Hi Dave!

It seems to me that the newer PICs (the ones I tested: PIC16F628,
PIC16F876) are much more sensitive to noise/glitches on the programming
lines than the old ones (PIC16C84, PIC16F84).

Right now I am working on my own little programmer and programming
software to adapt them for the PIC16F628 / PIC16F876.
The first big problem was that that the PIC16F628 read out false and
unstable data while the PIC16F84 read out and programmed just fine.
Solution for this was to take a shorter printer cable.
After that, the PIC16F876, accessed via ICSP had similiar problems -
symptoms are always as if the PICs programming interface gets out of
sync to the clock. On my osci I was able to see spikes in the
programming clock line (crosstalk between clock and data). Adding to
47pF capacitors to the clock and data line solved this problem.

This was never necessary with the PIC16F84 / PIC16C84.
The only explanation for me is that the PIC16F84 has a more robust (but
perhaps a bit slower) programming interface than the new PICs. The "old"
PICs ignored these spikes and glitches while the "new" don't.

As far as I know the PIC16F84A has the "new" programming interface
because it has a readable device ID word (at 2006h) just like the other
"new" PICs I tried to program.

Your solution by adding capacity to the mclr line fits perfect into
this image.

Florian

Dave Alger wrote:
>
> Hi all,
>
> Wonder if anyone can help.
>
> I designed a product using the PIC 16F84... we use a surface mount part and
> program it on the board. I had to introduce a jumper to allow me to isolate
> MCLR from the POR circuitry during programming and it worked fine.
>
> Recently we've had to switch to the F84A... for some reason that won't
> program on the board. I tried programming it on a bare board (just the PIC
> and connector pins) and it worked, but with other components it seemed to be
> useless.
>
> Strangely a guy here has found that by adding capacitance to the MCLR line
> during programming it can be made to work... but we're not sure why.
>
> Anyone got any ideas at all?
>
> Thanks in advance,
>
> Dave
>

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