UPD17015 datasheet(47/120 Pages) NEC

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µPD17015

10.2.6

Cautions for Using Basic Timer 0

(1) Correcting the basic timer 0 at a CE reset

The following paragraphs describe an example of correcting the timer at a CE reset.

As explained in the example, it is necessary to correct the timer at a CE reset, if the BTM0CY flag is used both

to detect a power failure and to control the clock timer.

The BTM0CY flag is cleared (0) when the supply voltage is first applied (at power-on reset). It is not set until

its contents have been read by an instruction listed in Table 10-1.

When the CE pin goes from a low level to a high level, a CE reset occurs in synchronization with the rising edge

of the BTM0CY flag set pulse, setting the BTM0CY flag to “1” and making it active.

Detecting the state of the BTM0CY flag at a system reset (power-on or CE reset) can therefore check for a power

failure. If the flag is “0”, it means that a power-on reset has occurred. If it is “1”, it means that a CE reset has

occurred (power failure detection).

In this case, a clock timer must keep operating even at a CE reset.

However, reading the BTM0CY flag for power failure detection clears the flag to 0 and makes it impossible to

detect the set (1) state of the flag for one cycle.

To solve this problem, it is necessary to update the clock timer if an attempt to detect a power failure detects

a CE reset. See Section 15.5 for power failure detection.

Example

Correcting the timer at a CE reset (when the BTM0CY flag is used for both power failure detection

and timer update)

START :

; Program address 0000H

Process A

; 1

SKT1

BTM0CY

; Built-in macro

; Checks the BTM0CY flag and branches to INITIAL

INITIAL

; if the flag is “0” (power failure detected).

BACKUP :

; 2

Timer update by 125 ms

; Timer correction because of backup (CE reset)

LOOP:

; 3

Process B

; While performing process B,

SKF1

BTM0CY

; tests the BTM0CY flag and updates the timer.

BACKUP

LOOP

INITIAL :

Process C

LOOP

Fig. 10-6 shows the timing chart for the above program.

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