Chapter 4. CPU OperationGFK-2222AD April 2018 1594.8.3 Watchdog TimerSoftware Watchdog TimerA software watchdog timer in the CPU is designed to detect failure to complete sweep conditions. Thetimer value for the software watchdog timer is set by using the programming software. The allowablerange for this timer is 10 ms to 2550 ms; the default value is 200 ms. The software watchdog timer alwaysstarts from zero at the beginning of each sweep.The software watchdog timer is useful in detecting abnormal operation of the application program thatprevents the CPU sweep from completing within the user-specified time. Examples of such abnormalapplication 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 loopWhen selecting a software watchdog value, always set the value higher than the longest expected sweeptime to prevent accidental expiration. For Constant Sweep mode, allowance for over sweep conditionsshould 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-EPCPE100/CPE115 Embedded Ethernet Interface.The watchdog timer continues during interrupt execution. Queuing of interrupts within a single sweepmay cause watchdog timer expiration.If the software watchdog timeout value is exceeded, the OK LED blinks, and the CPU goes to STOP-Haltmode73. Certain functions, however, are still possible. A fault is placed in the CPU fault table, and outputsgo to their default state. The CPU will only communicate with the programmer; no other communicationsor operations are possible. To recover, power must be cycled on the rack or backplane containing theCPU.To extend the current sweep beyond the software watchdog timer value, the application program mayrestart the software watchdog timer using Service Request function #8. However, the softwarewatchdog 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 GeniusBus Controller.Hardware Watchdog TimerA backup circuit provides additional protection for the CPU. If this backup circuit activates, the CPU isimmediately Reset. Outputs go to their default states, no communications of any kind are possible, andthe 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 resetresults 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 untilthe next power cycle. There is no automatic restart. If a charged battery is connected, the powercycle 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.
