The internal watchdog has two modes of operation: normal watchdog (WD) and windowed watchdog (WWD). The system integrator can select to use one mode or the other but not both at the same time. For details of programming the internal watchdogs, see the device-specific technical reference manual. The WD is a traditional single threshold watchdog. The user programs a timeout value to the watchdog and must provide a predetermined WDKEY to the watchdog before the timeout counter expires. Expiration of the timeout counter or an incorrect WDKEY triggers an error response. The WD can issue either a warm system reset or a CPU maskable interrupt upon detection of a failure. The WD is enabled after reset.

In case of WWD, user programs an upper bound and lower bound to create a time window during which the software must provide a predetermined WDKEY to the watchdog. Failure to receive the correct response within the time window or an incorrect WDKEY triggers an error response. The WWD can issue either a warm system reset or a CPU maskable interrupt upon detection of a failure. Normal WD operation is enabled by default after reset. Additional configuration need to be performed to enable the WWD operation. For details of programming the internal watchdogs, see the device-specific TRM. The use of the time window allows detection of additional clocking failure modes as compared to the WD implementation.