M41T81S

Clock operation

19/31

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.

Square wave output

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 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

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

10

0

1

64

Hz

10

1

0

32

Hz

10

1

1

16

Hz

1

1

008

Hz

1

1

014

Hz

1

1

102

Hz

1

1

111

Hz