nRF52840 Watchdog

[entr0p1]

Implement the nRF52840's built-in watchdog to reset the board if the firmware locks up for more than 30 seconds.

[entr0p1]

Happy to report this has worked perfectly. No erroneous resets overnight, and I was able to force the board to lock up by adding the below into the CLI:

I then called set wdt.test from the CLI, which caused the board to lock up completely:

I confirmed the board locked up completely, unable to log in over the mesh and the physical user button and UI stopped working:

After the watchdog timed out, the board reset as expected and booted up normally. Happy to mark this ready for review once I've added in the supported boards.

[4np]

What will happen with the tick() calls when the CPU is sleeping after #1353 is merged? Or does the NRF watchdog account for sleeping internally?

[fschrempf]

What will happen with the tick() calls when the CPU is sleeping after #1353 is merged? Or does the NRF watchdog account for sleeping internally?

I haven't checked, but I guess we will need to feed the watchdog in the wait for event loop.

[entr0p1]

What will happen with the tick() calls when the CPU is sleeping after #1353 is merged? Or does the NRF watchdog account for sleeping internally?

It does - the watchdog gets paused during sleep and halt states:

MeshCore/src/helpers/nrf52/Watchdog.cpp

Lines 22 to 24 in 2fc3e78

	// Configure WDT to pause during sleep and halt modes
	NRF_WDT->CONFIG = (WDT_CONFIG_SLEEP_Pause << WDT_CONFIG_SLEEP_Pos) |
	(WDT_CONFIG_HALT_Pause << WDT_CONFIG_HALT_Pos);

[entr0p1]

I've refactored this code and made it a lot tidier, plus rebased on dev (which I'll need to do again - meshcore moves fast!).

There's a compile flag to include WDT in the build, and a preference (disabled by default) with CLI command to enable WDT. The runtime code only exists in the repeater firmware but is simple to expand to others.