External Watchdog (Integrator Board)
The on-board external 1.6 second watchdog is a single chip part provided by ST STWD100PYW83F, the reset output of this part is connected to the Raspberry Pi Compute module. This is provided to give an extra layer of resilience over a system lockup in the event that the user considers the Raspberry Pi on-chip internal watchdog is unsuitable for their application.
The external watchdog device is driven by GPIO29 (/WD Enable) and GPIO28 (/WD Input), by default the watchdog is disabled. These GPIO lines are also shared with camera input CAM0 (secondary camera input) so cannot be used if 'Camera 0' is enabled
In order to enable the watchdog the below solder links need to be bridged -- this is a factory setting so please request with your order if you require these links to be pre-made for production quantities.
The circuit operates as below
Once the watchdog is enabled, by driving GPIO29 low, the watchdog input pin WDI (GPIO28) on the device must be togged H-L-H at least once per watchdog time-out period (1.6 seconds) and the low level pulse period must be >1uS long for the transition to be valid.
If the device sees a valid low-to-high transition on the input pin the internal 1.6 second countdown timer is reset and restarted. If the device does not see a valid input pulse within the watchdog time out period it will pull the RPi CPU module reset line low, which will also cause GPIO29 (/WD Enable) to be pulled high (as the Raspberry Pi CPU resets) and so disable the watchdog allowing the system to boot.
With this in mind a full hard reset of the Raspberry Pi Compute Module can be obtained by setting watchdog enable line low and then not toggling the watchdog input line..
The Reset lines for all other devices (including mPCIe) are available via separate, independent GPIO lines.
When the system hard resets in this manner all GPIO lines will revert back to their default state
Full datasheet for watchdog part: Download Link