mcf5407 Freescale Semiconductor, Inc, mcf5407 Datasheet - Page 221

• Manufacturer Part Number: mcf5407
• Description: Mcf5407 Coldfire Integrated Microprocessor User
• Manufacturer: Freescale Semiconductor, Inc

When the watchdog timer times out and SYPCR[SWRI] is programmed for a software
reset, an internal reset is asserted and RSR[SWTR] is set.
To prevent the watchdog timer from interrupting or resetting, the SWSR must be serviced
by performing the following sequence:
Both writes must occur in order before the timeout, but any number of instructions or
SWSR accesses can be executed between the two writes. This order allows interrupts and
exceptions to occur, if necessary, between the two writes.
Caution should be exercised when changing SYPCR values after the software watchdog
timer has been enabled with the setting of SYPCR[SWE], because it is difficult to
determine the state of the watchdog timer while it is running. The countdown value is
constantly compared with the timeout period specified by SYPCR[SWP,SWT]. Therefore,
altering SWP and SWT improperly causes unpredictable processor behavior. The following
steps must be taken to change SWP or SWT:
SYPCR[SWTAVAL]
Figure 6-4. MCF5407 Embedded System Recovery from Unterminated Access
1. Write 0x55 to SWSR.
2. Write 0xAA to the SWSR.
watchdog
timer IRQ
watchdog
Software
Software
timer TA
Code enables software watchdog timer interrupt and
SWTA functionality by writing SYPCR.
1
1
SWTAVAL is set if watchdog timer TA is asserted.
1. Watchdog timer times out due to unterminated bus
2. Watchdog timer interrupt cannot be serviced due to hung bus
Problem:
cycle. Wait for another timeout before setting SYPCR[SWTA].
Timeout
Chapter 6. SIM Overview
NOTE: The watchdog timer IRQ should
be set to the highest level in the system.
Timeout
3. TA held until another
bus cycle starts
Code in the watchdog timer interrupt
handler polls SYPCR[SWTAVAL] to
determine if SWT TA was needed. If so,
execute code to identify bad address.
Watchdog timer
IACK cycle
Programming Model
6-7