
System Interface Unit
MOTOROLA
MPC801 USER’S MANUAL
12-13
12
The timeout period is calculated as:
Solving this equation using a 32.768KHz external clock gives:
This provides a range from 122 microseconds with a PITC register of $0000, to 8 seconds
with a PITC of $FFFF.
12.9 THE SOFTWARE WATCHDOG TIMER
The software watchdog timer option prevents system lockout if the software becomes
trapped in loops without a controlled exit. The software watchdog timer is enabled after
system reset to cause a system reset if it times out. If you do not need the software watchdog
timer, the SWE bit in the system protection control register (SYPCR) must be cleared to
disable it. If you do need it, the software watchdog timer requires a special service sequence
to be executed on a periodic basis. If this periodic servicing does not occur, the software
watchdog timer times out and issues a reset or a nonmaskable interrupt, which is
programmed by the SWRI bit in the SYPCR register. Once the software writes the SYPCR
register, the state of the SWE bit cannot be changed. Refer to the system configuration and
protection registers for more information.
To service the software watchdog timer, write $556C and then $AA39 to the software
service register (SWSR). This clears the watchdog timer and the timing process begins
again. If any value other than $556C or $AA39 is written to the SWSR, the entire sequence
must be repeated. Although the writes must occur in the correct order before a timeout
occurs, any number of instructions can be executed between the writes. This allows
interrupts and exceptions to occur between the two writes when necessary. Refer to Figure
12-6 for more information.
PITperiod
1
+
Fpitrtclk
1
+
------------------------------------------
1or128
4
÷
-----------------PITC
=
=
PITperiod
1
+
PI8192
=