M48T212V datasheet(20/35 Pages) STMICROELECTRONICS

20 / 35 page

Clock operation

M48T212V

20/35

Figure 8.

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 Watchdog

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:

Accuracy of timer is within ± the selected resolution.

If the processor does not reset the timer within the specified period, the M48T212V sets the

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

is reset by reading the Flags Register (Address 0h).

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 pin when timed-out. When WDS is

set to a '1,' the watchdog will output a negative pulse on the RST pin for 40 to 200 ms. The

Watchdog register, AFE, ABE, and FT 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:

a transition (high-to-low or low-to-high) can be applied to the Watchdog Input pin (WDI)

the microprocessor can perform a WRITE of the Watchdog Register.

The time-out period then starts over. The WDI pin should be tied to VSS if not used. The

watchdog will be reset on each transition (edge) seen by the WDI pin. In the order to

AI03622

VCC

IRQ/FT

HIGH-Z

VPFD (max)

VPFD (min)

AFE Bit/ABE Bit

AF Bit in Flags Register

HIGH-Z

tREC

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