More news on my puzzle: I wrote a short piece of test code to double check my WDT reset functions. Basically initialize the WDT, and followed by some delays and turning on LEDs and resetting the WDT so I would have a definite sequence recognizable as the start of the code, followed finally by an infinite loop to let the WDT timeout. It worked as it should.

Then I added into this test program all the timer and serial i/o functions of my big project. It worked too: blink, blink, blink, pause, (timeout) blink, blink, blink, pause, (timeout) etc.

Then I took a copy of my big project, and at the very beginning, right after its timer and serial initialization I added this WDT test code, with the balance of the code following (but of course not executable because of the infinite loop in the way).

When I execute this, all I get is blink, blink, blink, pause... My serial i/o functions toggle the same LEDs as incoming data is received. If I reboot my system and send data into it, I can see the data blinking their LEDs (both UART ports) up to the time that the WDT times out, then everything stops.

So it looks like the reset is happening because the serial interrupts stopped. But the startup code at 0000 does not appear to have been executed.

Curiouser and curiouser...