M48T59 datasheet(17/32 Pages) STMICROELECTRONICS

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M48T59, M48T59Y, M48T59V

Clock operations

17/32

Figure 11.

Back-up mode alarm waveforms

3.6

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 eight-bit

Watchdog Register (Address 1FF7h). The five bits (BMB4-BMB0) that 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 x 1 or 3 seconds).

Note:

Accuracy of timer is within ± the selected resolution.

If the processor does not reset the timer within the specified period, the M48T59/Y/V sets

the WDF (Watchdog Flag) and generates a watchdog interrupt or a microprocessor reset.

WDF is reset by reading the Flags Register (Address 1FF0h).

Note:

User must transition address (or toggle chip enable) to see Flag Bit change.

The most significant bit of the Watchdog Register is the Watchdog Steering Bit. When set to

a '0,' the watchdog will activate the IRQ/FT pin when timed-out. When WDS is set to a '1,'

the watchdog will output a negative pulse on the RST pin for a duration of trec. The

Watchdog Register, the FT Bit, and the AFE and ABE 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 resets when the microprocessor performs a re-write of the Watchdog

value of 00000000 to the eight bits in the Watchdog Register.

The watchdog function is automatically disabled upon power-down and the Watchdog

frequency test function is activated, the watchdog or alarm function prevails and the

frequency test function is denied.

AI03254B

VCC

IRQ/FT

trec

VPFD (max)

VPFD (min)

ABE, AFE bit in Interrupt Register

AF bit in Flags Register

HIGH-Z

VSO

HIGH-Z

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