Hardware Manual for the PCD3 Series│Document 26/789; Version E 5│31.01.2005
Saia-Burgess
Controls Ltd.
Software watchdog
PCD3.Mxxx0 Classic CPUs and expansion housings
3-29
3
3.16
Software watchdog
The hardware watchdog provides maximum security. However, for non-critical
applications, a software watchdog may be sufficient, whereby the processor monitors
itself and the CPU is restarted in the event of a malfunction or a loop.
The core of the software watchdog is the instruction SYSWR K 1000. When this is
first issued, the software watchdog function is activated. This instruction must then be
issued at least every 200 ms, or the watchdog will trigger and restart the controller.
Instruction:
SYSWR
K 1000
; Software watchdog instruction
R/K x
; Parameters as per table below:
; K constant or value in
; register
x = 0
The software watchdog is deactivated
x = 1
The software watchdog is activated; if the instruction is not
repeated within 200 ms, there will be a cold start
x = 2
The software watchdog is activated; it the command is not
repeated within 200 ms, XOB 0 will be called, followed by a
cold start.
XOB 0 calls are entered in the PCD history as follows:
“XOB 0 WDOG START”
where XOB 0 has been invoked
by the software watchdog
“XOB 0 START EXEC”
where XOB 0 has been invoked
because of a supply fault
