Preventing inadvertent feeding of an external watchdog
One way to improve the robustness and reliability of the Watchdog triggering process is to leave the triggering GPIO pin in a HiZ state (configured as an input) by default. When you wish to "pet the dog", briefly configure the pin as an output, drive the pulse, then return the pin to the HiZ state. 

Think about this in terms of probabilities. While the probability of runaway code attempting to pulse the GPIO driver is pretty low, the probability of runaway code first configuring the trigger as an output and then pulsing it is vanishingly small.

Don't forget to use a pullup or pulldown to hold the trigger pin at the benign/inactive trigger voltge level when the GPIO is HiZ!