M48T212A datasheet(13/20 Pages) STMICROELECTRONICS

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M48T212A

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 7h.

Bits BMB4-BMB0 store a binary multiplier and the

two lower order bits RB1-RB0 select the resolu-

tion, 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 Reg-

ister = 3*1 or 3 seconds). If the processor does not

reset the timer within the specified period, the

M48T212A sets the WDF (Watchdog Flag) and

generates a watchdog interrupt or a microproces-

sor 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 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 and

the FT bit will reset to a ‘ 0’ at the end of a Watch-

dog 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. 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 perform a software reset

of the watchdog timer, the original time-out period

can be written into the Watchdog Register, effec-

tively 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 pin. This will also dis-

able the watchdog function until it is again pro-

grammed correctly. A read of the Flags Register

will reset the Watchdog Flag (Bit D7; Register 0h).

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 pin and the frequency test function is

activated, the watchdog function prevails and the

frequency test function is denied.

Figure 10. Back-Up Mode Alarm Waveforms

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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