NEC PD78F9488 - Manual (Page 162)

CHAPTER 8 WATCH TIMER
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User’s Manual U15331EJ4V1UD
(1) Watch timer
An interrupt request (INTWT) occurs at an interval of 0.5 second when using either the 4.19 MHz main
system clock or the 32.768 kHz subsystem clock.
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).
Caution An interval of 0.5 second cannot be created when using the 5.0 MHz main system clock.
Instead, switch to the 32.768 kHz subsystem clock, and then create the 0.5-second interval.
(2) Interval timer
An interrupt request (INTWTI) occurs at preset intervals.
Table 8-1. Interval Time of Interval Timer
Interval Time At f
X
= 5.0 MHz At f
X
= 4.19 MHz At f
XT
= 32.768 kHz At f
XT
/2 = 16.384 kHz
2
4
×1/f
W
409.6 µ s 488 µ s 488 µ s 976 µ s
2
5
×1/f
W
819.2 µ s 977 µ s 977 µ s 1.95 ms
2
6
×1/f
W 1.64 ms 1.95 ms 1.95 ms 3.90 ms
2
7
×1/f
W
3.28 ms 3.91 ms 3.91 ms 7.82 ms
2
8
×1/f
W
6.55 ms 7.81 ms 7.81 ms 15.6 ms
2
9
×1/f
W 13.1 ms 15.6 ms 15.6 ms 31.2 ms
Remarks 1. f
W
: Watch timer clock frequency (f
X
/2 7 , f
XT
, or f
XT
/2)
2. f
X
: Main system clock oscillation frequency
3. f
XT
: Subsystem clock oscillation frequency
8.2 Configuration of Watch Timer
The watch timer includes the following hardware.
Table 8-2. Configuration of Watch Timer
Item Configuration
Counter 5 bits × 1
Prescaler 9 bits × 1
Control registers Watch timer mode control register (WTM)
Watch timer interrupt time selection register (WTIM)