NVRAM supervisor. It is built in a low-power CMOS SRAM process and has a 64-byte
memory space with 44 bytes of NVRAM and 20 memory-mapped RTC registers (see
Table 2 on page 20). The RTC registers are configured in binary coded decimal (BCD)
format.
The M41ST85W combines a 400 kHz I2C serial RTC with an automatic backup battery
switchover circuit for powering an external LPSRAM as well as the internal RTC. When
power begins to fail, the switchover automatically connects to the backup battery to keep the
RTC and external LPSRAM alive in the absence of system power. Access to the LPSRAM is
also cut off via a chip-enable gate function, thereby write-protecting the memory. A
programmable watchdog and power-on reset/low voltage detect function are the key
elements in the microprocessor supervisor section.
The real-time clock includes a built-in 32.768 kHz oscillator (crystal-controlled), which
provides the time base for the timekeeping and calendar functions. Eight of the 20 clock
registers provide the basic clock/calendar functions while the other 12 bytes provide
status/control for the alarm, watchdog, and squarewave functions.
RTC addresses and data are transferred serially via the two-line, bidirectional I2C interface.
The built-in address register is incremented automatically after each WRITE or READ data
byte.
The M41ST85W has a built-in power sense circuit which detects power failures and
automatically switches to the backup battery when a power failure occurs. During an outage,
the power to sustain the SRAM and clock operations is typically supplied by a small lithium
button-cell battery as is the case when using the SNAPHAT® package option.
Functions available to the user include a non-volatile, time-of-day clock/calendar, alarm
interrupts, watchdog timer, and programmable squarewave generator. Other features
include a power-on reset as well as two additional debounce reset inputs (RSTIN1 and
RSTIN2) which can also generate an output reset (RST).
The eight registers for basic clock/calendar functions 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 M41ST85W is offered in two 28-lead SOIC packages. The 300 mil SOH28 SNAPHAT®
IC package mates with ST’s SNAPHAT battery/crystal top (ordered separately). SNAPHAT
battery options include 48 mAh and 120 mAh. ST’s 300 mil SOX28 embedded crystal IC
includes the 32 KHz crystal and is perfect for applications where a low profile is a must.
The SOH28 SNAPHAT SOIC includes sockets with gold plated contacts at both ends for
direct connection to the SNAPHAT top. The SNAPHAT battery/crystal top is inserted atop
the IC package after the completion of the surface mount assembly process which avoids
potential battery and crystal damage due to the high temperatures required for device
surface-mounting. The unique design allows the battery to be replaced, thus extending the
life of the RTC and NVRAM indefinitely.
The SNAPHAT top is keyed to prevent reverse insertion. The SNAPHAT IC and SNAPHAT
tops are shipped separately. The ICs are available in plastic anti-static tubes or in tape &
reel form. The SNAPHAT tops are shipped in plastic anti-static tubes. The part numbers are
M41ST85W Description
Doc ID 7531 Rev 11 7/43
M4T28-BR12SH1 (48 mAh) and M4T32-BR12SH1 (120 mAh). For the extended
temperature requirement, the 120 mAh M4T32-BR12SH6 is available. For more information,
see Table 21 on page 40.
Caution: Do not place the SNAPHAT® battery/crystal top in conductive foam, as this will drain the
lithium button-cell battery.
The 300 mil SOX embedded crystal SOIC typically requires a user-supplied battery for non-
volatile operation. Capacitor backup can also be implemented with this package.
Figure 1. Logic diagram
1. For 28-pin, 300 mil embedded crystal SOIC only.
AI03658
SCL
VCC
M41ST85W
EX
VSS
VBAT(1)
SDA
RSTIN1
IRQ/FT/OUT
SQW
WDI
RSTIN2
PFI
ECON
RST
PFO
VOUT
Description M41ST85W
8/43 Doc ID 7531 Rev 11
Table 1. Signal names
Figure 2. 28-pin SOIC connections
ECON Conditioned chip enable output
EX External chip enable
IRQ/FT/OUT Interrupt/frequency test/out output (open drain)
PFI Power fail input
PFO Power fail output
RST Reset output (open drain)
RSTIN1 Reset 1 Input
RSTIN2 Reset 2 Input
SCL Serial clock input
SDA Serial data input/output
SQW Square wave output
WDI Watchdog input
VCC Supply voltage
VOUT Voltage output
VSS Ground
VBAT(1)
1. For 28-pin, 300 mil embedded crystal SOIC only.
Battery supply voltage
NC No connect
NF No function
AI03659
8
2
3
4
5
6
7
9
10
11
12
13
14
22
21
20
19
18
17
16
15
28
27
26
25
24
23
1
RSTIN1
RSTIN2
NC
NC
NC
NC
WDI
NC
NC
IRQ/FT/OUT
NC
VOUT
NC
NC
EX
NC
PFI
SCL
NCNC
PFO
ECON
VSS
SDA
RST
NC
SQW VCC
M41ST85W
M41ST85W Description
Doc ID 7531 Rev 11 9/43
Figure 3. 28-pin, 300 mil SOIC connections
Note: No function (NF) pins should be tied to VSS. Pins 1, 2, 3, and 4 are internally
shorted together.
