Jameco Electronics Rabbit 3000 - Manual (Page 103)

94 Rabbit 3000 Microprocessor
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 7-13. Watchdog Timer Test Register (WDTTR adr = 0x09)
Bit(s) Value Description
7:0
0x51
Clock the least significant byte of the watchdog timer from the peripheral
clock. (Intended for chip test and code 0x54 below only.)
0x52
Clock the most significant byte of the watchdog timer from the peripheral
clock. (Intended for chip test and code 0x54 below only.)
0x53
Clock both bytes of the watchdog timer, in parallel, from the peripheral
clock. (Intended for chip test and code 0x54 below only.)
0x54
Disable the watchdog timer. This value, by itself, does not disable the
watchdog 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
watchdog timer. The watchdog timer will be re-enabled by any other write
to this register.
other
Normal clocking (32 kHz oscillator) for the watchdog timer. This is the
condition after reset.