The timed access procedure looks relatively straightforward. From page 34 of the user's guide:

1. Write 0xAA to TA (SFR 0xC7)
2. Write 0x55 to TA within 3 cycles
3. Modify timed-access protected register within 3 cycles. You want to clear bit 1 of WDCON. (SFR 0xD8).

If you feel compelled to used C code to do this, make sure you look at the disassembly to guarantee those three things happen within 3 instructions of one another.

Note that this procedure does not affect the content of the option control register, meaning if the watchdog timer was enabled, it will be again enabled at boot. You can do an in-application programming procedure if you want to manipulate the OCR in your software, however. Details for this are in the data sheet, rather than the manual that I've been citing all day.

Now, all discussion of timed access and IAP, have you done Erik's test to see if the watchdog is enabled?


Wow. That's not nice. Early revisions of all these parts, '420 included, seem to have their share of issues.


Have you looked at the errata sheet for the latest revision level of the '420? It's rather long. Perhaps what you really want is an A4 revision of the '450.

--Sasha Jevtic