AI06370d
8
2
3
4
5
6
7
9
10
11
12
13
14
22
21
20
19
18
17
16
15
28
27
26
25
24
23
1
RSTIN1
RSTIN2
NC
NF
NC
NC
SQW
WDI
NF
NC
IRQ/FT/OUT
NC
VOUT
NC
EX
PFI
SCL
NC
PFO
ECON
VSS VBAT
NC
SDA
RST
NF
NF VCC
M41ST85W
Description M41ST85W
10/43 Doc ID 7531 Rev 11
Figure 4. Block diagram
1. Open drain output.
2. Crystal integrated into SOIC package for MX package option.
AI03932
COMPARE
VPFD = 2.65V
VCC
COMPARE
VSO = 2.5V
VOUT
VBL= 2.5V BL
COMPARE
Crystal(2)
I2C
INTERFACE
REAL TIME CLOCK
CALENDAR
44 BYTES
USER RAM
RTC w/ALARM
& CALIBRATION
WATCHDOG
SQUARE WAVE
SDA
SCL
1.25V
PFI
PFO
RSTIN1
POR
SQW
RST(1)
WDI
WDS
AFE
IRQ/FT/OUT(1)
VBAT
32KHz
OSCILLATOR
COMPARE
RSTIN2
EX ECON
(Internal)
M41ST85W Description
Doc ID 7531 Rev 11 11/43
Figure 5. Hardware hookup
1. Required for embedded crystal (MX) package only.
AI03660
VCC
PFO
EX
SCL
M41ST85W
WDI
RSTIN1
RSTIN2
PFI
VSS
VBAT(1) IRQ/FT/OUT
SQW
RST
VOUT
ECON
SDA
Unregulated
Voltage
Regulator
VCC
VIN
Pushbutton
Reset
From MCU
LPSRAM
VCC
E
To RST
To LED Display
To NMI
To INT
R1
R2
Operating modes M41ST85W
12/43 Doc ID 7531 Rev 11
2 Operating modes
The M41ST85W 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 64 bytes
contained in the device can then be accessed sequentially in the following order:
1. Tenths/hundredths of a second register
2. Seconds register
3. Minutes register
4. Century/hours register
5. Day register
6. Date register
7. Month register
8. Year register
9. Control register
10. Watchdog register
11 - 16. Alarm registers
17 - 19. Reserved
20. Square wave register
21 - 64. User RAM
The M41ST85W clock continually monitors VCC for an out-of-tolerance condition. Should
VCC fall below VPFD, 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. When VCC falls
below VSO, the device automatically switches over to the battery and powers down into an
ultralow 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.
Write protection continues until VCC reaches VPFD(min) plus trec (min).
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 bi-
directional 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.
M41ST85W Operating modes
Doc ID 7531 Rev 11 13/43
Accordingly, the following bus conditions have been defined:
2.1.1 Bus not busy
Both data and clock lines remain high.
2.1.2 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.
Operating modes M41ST85W
14/43 Doc ID 7531 Rev 11
Figure 6. Serial bus data transfer sequence
Figure 7. Acknowledgement sequence
Figure 8. Write cycle timing: RTC & external SRAM control signals
AI00587
DATA
CLOCK
DATA LINE
STABLE
DATA VALID
START
CONDITION
CHANGE OF
DATA ALLOWED
STOP
CONDITION
AI00601
DATA OUTPUT
BY RECEIVER
DATA OUTPUT
BY TRANSMITTER
SCL FROM
MASTER
START
CLOCK PULSE FOR
ACKNOWLEDGEMENT
12 89
MSB LSB
AI03663
EX
ECON
tEXPD
tEXPD
M41ST85W Operating modes
Doc ID 7531 Rev 11 15/43
2.2 Read mode
In this mode the master reads the M41ST85W slave after setting the slave address (see
Figure 9). 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 (see Figure 10 on page 16). The address pointer is
only incremented on reception of an acknowledge clock. The M41ST85W 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 either due to a stop condition or when
the pointer increments to a non-clock or RAM address.
Note: This is true both in READ mode and WRITE mode.
An alternate READ mode may also be implemented whereby the master reads the
M41ST85W 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 11 on page 16).
Figure 9. Slave address location
AI00602
R/W
SLAVE ADDRESS
START A
0100011
MSB
LSB
Operating modes M41ST85W
16/43 Doc ID 7531 Rev 11
Figure 10. Read mode sequence
Figure 11. Alternate read mode sequence
AI00899
BUS ACTIVITY:
ACK
S
ACK
ACK
ACK
NO ACK STOP
START
P
SDA LINE
BUS ACTIVITY:
MASTER
R/W
DATA n DATA n+1
DATA n+X
WORD
ADDRESS (An)
SLAVE
ADDRESS
S
START
R/W
SLAVE
ADDRESS
ACK
AI00895
BUS ACTIVITY:
ACK
S
ACK
ACK
ACK
NO ACK STOP
START
PSDA LINE
BUS ACTIVITY:
MASTER
R/W
DATA n DATA n+1 DATA n+X
SLAVE
ADDRESS
M41ST85W Operating modes
Doc ID 7531 Rev 11 17/43
2.3 Write mode
In this mode the master transmitter transmits to the M41ST85W slave receiver. Bus protocol
is shown in Figure 12. 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
memory location within the RAM on the reception of an acknowledge clock. The M41ST85W
slave receiver will send an acknowledge clock to the master transmitter after it has received
the slave address and again after it has received the word address and each data byte.
Figure 12. Write mode sequence
2.4 Data retention mode
With valid VCC applied, the M41ST85W can be accessed as described above with READ or
WRITE cycles. Should the supply voltage decay, the M41ST85W will automatically deselect,
write protecting itself (and any external SRAM) when VCC falls between VPFD(max) and
VPFD(min). This is accomplished by internally inhibiting access to the clock registers. At this
time, the reset pin (RST) is driven active and will remain active until VCC returns to nominal
levels. External RAM access is inhibited in a similar manner by forcing ECON to a high level.
