SYSTEM INTEGRATION MODULE
MC68332
4-6
USER’S MANUAL
Perform a software watchdog service sequence as follows:
1. Write $55 to SWSR.
2. Write $AA to SWSR.
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
. 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
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.
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 SWP
0 (External Clock)
1 (
÷
512)
1 (Internal Clock)
0 (
÷
1)
Table 4-4 Software Watchdog Ratio
SWP
SWT Ratio
0
00
2
9
0
01
2
11
0
10
2
13
0
11
2
15
1
00
2
18
1
01
2
20
1
10
2
22
1
11
2
24
Time-out Period
1
EXTAL Frequency Divide Ratio
⁄
-------------------------------------------------------------------------------------
=
Time-out Period
Divide Ratio
EXTAL Frequency
-------------------------------------------------
=
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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