M41ST87W datasheet(36/52 Pages) STMICROELECTRONICS

36 / 52 page

Clock operation

M41ST87Y, M41ST87W

36/52

Doc ID 9497 Rev 8

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) but does not clear the IRQ/OUT pin.

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 44). 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 44).

Note:

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

up resistor.

Table 9.

Square wave output frequency

Square wave bits

Square wave

RS3

RS2

RS1

RS0

Frequency

Units

0000

None

–

0001

32.768

kHz

0010

8.192

kHz

0011

4.096

kHz

0100

2.048

kHz

0101

1.024

kHz

0110

512

0111

256

1000

128

1001

1010

1011

11008

11014

11102

11111

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