This level is within 0.2 volts of the VBAT
. ECON will remain at this level as long as VCC
remains at an out-of-tolerance condition. When VCC falls below the battery backup
switchover voltage (VSO), power input is switched from the VCC pin to the SNAPHAT®
battery, and the clock registers and external SRAM are maintained from the attached
battery supply.
All outputs become high impedance. The VOUT pin is capable of supplying 100 µA of current
to the attached memory with less than 0.3 volts drop under this condition. On power-up,
when VCC returns to a nominal value, write protection continues for trec by inhibiting ECON.
The RST signal also remains active during this time (see Figure 20 on page 33).
Note: Most low power SRAMs on the market today can be used with the M41ST85W RTC
SUPERVISOR. There are, however some criteria which should be used in making the final
choice of an SRAM to use. The SRAM must be designed in a way where the chip enable
input disables all other inputs to the SRAM. This allows inputs to the M41ST85W and
SRAMs to be “Don’t Care” once VCC falls below VPFD(min). The SRAM should also
guarantee data retention down to VCC = 2.0 volts. The chip enable access time must be
sufficient to meet the system needs with the chip enable output propagation delays
included. If the SRAM includes a second chip enable pin (E2), this pin should be tied to
VOUT
.
AI00591
BUS ACTIVITY:
ACK
S
ACK
ACK
ACK
ACK STOP
START
PSDA LINE
BUS ACTIVITY:
MASTER
R/W
DATA n DATA n+1 DATA n+X
WORD
ADDRESS (An)
SLAVE
ADDRESS
Operating modes M41ST85W
18/43 Doc ID 7531 Rev 11
If data retention lifetime is a critical parameter for the system, it is important to review the
data retention current specifications for the particular SRAMs being evaluated. Most SRAMs
specify a data retention current at 3.0 volts. Manufacturers generally specify a typical
condition for room temperature along with a worst case condition (generally at elevated
temperatures). The system level requirements will determine the choice of which value to
use. The data retention current value of the SRAMs can then be added to the IBAT value of
the M41ST85W to determine the total current requirements for data retention. The available
battery capacity for the SNAPHAT® top of your choice can then be divided by this current to
determine the amount of data retention available (see Table 21 on page 40).
For a further more detailed review of lifetime calculations, please see application note
AN1012.
M41ST85W Clock operation
Doc ID 7531 Rev 11 19/43
3 Clock operation
The eight byte clock register (see Table 2 on page 20) 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: A WRITE to any clock register will result in the tenths/hundredths of seconds being reset to
“00,” and 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. The
control register (address location 08h) may be accessed independently. 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 halt update bit (HT) 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 TIMEKEEPER® registers
The M41ST85W 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™
TIMEKEEPER 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 a non-clock or RAM address.
TIMEKEEPER and alarm registers store data in BCD. control, watchdog and square wave
registers store data in binary format.
Clock operation M41ST85W
20/43 Doc ID 7531 Rev 11
Table 2. TIMEKEEPER® register map
Address
Data Function/range
BCD format
D7 D6 D5 D4 D3 D2 D1 D0
00h 0.1 seconds 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 10 hours Hours (24-hour format) Century/hours 0-1/00-23
04h TR 0 0 0 0 Day of week Day 01-7
05h 0 0 10 date Date: day of month Date 01-31
06h 0 0 0 10M Month Month 01-12
07h 10 years Year Year 00-99
08h OUT FT S Calibration Control
09h WDS BMB4 BMB3 BMB2 BMB1 BMB0 RB1 RB0 Watchdog
0Ah AFE SQWE ABE Al 10M Alarm month Al month 01-12
0Bh RPT4 RPT5 AI 10 date Alarm date Al date 01-31
0Ch RPT3 HT AI 10 hour Alarm hour Al hour 00-23
0Dh RPT2 Alarm 10 minutes Alarm minutes Al min 00-59
0Eh RPT1 Alarm 10 seconds Alarm seconds Al sec 00-59
0Fh WDF AF 0 BL 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
Keys: S = Sign bit RB0-RB1 = Watchdog resolution bits
FT = Frequency test bit WDS = Watchdog steering bit
ST = Stop bit ABE = Alarm in battery backup mode enable bit
0 = Must be set to zero RPT1-RPT5 = Alarm repeat mode bits
BL = Battery low flag (read only) WDF = Watchdog flag (read only)
BMB0-BMB4 = Watchdog multiplier bits AF = Alarm flag (read only)
CEB = Century enable bit SQWE = Square wave enable
CB = Century bit RS0-RS3 = SQW frequency
OUT = Output level HT = Halt update bit
AFE = Alarm flag enable flag TR = trec bit
M41ST85W Clock operation
Doc ID 7531 Rev 11 21/43
3.3 Calibrating the clock
The M41ST85W 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 +/–1.53 minutes per month. 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 (see Figure 13 on page 22).
Therefore, the M41ST85W 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 14 on page 22. 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. 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 M41ST85W 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 pin. The pin will toggle at 512 Hz, 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 watchdog steering bit (WDS, D7 of 09h) is '1' or 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.
The IRQ/FT/OUT pin is an open drain output which requires a pull-up resistor to VCC for
proper operation. A 500 to 10 k resistor is recommended in order to control the rise time.
The FT bit is cleared on power-down.
Clock operation M41ST85W
22/43 Doc ID 7531 Rev 11
Figure 13. Crystal accuracy across temperature
Figure 14. Calibration waveform
AI07888
–160
0 10203040506070
Frequency (ppm)
Temperature °C
80–10–20–30–40
–100
–120
–140
–40
–60
–80
20
0
–20
= –0.036 ppm/°C2 ± 0.006 ppm/°C2
K
ΔF= K x (T – TO)2
F
TO = 25°C ± 5°C
AI00594B
NORMAL
POSITIVE
CALIBRATION
NEGATIVE
CALIBRATION
M41ST85W Clock operation
Doc ID 7531 Rev 11 23/43
3.4 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
M41ST85W is in the battery backup to serve as a system wake-up call.
