UPD17708 datasheet(328/356 Pages) NEC

328 / 356 page

328

µPD17704, 17705, 17707, 17708, 17709

21.7.2 Cautions on detecting power failure by BTM0CY flag

The following points must be noted when counting the watch timer by using the BTM0CY flag.

(1) Updating watch

When creating a watch program using the timer carry, the watch must be updated after a power failure has

been detected.

This is because the BTM0CY flag is reset to 0 because it is read after a power failure has been detected. As

a result, counting of the watch is overlooked once.

(2) Watch updating processing time

Updating the watch must be completed before the next basic timer 0 carry FF setting pulse rises.

This is because CE reset is executed before the watch updating processing has been completed if the CE pin

goes high during watch updating processing.

For the details of (1) and (2), refer to (3) Compensating basic timer 0 carry at CE reset in 13.2.6.

The following points must be noted when performing processing in case of a power failure.

(3) Timing to detect power failure

When counting the watch by using the BTM0CY flag, the BTM0CY flag must be read to detect a power failure

before the next basic timer 0 carry FF setting pulse rises after the program has been started from address

0000H.

This is because, if the basic timer 0 carry FF setting time is set to, say, 10 ms, and if the power failure is detected

11 ms after the program has been started, the BTM0CY flag is overlooked once.

For further information, refer to (3) Compensating basic timer 0 carry at CE reset in 13.2.6.

Power failure detection and initial processing must be performed within the time in which the basic timer 0 carry

FF is set, as shown in the example below.

This is because, if the CE pin rises and CE reset is executed during power failure processing or initial processing,

the processing is stopped in midway, causing a problem.

To update the basic timer 0 carry FF setting time in the initial processing, the instruction that changes the setting

time must be executed at the end of the initial processing.

This is because, if the basic timer 0 carry FF setting time is changed before the initial processing, the initial

processing may not be executed to the end because CE reset may be executed.

UPD17708 Datasheet(PDF) 328 Page - NEC