MPC8280 datasheet(178/1386 Pages) FREESCALE

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System Interface Unit (SIU)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

4-6

Freescale Semiconductor

4.1.5

Software Watchdog Timer

The SIU provides the software watchdog timer option to prevent system lock in case the software becomes

trapped in loops with no controlled exit. Watchdog timer operations are configured in the SYPCR,

described in Section 4.3.2.8, “System Protection Control Register (SYPCR).”

The software watchdog timer is enabled after reset to cause a hard reset if it times out. If the software

watchdog timer is not needed, the user must clear SYPCR[SWE] to disable it. If used, the software

watchdog timer requires a special service sequence to be executed periodically. Without this periodic

servicing, the software watchdog timer times out and issues a reset or a nonmaskable interrupt,

programmed in SYPCR[SWRI]. Once software writes SWRI, the state of SWE cannot be changed.

The software watchdog timer service sequence consists of the following two steps:

1. Write 0x556C to the software service register (SWSR)

2. Write 0xAA39 to SWSR

The service sequence clears the watchdog timer and the timing process begins again. If a value other than

0x556C or 0xAA39 is written to the SWSR, the entire sequence must start over. Although the writes must

occur in the correct order before a time-out, any number of instructions can be executed between the

writes. This allows interrupts and exceptions to occur between the two writes when necessary. Figure 4-6

shows a state diagram for the watchdog timer.

Figure 4-6. Software Watchdog Timer Service State Diagram

Although most software disciplines permit or even encourage the watchdog concept, some systems require

a selection of time-out periods. For this reason, the software watchdog timer must provide a selectable

range for the time-out period. Figure 4-7 shows how to handle this need.

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

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