Bits RPT5–RPT1 put the alarm in the repeat mode of operation. Ta bl e 3 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, the
alarm condition activates the IRQ/FT/OUT pin as shown in Figure 15. To disable alarm,
write '0' to the alarm date register and to RPT5–RPT1.
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 output is cleared by a READ to the flags register. 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 pin can also be activated in the battery backup mode. The IRQ/FT/OUT
will go low if an alarm occurs and both ABE (alarm in battery backup mode enable) and AFE
are set. The ABE and AFE bits are reset during power-up, therefore an alarm generated
during power-up will only set AF. The user can read the flag register at system boot-up to
determine if an alarm was generated while the M41ST85W was in the deselect mode during
power-up. Figure 16 on page 24 illustrates the backup mode alarm timing.
Figure 15. Alarm interrupt reset waveform
Table 3. Alarm repeat modes
RPT5 RPT4 RPT3 RPT2 RPT1 Alarm setting
11111Once per second
11110Once per minute
11100Once per hour
11000Once per day
10000Once per month
00000Once per year
AI03664
IRQ/FT/OUT
ACTIVE FLAG
0Fh0Eh 10h
HIGH-Z
Clock operation M41ST85W
24/43 Doc ID 7531 Rev 11
Figure 16. Backup mode alarm waveform
3.5 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).
Note: The accuracy of the timer is within ± the selected resolution.
If the processor does not reset the timer within the specified period, the M41ST85W sets the
WDF (watchdog flag) and generates a watchdog interrupt or a microprocessor reset.
The most significant bit of the watchdog register is the watchdog steering bit (WDS). When
set to a '0,' the watchdog will activate the IRQ/FT/OUT pin when timed-out. When WDS is
set to a '1,' the watchdog will output a negative pulse on the RST pin for trec. The watchdog
register, FT, AFE, ABE and SQWE bits will reset to a '0' at the end of a watchdog time-out
when the WDS bit is set to a '1.'
The watchdog timer can be reset by two methods: 1) a transition (high-to-low or low-to-high)
can be applied to the watchdog input pin (WDI) or 2) the microprocessor can perform a
WRITE of the watchdog register. The time-out period then starts over.
Note: The WDI pin should be tied to VSS if not used.
In order to perform a software reset of the watchdog timer, the original time-out period can
be written into the watchdog register, effectively restarting the count-down cycle.
Should the watchdog timer time-out, and the WDS bit is programmed to output an interrupt,
a value of 00h needs to be written to the watchdog register in order to clear the IRQ/FT/OUT
AI03920
VCC
IRQ/FT/OUT
VPFD
ABE, AFE Bits in Interrupt Register
AF bit in Flags Register
HIGH-Z
VSO
HIGH-Z
trec
M41ST85W Clock operation
Doc ID 7531 Rev 11 25/43
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 to output to the IRQ/FT/OUT pin and the frequency
test function is activated, the watchdog function prevails and the frequency test function is
denied.
3.6 Square wave output
The M41ST85W 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 Ta bl e 4 . Once the selection of the SQW frequency has been
completed, the 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
3.7 Power-on reset
The M41ST85W continuously monitors VCC. When VCC falls to the power fail detect trip
point, the RST pulls low (open drain) and remains low on power-up for trec after VCC passes
VPFD(max). The RST pin is an open drain output and an appropriate pull-up resistor should
be chosen to control rise time.
Square wave bits Square wave
RS3 RS2 RS1 RS0 Frequency Units
0000None–
000132.768kHz
00108.192kHz
00114.096kHz
01002.048kHz
01011.024kHz
0110512Hz
0111256Hz
1000128Hz
100164Hz
101032Hz
101116Hz
11008Hz
11014Hz
11102Hz
11111Hz
Clock operation M41ST85W
26/43 Doc ID 7531 Rev 11
3.8 Reset inputs (RSTIN1 & RSTIN2)
The M41ST85W provides two independent inputs which can generate an output reset. The
duration and function of these resets is identical to a reset generated by a power cycle.
Ta bl e 5 and Figure 17 illustrate the AC reset characteristics of this function. Pulses shorter
than tRLRH1 and tRLRH2 will not generate a reset condition. RSTIN1 and RSTIN2 are each
internally pulled up to VCC through a 100 kΩ resistor.
Figure 17. RSTIN1 & RSTIN2 timing waveforms
Note: With pull-up resistor
Table 5. Reset AC characteristics
3.9 Power-fail input/output
The power-fail input (PFI) is compared to an internal reference voltage (1.25 V). If PFI is less
than the power-fail threshold (VPFI), the power-fail output (PFO) will go low. This function is
intended for use as an undervoltage detector to signal a failing power supply. Typically PFI is
connected through an external voltage divider (see Figure 5 on page 11) to either the
unregulated DC input (if it is available) or the regulated output of the VCC regulator. The
voltage divider can be set up such that the voltage at PFI falls below VPFI several
milliseconds before the regulated VCC input to the M41ST85W or the microprocessor drops
below the minimum operating voltage.
During battery backup, the power-fail comparator turns off and PFO goes (or remains) low.
This occurs after VCC drops below VPFD(min). When power returns, PFO is forced high,
irrespective of VPFI for the write protect time (trec), which is the time from VPFD(max) until the
inputs are recognized. At the end of this time, the power-fail comparator is enabled and PFO
Symbol Parameter(1)
1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.7 to 3.6 V (except where noted).
