m41t81s STMicroelectronics, m41t81s Datasheet - Page 19

• Manufacturer Part Number: m41t81s
• Description: Serial Access Real-time Clock With Alarms
• Manufacturer: STMicroelectronics

Square wave output
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 M41T81S 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.
The M41T81S 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
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.
RS3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
Square Wave Output Frequency
RS2
Square Wave Bits
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
Table
RS1
4. Once the selection of the SQW frequency has been
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
RS0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Frequency
32.768
8.192
4.096
2.048
1.024
None
512
256
128
64
32
16
8
4
2
1
Square Wave
Units
kHz
kHz
kHz
kHz
kHz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
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