Watchdog Timer - GE PACSystems RX7i Reference Manual

4.8.3 Watchdog Timer

Software Watchdog Timer
A software watchdog timer in the CPU is designed to detect failure to complete sweep conditions. The
timer value for the software watchdog timer is set by using the programming software. The allowable
range for this timer is 10 ms to 2550 ms; the default value is 200 ms. The software watchdog timer always
starts from zero at the beginning of each sweep.
The software watchdog timer is useful in detecting abnormal operation of the application program that
prevents the CPU sweep from completing within the user-specified time. Examples of such abnormal
application program conditions are as follows:
▪
Excessive recursive calling of a block
▪
Excessive looping (large loop count or large amounts of execution time for each iteration)
▪
Infinite execution loop
When selecting a software watchdog value, always set the value higher than the longest expected sweep
time to prevent accidental expiration. For Constant Sweep mode, allowance for over sweep conditions
should be considered when selecting the software watchdog timer value.
Refer to Section, A-3.6 for EGD Sweep Impact for RX3i CPE302/CPE305/CPE310 and RSTi-EP
CPE100/CPE115 Embedded Ethernet Interface.
The watchdog timer continues during interrupt execution. Queuing of interrupts within a single sweep
may cause watchdog timer expiration.
If the software watchdog timeout value is exceeded, the OK LED blinks, and the CPU goes to STOP-Halt
73
mode . Certain functions, however, are still possible. A fault is placed in the CPU fault table, and outputs
go to their default state. The CPU will only communicate with the programmer; no other communications
or operations are possible. To recover, power must be cycled on the rack or backplane containing the
CPU.
To extend the current sweep beyond the software watchdog timer value, the application program may
restart the software watchdog timer using Service Request function #8. However, the software
watchdog timer value may only be changed from the configuration software.
Note that Service Request Function #8 does not reset the output scan timer implemented on the Genius
Bus Controller.
Hardware Watchdog Timer
A backup circuit provides additional protection for the CPU. If this backup circuit activates, the CPU is
immediately Reset. Outputs go to their default states, no communications of any kind are possible, and
the CPU halts. The recovery procedure is documented below.
There are two basic forms of hardware watchdog:
1) for RX3i CPE302, CPE305, CPE310, CPE330, CPE400 and RSTi-EP CPE100/CPE115, a watchdog reset
results in an automatic restart into STOP-Halt mode;
2) for RX3i CPU310, CPU315, CPU320 and all RX7i CPUs, the watchdog reset holds the CPU in reset until
the next power cycle. There is no automatic restart. If a charged battery is connected, the power
cycle will result in a restart into STOP-Halt mode.
73
RSTi-EP CPE100/CPE115 may remain in STOP-HALT mode even after power cycle. To recover it from STOP-HALT mode,
power down the module after pressing and holding the membrane switch.
GFK-2222AD April 2018
Chapter 4. CPU Operation
159