M41ST87YMX6 datasheet(34/48 Pages) STMICROELECTRONICS

34 / 48 page

Clock operation

M41ST87Y, M41ST87W

34/48

3.4

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

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

Note:

The accuracy of the timer is within ± the selected resolution.

If the processor does not reset the timer within the specified period, the M41ST87Y/W sets

the WDF (watchdog flag) and generates a watchdog interrupt or a microprocessor 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/OUT pin when timed-out. When WDS is set to

a '1,' the watchdog will output a negative pulse on the RST pin for trec. The watchdog

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.

Note:

The WDI pin should be tied to VSS if not used.

In order to perform a software reset of the watchdog timer, the original time-out period can

be written into the watchdog register, effectively restarting the count-down cycle.

Should the watchdog timer time-out, and the WDS bit is programmed to output an interrupt,

either a transition of the WDI pin, or a value of 00h needs to be written to the watchdog

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.

3.5

Square wave output

The M41ST87Y/W offers the user a programmable square wave function which is output on

the SQW/FT pin. RS3-RS0 bits located in 13h establish the square wave output frequency.

These frequencies are listed in Table 9. Once the selection of the SQW frequency has been

completed, the SQW/FT pin can be turned on and off under software control with the square

wave enable bit (SQWE) located in Register 0Ah.

The SQW/FT output is programmable as an N-channel, open drain output driver, or a full-

CMOS output driver. By setting the square wave open drain bit (SQWOD) to a '1,' the output

will be configured as an open drain (with IOL as specified in Table 17 on page 42). When

SQWOD is set to '0,' the output will be configured as full-CMOS (sink and source current as

specified in Table 17 on page 42).

Note:

When configured as open drain (SQWOD = '1'), the SQW/FT pin requires an external pull-

up resistor.

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