A watchdog-controlled power cycle

Using an external watchdog circuit to cycle the power to all your on-board circuity is a rather good idea. It helps to cover cases where some stray external event has caused an upset on some silicon chip circuit. Sometimes these events, whether they be high voltage spikes, reverse bias on signal inputs or even alpha particles can cause localized latchup on-board the chip and the only realistic way to clear such fault is to cycle the power.

Using the scheme to connect the WDT to the enable of the voltage regulator does require some due diligence on your part.

a) Make sure the WDT circuit is not powered by the same voltage rails as the disabled regulator.

b) Make sure that the length of time that the regulator is disabled is long enough. Bypass and filter capacitors on the output of the regulator need to discharge to the greatest degree possible in order to qualify as a power cycle to the silicon. In many instances a millisecond or so of disable time will only allow a partial discharge of the output rails.

Ok, I now have this working properly. And here's how:

1. Read the manual.
2. Throw the manual away.
3. Write a working program in a different IDE / compiler.
4. Disassemble the resultant program and see how it should be done.

XC32 builds in code to handle the NMI into its reset handler for you, and the code it generates bears no resemblance to what the manual says you should do. Instead it examines the NMI status of C0. If it was an NMI then do an eret, otherwise start the program normally.

Here's the relevant bits of the program disassembled:

bfc00000:   401a6000    mfc0    k0,c0_status
bfc00004:   7f5a04c0    ext k0,k0,0x13,0x1
bfc00008:   13400005    beqz    k0,bfc00020 <_no_nmi>
bfc0000c:   00000000    nop
bfc00010:   3c1a9d00    lui k0,0x9d00
bfc00014:   275a02a8    addiu   k0,k0,680
bfc00018:   03400008    jr  k0 <_nmi_handler>
bfc0001c:   00000000    nop

9d0002a8 <_nmi_handler>:
9d0002a8:   401a6000    mfc0    k0,c0_status
9d0002ac:   3c1bffbf    lui k1,0xffbf
9d0002b0:   377bffff    ori k1,k1,0xffff
9d0002b4:   035bd024    and k0,k0,k1
9d0002b8:   409a6000    mtc0    k0,c0_status
9d0002bc:   42000018    eret

So you see it doesn't care why the NMI happened - it just erets regardless.

With that style of doing it in my startup routine it now works!