Min Max Unit
tRLRH1(2)
2. Pulse width less than 50 ns will result in no RESET (for noise immunity).
RSTIN1 low to RSTIN1 high 200 ns
tRLRH2(3)
3. Pulse width less than 20 ms will result in no RESET (for noise immunity).
RSTIN2 low to RSTIN2 high 100 ms
tR1HRH(4)
4. Programmable (see Table 6 on page 28).
RSTIN1 high to RST high 40 200 ms
tR2HRH(4) RSTIN2 high to RST high 40 200 ms
AI03665
RSTIN2
RST (1)
RSTIN1
tRLRH1
tRLRH2
tR1HRH tR2HRH
M41ST85W Clock operation
Doc ID 7531 Rev 11 27/43
follows PFI. If the comparator is unused, PFI should be connected to VSS and PFO left
unconnected.
3.10 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.
3.11 Output driver pin
When the FT bit, AFE bit and watchdog register are not set, the IRQ/FT/OUT 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 pin will
be driven low.
Note: The IRQ/FT/OUT pin is an open drain which requires an external pull-up resistor.
3.12 Battery low warning
The M41ST85W automatically performs battery voltage monitoring upon power-up and at
factory-programmed time intervals of approximately 24 hours. The battery low (BL) bit, bit
D4 of flags register 0Fh, will be asserted if the battery voltage is found to be less than
approximately 2.5 V. The BL bit will remain asserted until completion of battery replacement
and subsequent battery low monitoring tests, either during the next power-up sequence or
the next scheduled 24-hour interval.
If a battery low is generated during a power-up sequence, this indicates that the battery is
below approximately 2.5 volts and may not be able to maintain data integrity in the SRAM.
Data should be considered suspect and verified as correct. A fresh battery should be
installed.
If a battery low indication is generated during the 24-hour interval check, this indicates that
the battery is near end of life. However, data is not compromised due to the fact that a
nominal VCC is supplied. In order to insure data integrity during subsequent periods of
battery backup mode, the battery should be replaced. The SNAPHAT® top may be replaced
while VCC is applied to the device.
Note: This will cause the clock to lose time during the interval the SNAPHAT battery/crystal top is
disconnected.
The M41ST85W only monitors the battery when a nominal VCC is applied to the device.
Thus applications which require extensive durations in the battery backup mode should be
powered-up periodically (at least once every few months) in order for this technique to be
beneficial. Additionally, if a battery low is indicated, data integrity should be verified upon
power-up via a checksum or other technique.
Clock operation M41ST85W
28/43 Doc ID 7531 Rev 11
3.13 trec bit
Bit D7 of clock register 04h contains the trec bit (TR). trec refers to the automatic
continuation of the deselect time after VCC reaches VPFD. This allows for a voltage settling
time before WRITEs may again be performed to the device after a power-down condition.
The trec bit will allow the user to set the length of this deselect time as defined by Ta bl e 6 .
3.14 Initial power-on defaults
Upon initial application of power to the device, the following register bits are set to a '0' state:
watchdog register, FT, AFE, ABE, SQWE, and TR. The following bits are set to a '1' state:
ST, OUT, and HT (see Ta bl e 7 ).
Table 6. trec definitions
Table 7. Default values
trec bit (TR) STOP bit (ST)
trec time
Units
Min Max
009698ms
0140200
(1)
1. Default setting
ms
1 X 50 2000 µs
Condition TR ST HT Out FT AFE ABE SQWE Watchdog
register(1)
1. WDS, BMB0-BMB4, RB0, RB1.
Initial power-up(2)
2. State of other control bits undefined.
0111000 0 0
Subsequent power-up
(with battery backup)(3)
3. UC = Unchanged
UC UC 1 UC 0 0 0 0 0
M41ST85W Maximum ratings
Doc ID 7531 Rev 11 29/43
4 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. Refer also to the STMicroelectronics SURE
Program and other relevant quality documents.
Table 8. Absolute maximum ratings
Caution: Negative undershoots below –0.3 V are not allowed on any pin while in the battery backup
mode.
Caution: Do NOT wave solder SOIC to avoid damaging SNAPHAT sockets.
Symbol Parameter Value Unit
TSTG
Storage temperature (VCC off, oscillator
off)
SNAPHAT®–40 to 85 °C
SOIC –55 to 150 °C
TSLD Lead solder temperature for 10 seconds SOH28(1)
1. For SOH28 package, lead-free (pb-free) lead finish: reflow at peak temperature of 260°C (the time above
255°C must not exceed 30 seconds).
260 °C
SOX28(2)
2. The SOX28 package has lead-free (pb-free) lead finish, but cannot be exposed to peak reflow temperature
in excess of 240°C. Reflow at peak temperature of 240°C (the time above 235°C must not exceed 20
seconds).
240 °C
VIO Input or output voltage –0.3 to VCC+0.3 V
VCC Supply voltage –0.3 to 4.6 V
IOOutput current 20 mA
PDPower dissipation 1 W
DC and AC parameters M41ST85W
30/43 Doc ID 7531 Rev 11
5 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 9. DC and AC measurement conditions
Note: Output High Z is defined as the point where data is no longer driven.
