NEC PD17062 User Manual

Page 143

143

PD17062

12.4.2 Correcting the Timer Carry FF at a CE reset

This section describes an example of correcting the timer at a CE reset.

If the timer carry FF is used both to check for power failure and as a timer, it is necessary to correct the

timer at a CE reset, as explained in the following example.

The timer carry FF is reset to 0 at a power-on reset, and it is kept from being set until the BTM0CY flag is

read-accessed using a PEEK instruction.

When the CE pin goes from a low to a high, a CE reset occurs in synchronization with the positive-going

edge of the timer carry FF set pulse. At this point, the BTM0CY flag is set to 1 and becomes active.

Therefore, checking the state of the BTM0CY flag at a system reset (power-on reset or CE reset) can

discriminate between a power-on reset and CE reset; if the flag is 0, it indicates a power-on reset, and if 1,

it indicates a CE reset (power failure check).

A timer for ordinary time measurement must continue to operate even at a CE-reset.

Reading the BTM0CY flag for a power failure check could reset the BTM0CY flag to 0, thus losing a chance

of detecting a set (1) state of the flag.

To skirt the above problem, it is necessary to update the timer for time measurement at a CE reset that occurs

because of power failure.

See also Section 14.6 for details about a power failure check.

Example Correcting the timer at a CE reset

When using the timer carry FF for a power failure check and clock update

START:

; Program address 0000H

Process A

;

SKT1

BTM0CY

; Built-in macro

; Tests the BTM0CY flag.

INITIAL

; Branches to INITIAL if 0 (power failure check)

BACKUP:

;

Update the clock by 100 ms ; Correct the clock because of a backup (CE reset).

LOOP:

;

Process B

; While performing process B,

SKF1

BTM0CY

; tests the BTM0CY flag and updates the clock.

BACKUP

LOOP

INITIAL:

INITFLG NOT BTM0ZX, NOT BTM0CK2, NOT BTM0CK1, NOT BTM0CK0

; Built-in macro

; Because a power failure (power-on reset) is detected, the timer

; carry FF setting time interval is set to 100 ms, and passes

; control to process C.

Process C

LOOP

Fig. 12-6 shows the timing chart for the above program.