MPC555
/
MPC556
SYSTEM CONFIGURATION AND PROTECTION
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
6-17
Although the writes must occur in the correct order prior to time-out, any number of
instructions may be executed between the writes. This allows interrupts and excep-
tions to occur, if necessary, between the two writes.
Figure 6-8 SWT Interrupts and Exceptions
Although most software disciplines support the watchdog concept, different systems
require different time-out periods. For this reason, the software watchdog provides a
selectable range for the time-out period.
In
, the range is determined by the value SWTC field. The value held in the
SWTC field is then loaded into a 16-bit decrementer clocked by the system clock. An
additional divide by 2048 prescaler is used if necessary. The decrementer begins
counting when loaded with a value from the software watchdog timing count field
(SWTC). After the timer reaches 0x0, a software watchdog expiration request is issued
to the reset or NMI control logic.
Upon reset, the value in the SWTC is set to the maximum value and is again loaded
into the software watchdog register (SWR), starting the process over. When a new val-
ue is loaded into the SWTC, the software watchdog timer is not updated until the ser-
vicing sequence is written to the SWSR. If the SWE is loaded with the value zero, the
modulus counter does not count.
0x556C / Don’t Reload
Reset
0xAA39 / Reload
State 0
Waiting for 0x556C
State 1
Waiting for 0xAA39
Not 0xAA39 / Don’t Reload
Not 0x556C / Don’t Reload
Not 0x556C / Don’t Reload
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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