
MOTOROLA
4-6
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
4
Both writes must occur before time-out in the order listed, but any number of instruc-
tions can be executed between the two writes.
Watchdog clock rate is affected by the software watchdog prescale (SWP) and soft-
ware watchdog timing (SWT) fields in SYPCR.
SWP determines system clock prescaling for the watchdog timer and determines that
one of two options, either no prescaling or prescaling by a factor of 512, can be select-
ed. The value of SWP is affected by the state of the MODCLK pin during reset, as
shown in
Table 4-3
. System software can change SWP value.
The SWT field selects the divide ratio used to establish software watchdog time-out
period. Time-out period is given by the following equations.
or
Table 4-4
shows the ratio for each combination of SWP and SWT bits. When SWT[1:0]
are modified, a watchdog service sequence must be performed before the new time-
out period can take effect.
Figure 4-3
is a block diagram of the watchdog timer and the clock control for the pe-
riodic interrupt timer.
Table 4-3 MODCLK Pin and SWP Bit During Reset
MODCLK
0 (External Clock)
1 (Internal Clock)
SWP
1 (
÷
512)
0 (
÷
1)
Table 4-4 Software Watchdog Ratio
SWP
0
0
0
0
1
1
1
1
SWT
00
01
10
11
00
01
10
11
Ratio
2
9
2
11
2
13
2
15
2
18
2
20
2
22
2
24
Time-out Period
1
EXTAL Frequency Divide Ratio
------------------------------------------------------------------------------------
=
Time-out Period
Divide Ratio
EXTAL Frequency
------------------------------------------------
=