Figure 18. AC testing input/output waveforms
Table 10. Capacitance
Parameter M41ST85W
VCC supply voltage 2.7 to 3.6 V
Ambient operating temperature –40 to 85°C
Load capacitance (CL)50 pF
Input rise and fall times ≤ 50 ns
Input pulse voltages 0.2 to 0.8VCC
Input and output timing ref. voltages 0.3 to 0.7VCC
Symbol Parameter(1)(2)
1. Effective capacitance measured with power supply at 5 V. Sampled only, not 100% tested.
2. At 25°C, f = 1 MHz.
Min Max Unit
CIN Input capacitance 7 pF
COUT(3)
3. Outputs are deselected.
Output capacitance 10 pF
tLP Low-pass filter input time constant (SDA and SCL) 50 ns
AI02568
0.8VCC
0.2VCC
0.7VCC
0.3VCC
M41ST85W DC and AC parameters
Doc ID 7531 Rev 11 31/43
Table 11. DC characteristics
Sym Parameter Test condition(1)
1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.7 to 3.6 V (except where noted).
M41ST85W
Unit
Min Typ Max
IBAT(2)
2. Measured with VOUT and ECON open.
Battery current OSC ON TA = 25°C, VCC = 0 V, VBAT = 3 V 400 500 nA
Battery current OSC OFF 50 nA
ICC1 Supply current f = 400 kHz 0.75 mA
ICC2 Supply current (standby) SCL, SDA = VCC – 0.3 V
or VSS + 0.3 V 0.50 mA
ILI(3)
3. RSTIN1 and RSTIN2 internally pulled-up to VCC through 100 kΩ resistor. WDI internally pulled-down to VSS through
100 kΩ resistor.
Input leakage current 0V ≤ VIN ≤ VCC ±1 µA
Input leakage current (PFI) –25 2 25 nA
ILO(4)
4. Outputs deselected.
Output leakage current 0V ≤ VIN ≤ VCC ±1 µA
IOUT1(5)
5. External SRAM must match RTC supervisor chip VCC specification.
VOUT current (active) VOUT1 > VCC – 0.3 V 100 mA
IOUT2 VOUT current (battery backup) VOUT2 > VBAT – 0.3 V 100 µA
VIH Input high voltage 0.7VCC VCC + 0.3 V
VIL Input low voltage –0.3 0.3VCC V
VBAT Battery voltage 2.5 3.0 3.5(6)
6. For rechargeable backup, VBAT (max) may be considered VCC.
V
VOH Output high voltage(7)
7. For PFO and SQW pins (CMOS).
IOH = –1.0 mA 2.4 V
Pull-up supply voltage (open drain) RST, IRQ/FT/OUT 3.6 V
VOHB(8)
8. Conditioned output (ECON) can only sustain CMOS leakage current in the battery backup mode. Higher leakage currents
will reduce battery life.
VOH (battery backup) IOUT2 = –1.0 µA 2.5 2.9 3.5 V
VOL
Output low voltage IOL = 3.0 mA 0.4 V
Output low voltage (open drain)(9)
9. For IRQ/FT/OUT, RST pins (open drain): if pulled-up to supply other than VCC, this supply must be equal to, or less than
3.0 V when VCC = 0 V (during battery backup mode).
IOL = 10 mA 0.4 V
VPFD Power fail deselect 2.55 2.60 2.70 V
VPFI
PFI input threshold VCC = 3 V(W) 1.225 1.250 1.275 V
PFI hysteresis PFI rising 20 70 mV
VSO Battery backup switchover 2.5 V
DC and AC parameters M41ST85W
32/43 Doc ID 7531 Rev 11
Figure 19. Bus timing requirements sequence
Table 12. AC characteristics
Symbol Parameter(1)
1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.7 to 3.6 V (except where otherwise noted).
Min Max Unit
fSCL SCL clock frequency 0 400 kHz
tBUF Time the bus must be free before a new transmission can start 1.3 µs
tEXPD EX to ECON propagation delay 15 ns
tFSDA and SCL fall time 300 ns
tHD:DAT(2)
2. Transmitter must internally provide a hold time to bridge the undefined region (300 ns max) of the falling edge of SCL.
Data hold time 0 µs
tHD:STA START condition hold time (after this period the first clock pulse is generated) 600 ns
tHIGH Clock high period 600 ns
tLOW Clock low period 1.3 µs
tRSDA and SCL rise time 300 ns
tSU:DAT Data setup time 100 ns
tSU:STA START condition setup time (only relevant for a repeated start condition) 600 ns
tSU:STO STOP condition setup time 600 ns
AI00589
SDA
P
tSU:STOtSU:STA
tHD:STA
SR
SCL
tSU:DAT
tF
tHD:DAT
tR
tHIGH
tLOW
tHD:STAtBUF
SP
M41ST85W DC and AC parameters
Doc ID 7531 Rev 11 33/43
Figure 20. Power down/up mode AC waveforms
Table 13. Power down/up AC characteristics
Symbol Parameter(1)
1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.7 to 3.6 V (except where otherwise noted).
Min Typ Max Unit
tF(2)
2. VPFD(max) to VPFD(min) fall time of less than tF may result in deselection/write protection not occurring until
200 µs after VCC passes VPFD(min).
VPFD(max) to VPFD(min) VCC fall time 300 µs
tFB(3)
3. VPFD(min) to VSS fall time of less than tFB may cause corruption of RAM data.
VPFD(min) to VSS VCC fall time 10 µs
tPD EX at VIH before power down 0 µs
tPFD PFI to PFO propagation delay 15 25 µs
tRVPFD(min) to VPFD(max) VCC rise time 10 µs
tRB VSS to VPFD(min) VCC rise time 1 µs
trec(4)
4. Programmable (see Table 6 on page 28)
Power-up deselect time 40 200 ms
AI03661
VCC
INPUTS
(PER CONTROL INPUT)
OUTPUTS
DON'T CARE
HIGH-Z
tF
tFB
tR
tPD
tRB
tDR
VALID VALID
(PER CONTROL INPUT)
RECOGNIZEDRECOGNIZED
VPFD (max)
VPFD (min)
VSO
trec
RST
ECON
PFO
Package mechanical data M41ST85W
34/43 Doc ID 7531 Rev 11
6 Package mechanical data
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.
