SYSTEM INTEGRATION MODULE
M68HC16 Z SERIES
5-26
USER’S MANUAL
Both writes must occur before time-out in the order listed. Any number of instructions
can be executed between the two writes.
Watchdog clock rate is affected by the software watchdog prescale (SWP) bit and the
software watchdog timing (SWT[1:0]) field 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.
SWT[1:0] selects the divide ratio used to establish the software watchdog time-out
period.
The following equation calculates the time-out period for a slow reference frequency,
where f
ref
is equal to the EXTAL crystal frequency.
The following equation calculates the time-out period for a fast reference frequency,
where f
ref
is equal to the EXTAL crystal frequency.
The following equation calculates the time-out period for an externally input clock fre-
quency on both slow and fast reference frequency devices, when f
sys
is equal to the
system clock frequency.
shows the divide ratio for each combination of SWP and SWT[1:0] bits.
When SWT[1:0] are modified, a watchdog service sequence must be performed be-
fore the new time-out period can take effect.
Table 5-9 MODCLK Pin and SWP Bit During Reset
MODCLK
SWP
0 (External Clock)
1 (
÷
512)
1 (Internal Clock)
0 (
÷
1)
Time-Out Period
Divide Ratio Specified by SWP and SWT[1:0]
f
ref
------------------------------------------------------------------------------------------------------------------------
=
Time-Out Period
128
(
)
Divide Ratio Specified by SWP and SWT[1:0]
(
)
f
ref
--------------------------------------------------------------------------------------------------------------------------------------------
=
Time-Out Period
Divide Ratio Specified by SWP and SWT[1:0]
f
sys
------------------------------------------------------------------------------------------------------------------------
=
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..