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On Tue, Jun 12, 2007 at 04:12:08PM +0100, Alan B. Pearce wrote:
> >Also if I wait a day, burning works fine again,
> >for about twenty or so burns.
> 
> This strikes me as a problem with LV programming bit being set, and the PGM 
> pin not being held in the correct state.
> 
> What programmer are you using? I seem to remember someone recently having a 
> problem where they found their programmer was illegally changing some bits 
> in the config word from what was specified, and doing something like this. 

The problem appears with both a PICStart+ clone and a PICKit2...

That said, sure enough I found it. Turns out the errata was applicable,
namely that switching to the internal oscillator to 8mhz too soon
actually causes the oscillator to overshoot, resulting in an out of spec
condition. This didn't seem to affect my app at all, but it did cause
programming to fail. Adding a half second delay at the very top of the
program fixed the issue. Anything I did after the oscillator switch had
no effect, for instance an infinit loop directly after the switch to
8mhz still failed, which the same infinit loop, with a five second delay
before the switch worked fine.

What's really weird is it took a good 6 months of on and off development
to even run into the problem. Yet I the code to switch to 8mhz was in
there from the very start, and indeed was copied from another 16lf819
using project.

Even weirder though was I was trying some months old firmware versions
out, that now suddenly, and reliably, have the problem! It's like
running into it once in a day causes the programmer, even after being
turned off, to trigger it even with firmware that previously worked
fine. All the time I was using the same three 16lf819's, and my "clean"
ones that I could reproduce the problem with too were all from the same
rail.

Embedded programming is downright wacky!

- -- 
http://petertodd.org
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