Panasonic MN101C77C, F77G user manual Watchdog timer cannot stop, once it starts operation

Chapter 9 Watchdog Timer

9-3 Operation

9-3-1 Operation

The watchdog timer counts system clock (fs) as a clock source. If the watchdog timer is overflowes, the watchdog interrupt (WDIRQ) is generated as an non maskable interrupt (NMI). At reset, the watchdog timer is stopped, but once the operation is enabled, it cannot be stopped except at reset. The watchdog timer control register (WDCTR) sets when the watchdog timer is released or how long the time-out period should be.

This watchdog timer can detect such that the watchdog timer clear is repeated in short cycle. If the watchdog timer clear is repeated in shorter cycle than the set time (the lowest value of watchdog timer clear possible), it is regarded as an error and the watchdog interrupt (WDIRQ) is generated.

If the watchdog interrupt (WDIRQ) is generated twice consecutively, it is regarded to be an indication that the software cannot execute in the intended sequence; thus, a system reset is initiated by the hardware.

The watchdog timer cannot stop, once it starts operation.

„Usage of Watchdog Timer

When the watchdog timer is used, constant clear in program is needed to prevent an overflow of the watchdog timer. As a result of the software failure, the software cannot execute in the intended se- quence, thus the watchdog timer overflows and error is detected.

Programming of the watchdog timer is generally done in the last step of its programming.

„How to Detect Incorrect Code Execution

The watchdog timer is executed to be cleared in the certain cycle on the correct code execution. On this LSI, the watchdog timer detects errors when,

(1)the watchdog timer overflows.

(2)the watchdog timer clear happens in the shorter cycle than the watchdog timer clear possible lowest value, set in the watchdog timer control register (WDCTR).

When the watchdog timer detects any error, the watchdog interrupt (WDIRQ) is generated as a non maskable interrupt (NMI).

IX - 4 Operation