UPD17708 datasheet(160/356 Pages) NEC

160 / 356 page

µPD17704, 17705, 17707, 17708, 17709

160

Figure 13-9. Timing Chart

As shown in Figure 13-9, the program is started from address 0000H because the internal 10-Hz pulse

rises when supply voltage VDD is first applied.

When the BTM0CY flag is detected at point A, it is judged that the BTM0CY flag is reset (to 0) and that

a power failure (power-ON reset) has occurred because the power has just been applied.

Therefore, “processing C” is executed, and the BTM0CY flag setting pulse is set to 100 ms.

Because the content of the BTM0CY flag is read once at point A, the BTM0CY flag will be set to 1 every

100 ms.

Next, even if the CE pin goes low at point B and high at point C, the program counts up the watch timer

while executing “processing B”, unless the clock stop instruction is executed.

At point C, because the CE pin goes high, CE reset is effected at point D at which the BTM0CY flag setting

pulse rises next time, and the program is started from address 0000H.

When the BTM0CY flag is detected at point E at this time, it is set to 1. Therefore, this is judged to be

a back up (CE reset).

As is evident from the above figure, unless the watch is updated by 100 ms at point E, the watch is delayed

by 100 ms each time CE reset is effected.

If processing A takes longer than 100 ms when a power failure is detected at point E, the setting of the

BTM0CY flag is overlooked two times. Therefore, processing A must be completed within 100 ms.

The above description also applies when the BTM0CY flag setting pulse is set to 50, 20, or 10 ms.

Therefore, the BTM0CY flag must be detected for power failure detection within the BTM0CY flag setting

time after the program has been started from address 0000H.

Power-ON reset

Start from address 0

CE reset

Start from address 0

Application of

supply voltage

BTM0CY flag detected

5 V

VDD

BTM0CY flag setting

pulse (10 Hz)

BTM0CY flag

Program processing

Program instruction

0 V

BBB

<3>

Watch UP

<3>

Watch UP

<3>

Watch UP

<3>

<1>

Watch UP

<3>

<1>

Updates watch timer because

setting of BTM0CY flag (to 1)

is detected

Point A

Point B

Point C

Point D

Point E

UPD17708 Datasheet(PDF) 160 Page - NEC

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