NEC mPD780024AS Series, Preliminary User'S Manual

Page: 155 / 311

155
CHAPTER 8 WATCH TIMER
Preliminary User’s Manual U16035EJ1V0UM
8.4 Operations of Watch Timer
8.4.1 Watch timer operation
The watch timer generates an interrupt request (INTWT) at a specific time interval (2
14
/f
W
seconds) by using the
main system clock or subsystem clock.
The interrupt request is generated at the following time interval.
• If main system clock (8.38 MHz) is selected: 0.25 seconds
• If subsystem clock (32.768 kHz) is selected: 0.5 seconds
The watch timer generates an interrupt request (INTWT) at a specific time interval.
When bit 0 (WTM0) and bit 1 (WTM1) of the watch timer operation mode register (WTM) are set to 1, the count
operation starts. When these bits are set to 0, the 5-bit counter is cleared and the count operation stops.
When the interval timer is simultaneously operated, zero-second start can be achieved only for the watch timer
by setting WTM1 to 0. In this case, however, the 9-bit prescaler is not cleared. Therefore, an error up to 2
9 ×
1/f
W
seconds occurs in the first overflow (INTWT) after zero-second start.
Remark f
X
: Main system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
f
W
: Watch timer clock frequency (f
X
/2
7
or f
XT
)
8.4.2 Interval timer operation
The watch timer operates as interval timer which generates interrupt requests (INTWTI) repeatedly at an interval
of the preset count value.
The interval time can be selected with bits 4 to 6 (WTM4 to WTM6) of the watch timer operation mode register
(WTM).
Table 8-3. Interval Timer Interval Time
WTM6 WTM5 WTM4 Interval Time
When Operated at When Operated at When Operated at
f
X
= 8.38 MHz f
X
= 4.19 MHz f
XT
= 32.768 kHz
0 0 0 2
4
×
1/f
W
244
µ
s 489
µ
s 488
µ
s
0 0 1 2
5 ×
1/f
W
489
µ
s 978
µ
s 977
µ
s
0 1 0 2
6
×
1/f
W
978
µ
s 1.96 ms 1.95 ms
0 1 1 2
7 ×
1/f
W
1.96 ms 3.91 ms 3.91 ms
1 0 0 2
8
×
1/f
W
3.91 ms 7.82 ms 7.81 ms
1 0 1 2
9
×
1/f
W
7.82 ms 15.6 ms 15.6 ms
Other than above Setting prohibited
Remark f
X
: Main system clock oscillation frequency
f
XT
: Subsystem clock oscillation frequency
f
W
: Watch timer clock frequency (f
X
/2 7 or f
XT
)