Figure 21. SOH28 – 28-lead plastic small outline, battery SNAPHAT®, package outline
Note: Drawing is not to scale.
SOH-A
E
N
D
C
LA1 α
1
H
A
CP
Be
A2
eB
Table 14. SOH28 – 28-lead plastic small outline, battery SNAPHAT®, package mechanical data
Symbol
millimeters inches
Typ Min Max Typ Min Max
A 3.05 0.120
A1 0.05 0.36 0.002 0.014
A2 2.34 2.69 0.092 0.106
B 0.36 0.51 0.014 0.020
C 0.15 0.32 0.006 0.012
D 17.71 18.49 0.697 0.728
E 8.23 8.89 0.324 0.350
e 1.27 – – 0.050 – –
eB 3.20 3.61 0.126 0.142
H 11.51 12.70 0.453 0.500
L 0.41 1.27 0.016 0.050
a0°8°0°8°
N 28 28
CP 0.10 0.004
M41ST85W Package mechanical data
Doc ID 7531 Rev 11 35/43
Figure 22. 4-pin SNAPHAT® housing for 48 mAh battery & crystal, package outline
Note: Drawing is not to scale.
SHTK-A
A1 A
D
E
eA
eB
A2
BL
A3
Table 15. 4-pin SNAPHAT® housing for 48 mAh battery & crystal, mechanical data
Symbol
millimeters inches
Typ Min Max Typ Min Max
A 9.78 0.3850
A1 6.73 7.24 0.2650 0.2850
A2 6.48 6.99 0.2551 0.2752
A3 0.38 0.0150
B 0.46 0.56 0.0181 0.0220
D 21.21 21.84 0.8350 0.8598
E 14.22 14.99 0.5598 0.5902
eA 15.55 15.95 0.6122 0.6280
eB 3.20 3.61 0.1260 0.1421
L 2.03 2.29 0.0799 0.0902
Package mechanical data M41ST85W
36/43 Doc ID 7531 Rev 11
Figure 23. 4-pin SNAPHAT® housing for 120 mAh battery & crystal, package outline
Note: Drawing is not to scale.
SHTK-A
A1 A
D
E
eA
eB
A2
BL
A3
Table 16. 4-pin SNAPHAT® housing for 120 mAh battery & crystal, mechanical data
Symbol
millimeters inches
Typ Min Max Typ Min Max
A 10.54 0.415
A1 8.00 8.51 0.315 0.335
A2 7.24 8.00 0.285 0.315
A3 0.38 0.015
B 0.46 0.56 0.018 0.022
D 21.21 21.84 0.835 0.860
E 17.27 18.03 0.680 0.710
eA 15.55 15.95 0.612 0.628
eB 3.20 3.61 0.126 0.142
L 2.03 2.29 0.080 0.090
M41ST85W Package mechanical data
Doc ID 7531 Rev 11 37/43
Figure 24. SOX28 – 28-lead plastic small outline, 300 mils, embedded crystal, package outline
Note: Drawing is not to scale.
E
14
e
D
C
H
15 28
1
B
SO-E
A1 LA1 α
h x 45°
AA2
ddd
Table 17. SOX28 – 28-lead plastic small outline, 300 mils, embedded crystal, mechanical data
Symbol
millimeters inches
Typ Min Max Typ Min Max
A 2.44 2.69 0.096 0.106
A1 0.15 0.31 0.006 0.012
A2 2.29 2.39 0.090 0.094
B 0.41 0.51 0.016 0.020
C 0.20 0.31 0.008 0.012
D 17.91 18.01 0.705 0.709
ddd 0.10 0.004
E 7.57 7.67 0.298 0.302
e 1.27 – – 0.050 – –
H 10.16 10.52 0.400 0.414
L 0.51 0.81 0.020 0.032
a0°8°0°8°
N 28 28
Package mechanical data M41ST85W
38/43 Doc ID 7531 Rev 11
Figure 25. Carrier tape for SOH28 and SOX28 package
T
K0
P1
A0
B0
P2
P0
CENTER LINES
OF CAVITY
W
E
F
D
TOP COVER
TAPE
USER DIRECTION OF FEED
AM03073v1
Table 18. Carrier tape dimensions for SOH28 and SOX28 packages
Package W D E P0P2FA
0B0K0P1TUnit
Bulk
Qty
SOH28 24.00
±0.30
1.50
+0.10/
–0.00
1.75
±0.10
4.00
±0.10
2.00
±0.10
11.50
±0.10
12.90
±0.10
18.70
±0.10
3.20
±0.10
16.00
±0.10
0.30
±0.05 mm 936
SOX28 24.00
±0.30
1.50
+0.10/
–0.00
1.75
±0.10
4.00
±0.10
2.00
±0.10
11.50
±0.10
12.70
±0.10
18.20
±0.10
3.20
±0.10
16.00
±0.10
0.30
±0.05 mm 1000
M41ST85W Package mechanical data
Doc ID 7531 Rev 11 39/43
Figure 26. Reel schematic
Note: The dimensions given in Ta b l e 1 9 incorporate tolerances that cover all variations on critical
parameters.
A
D
B
Full radius
Tape slot
In core for
Tape start
2.5mm min.width
G measured
At hub
C
N
40mm min.
