Rabbit 2000, User Manual

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User’s Manual 81
The code to do this may also hit the watchdog with a 0.25-second period to speed up the
reset. Such watchdog code must be written so that it is highly unlikely that a crash will
incorporate the code and continue to hit the watchdog in an endless loop. The following
suggestions will help.
1. Place a jump to self before the entry point of the watchdog hitting routines. This pre-
vents entry other than by a direct call or jump to the routine.
2. Before calling the routine, set a data byte to a special value and then check it in the rou-
tine to make sure the call came from the right caller. If not, go into an endless loop with
interrupts disabled.
3. Maintain data corruption flags and/or checksums. If these go wrong, go into an endless
loop with interrupts off.
Table 7-9. Watchdog Timer Test Register (WDTTR adr = 0x09)
Bit(s) Value Description
7:0 0x51
Clock the least significant byte of the WDT timer from the peripheral
clock. (Intended for chip test and code 0x54 below only.)
0x52
Clock the most significant byte of the WDT timer from the peripheral
clock. (Intended for chip test and code 0x54 below only.)
0x53
Clock both bytes of the WDT timer, in parallel, from the peripheral clock.
(Intended for chip test and code 0x54 below only.)
0x54
Disable the WDT timer. This value, by itself, does not disable the WDT
timer. Only a sequence of two writes, where the first write is 0x51, 0x52 or
0x53, followed by a write of 0x54, actually disables the WDT timer. The
WDT timer will be re-enabled by any other write to this register.
other
Normal clocking (32 kHz oscillator) for the WDT timer. This is the
condition after reset.