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4 watch timer operation, 1 operation as watch timer, 2 operation as interval timer – NEC PD78F9488 User Manual

Page 165

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CHAPTER 8 WATCH TIMER

User’s Manual U15331EJ4V1UD

165

8.4 Watch Timer Operation

8.4.1 Operation as watch timer

The main system clock (4.19 MHz) or subsystem clock (32.768 kHz) is used to enable the watch timer to operate

at 0.5-second intervals.

Also, an interrupt request (INTWT) occurs at an interval of 1.0 seconds when using the 32.768 kHz subsystem

clock via a setting in the watch timer interrupt time selection register (WTIM).

The watch timer is used to generate an interrupt request at specified intervals.
By setting bits 0 and 1 (WTM0 and WTM1) of the watch timer mode control register (WTM) to 1, the watch timer

starts counting. By setting them to 0, the 5-bit counter is cleared and the watch timer stops counting.

It is possible to start the watch timer from zero seconds by clearing WTM1 to 0 when the interval timer and watch

timer operate at the same time. In this case, however, an error of up to 2

9

× 1/f

W

seconds may occur in the overflow

(INTWT) after the zero-second start of the watch timer because the 9-bit prescaler is not cleared to 0.

8.4.2 Operation as interval timer

The interval timer is used to repeatedly generate an interrupt request at the interval specified by a preset count

value.

The interval can be selected by bits 4 to 6 (WTM4 to WTM6) of the watch timer mode control register (WTM).

Table 8-3. Interval Time of Interval Timer

WTM6 WTM5 WTM4 Interval Time

At f

X

= 5.0 MHz

At f

X

= 4.19 MHz

At f

XT

= 32.768

kHz

At f

XT

= 16.384

kHz

0 0 0

2

4

× 1/f

W

409.6

µ

s 488

µ

s 488

µ

s 976

µ

s

0 0 1

2

5

× 1/f

W

819.2

µ

s 977

µ

s 977

µ

s 1.95

ms

0 1 0

2

6

× 1/f

W

1.64 ms

1.95 ms

1.95 ms

3.90 ms

0 1 1

2

7

× 1/f

W

3.28 ms

3.91 ms

3.91 ms

7.82 ms

1 0 0

2

8

× 1/f

W

6.55 ms

7.81 ms

7.81 ms

15.6 ms

1 0 1

2

9

× 1/f

W

13.1 ms

15.6 ms

15.6 ms

31.2 ms

Other than above

Setting prohibited

Remarks 1. f

X

: Main system clock oscillation frequency

2. f

XT

: Subsystem clock oscillation frequency

3. f

W

: Watch timer clock frequency

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