Motorola ColdFire MCF5281 - Manual (Page 158)

System Control Module (SCM)
8-6 Freescale Semiconductor
When the core watchdog timer times out and CWCR[CWRI] is programmed for a software reset, an
internal reset is asserted and CRSR[CWDR] is set. To prevent the core watchdog timer from interrupting
or resetting, the CWSR must be serviced by performing the following sequence:
1. Write 0x55 to CWSR.
2. Write 0xAA to the CWSR.
Both writes must occur in order before the time-out, but any number of instructions 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 CWCR values after the software watchdog timer has
been enabled with the setting of CWCR[CWE], because it is difficult to determine the state of the core
watchdog timer while it is running. The countdown value is constantly compared with the time-out period
specified by CWCR[CWT]. The following steps must be taken to change CWT:
1. Disable the core watchdog timer by clearing CWCR[CWE].
2. Reset the counter by writing 0x55 and then 0xAA to CWSR.
3. Update CWCR[CWT].
4. Re-enable the core watchdog timer by setting CWCR[CWE]. This step can be performed in step
3.
The CWCR controls the software watchdog timer, time-out periods, and software watchdog timer transfer
acknowledge. The register can be read at any time, but can be written only if the CWT is not pending. At
system reset, the software watchdog timer is disabled.
7 6 5 3 2 1 0
Field CWE CWRI CWT[2:0] CWTA CWTAVAL CWTIC
Reset 0000_0000
R/W R/W
Address IPSBAR + 0x011
Figure 8-4. Core Watchdog Control Register (CWCR)
Table 8-5. CWCR Field Description
Bits Name Description
7 CWE Core watchdog enable.
0 SWT disabled.
1 SWT enabled.
6 CWRI Core watchdog reset/interrupt select.
0 If a time-out occurs, the CWT generates an interrupt to the processor core. The interrupt level for
the CWT is programmed in the interrupt control register 8 (ICR8) of INTC0.
1 Reserved; do not use.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3