Access hole
At slot location
T
AM04928v1
Table 19. Reel dimensions for 24 mm carrier tape (SOH28 and SOX28 packages)
Carrier tape A
(max)
B
(min) CD
(min)
N
(min) GT
(max)
24 mm
(SOH28, SOX28)
330 mm
(13-inch) 1.5 mm 13 mm
± 0.2 mm 20.2 mm 60 mm 24.4 mm
+ 2/–0 mm 30.4 mm
Part numbering M41ST85W
40/43 Doc ID 7531 Rev 11
7 Part numbering
Table 20. Ordering information scheme
Caution: Do not place the SNAPHAT® battery package “M4Txx-BR12SH” in conductive foam as it will
drain the lithium button-cell battery.
For other options, or for more information on any aspect of this device, please contact the ST sales office
nearest you.
Table 21. SNAPHAT® battery/crystal table
Example: M41ST 85W MH 6 E
Device type
M41ST
Supply voltage and write protect voltage
85W = VCC = 2.7 to 3.6 V; 2.55V ≤ VPFD ≤ 2.70 V
Package
MH(1) = SOH28
1. The 28-pin SOIC package (SOH28) requires the SNAPHAT® battery/crystal package which is ordered
separately under the part number “M4TXX-BR12SHx” in plastic tube (see Table 21).
MX(2) = SOX28
2. The SOX28 package includes an embedded 32,768 Hz crystal.
Temperature range
6 = –40 to 85°C
Shipping method
For SOH28:
E = ECOPACK® package, tubes(3)
3. Not recommended for new design. Contact local ST sales office for availability.
F = ECOPACK® package, tape & reel
For SOX28:
Blank = ECOPACK® package, tubes(3)
TR = ECOPACK® package, tape & reel
Part number Description Package
M4T28-BR12SH Lithium battery (48 mAh) and crystal SNAPHAT® top SH
M4T32-BR12SH Lithium battery (120 mAh) and crystal SNAPHAT® top SH
M41ST85W Environmental information
Doc ID 7531 Rev 11 41/43
8 Environmental information
Figure 27. Recycling symbols
This product contains a non-rechargeable lithium (lithium carbon monofluoride chemistry)
button cell battery fully encapsulated in the final product.
Recycle or dispose of batteries in accordance with the battery manufacturer's instructions
and local/national disposal and recycling regulations.
Revision history M41ST85W
42/43 Doc ID 7531 Rev 11
9 Revision history
Table 22. Document revision history
Date Revision Changes
Aug-2000 1 First issue
24-Aug-2000 1.1 Block diagram added (Figure 4)
12-Oct-2000 1.2 trec table removed, cross references corrected
18-Dec-2000 2 Reformatted, TOC added, and PFI input leakage current added (Ta bl e 1 1 )
18-Jun-2001 2.1
Addition of trec information, table changed, one added (Ta bl e 2 , 6); changed PFI/PFO
graphic (see Figure 4); change to DC and AC characteristics, order information
(Ta b l e 1 1 , 12, 20); note added to “Setting Alarm Clock Registers” section; added
temp./voltage info. to tables (Ta b l e 1 0 , 11, 6, 12, 13); addition of default values
(Ta b l e 7 )
22-Jun-2001 2.2 Note added to clock operation section
26-Jul-2001 3 Change in product maturity
07-Aug-2001 3.1 Improve text in “Setting the Alarm Clock” section
20-Aug-2001 3.2 Change VPFD values in document
06-Sep-2001 3.3 DC characteristics VBAT changed; VOHB changed; PFI hysteresis (PFI Rising) spec.
added; and crystal electrical characteristics table removed (Tabl e 1 1 , 6)
03-Dec-2001 3.4 Changed READ/WRITE mode sequences (Figure 10, 12); change in VPFD lower limit
for 5V (M41ST85Y) part only (Ta bl e 1 1 , 20)
01-May-2002 3.5 Change trec definition (Ta bl e 6 ); modify reflow time and temperature footnote (Ta bl e 8 )
03-Jul-2002 3.6 Modify DC characteristics table footnote, default values (Ta bl e 1 1 , 7)
15-Nov-2002 3.7 Added embedded crystal (MX) package option; corrected initial power-up condition
(cover page, Figure 1, 3, 4, 5, 24, Ta bl e 1 , 7, 20, )
24-Jan-2003 3.8 Update diagrams (Figure 4, 5, 24); update values (Ta bl e 1 3 , 5, 6, 7, )
25-Feb20-03 4 New Si changes (Ta b l e 1 3 , 5, 6); corrected dimensions (Figure 24; Ta b l e )
20-May-2004 5 Reformatted; correct dimensions; update Lead-free information (Figure 20, 13, 16;
Ta b le 8 , 16, 20)
15-Jun-2004 6 Update characteristics; add package shipping (Figure 3; Ta b l e 1 , 11, 20)
13-Sep-2004 7 Update maximum ratings (Ta bl e 8 )
10-Jan-2006 8 Updated template, Lead-free text, removed 5V references (Figure 1, 2, 3, 4, 5; Tabl e 5,
8, 9, 11, 12, 13, 20, 21)
03-Oct-2007 9
Reformatted document, minor textual changes; added lead-free second level
interconnect information to cover page and Section 6: Package mechanical data;
updated Ta b l e 8 .
16-Dec-2008 10 Added Section 8: Environmental information; updated Ta b l e 8 , Section 6: Package
mechanical data; minor reformatting.
03-Oct-2011 11
Updated Table 20: Ordering information scheme as shipping in tubes is not
recommended for new design; updated Section 8: Environmental information; added
Figure 25, 26 and Ta b l e 1 8 , 19.
M41ST85W
Doc ID 7531 43/43
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