UPD17708 datasheet(158/356 Pages) NEC

158 / 356 page

µPD17704, 17705, 17707, 17708, 17709

158

Example Example of timer updating processing and BTM0CY flag detection time interval

START:

CLR2

BTM0CK1, BTM0CK0

; Sets BTM0CY flag setting pulse to 10 Hz (100 ms)

BTIMER:

; <1>

SKT1

BTM0CY

; Updates timer if BTM0CY flag is “1”

AAA

Timer updating

BTIMER

AAA:

Processing A

BTIMER

The timing chart of the above program is shown below.

(3) Compensating basic timer 0 carry at CE reset

Next, an example of compensating the timer at CE reset is described below.

As shown in the example below, the timer must be compensated at CE reset “if the BTM0CY flag is used

for power failure detection and if the BTM0CY flag is used for a watch timer”.

The BTM0CY flag is reset (to 0) first on power application (power-ON reset), and is disabled from being

set until it is read once by the “PEEK” instruction.

If the CE pin goes high, CE reset is effected in synchronization with the rising edge of the BTM0CY flag

setting pulse. At this time, the BTM0CY flag is set (to 1) and the timer is started.

By detecting the status of the BTM0CY flag at system reset (power-ON reset or CE reset), therefore, it

can be identified whether a power-ON reset or CE reset has been effected (power failure detection). That

is, power-ON reset has been effected if the flag is “0”, and CE reset has been effected if it is “1”.

At this time, the watch timer must continue operating even if CE reset has been effected.

However, because the BTM0CY flag is reset to 0 when it is read to detect a power failure, the set status

(1) of the BTM0CY flag is overlooked once. If the delay function of CE reset is used, the value set to

the CE reset timer carry counter (control register address 06H) is overlooked.

Consequently, the watch timer must be updated if CE reset is identified by means of power failure

detection.

For the details of power failure detection, refer to 21. RESET.

tSET

CE pin

BTM0CY detection interval

tCHECK

Timer updating processing

tTIMER

BTM0CY flag

setting pulse

<1> SKT1

BTM0CY

<2> SKT1

BTM0CY

CE reset

If this timer updating processing

time is too long, CE reset is effected

during processing.

UPD17708 Datasheet(PDF) 158 Page - NEC

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