Stephen D. Barnes wrote:
>> Just to pipe up, I don't know if I was the one you were talking to (I
>> did email you a couple things), but I did get my Warp13 working with
>> the 2011. Worked quite well too, once I added the Olin "cap and
>> resistor" thing.
>>
>> Of course, shortly after I got it working I received an ICD2 (which
>> also required the "cap and resistor" thing). I'm not sure I'd
>> consider having an ICD a REQUIREMENT of using the dsPIC, but it sure
>> does make life easier.
>
> I have done some searching and cannot find out what this "cap and
> resistor" thing is.
> anyone care to elaborate or post a link?

Programming is done with two digital lines called PGC (clock) and PGD
(data).  These lines are right next to each other in the standard Microchip
programming cable with RJ-12 plugs on each end.

The problem comes from edges on one of these lines coupling onto the other.
Since the programmer always drives the PGC line, this can be handled in the
programmer by a little filtering.  However, during readback the target chip
drives the PGD line.  Sharp eges on that can and do couple onto the PGC
line, especially when a dsPIC is driving the line, probably because these
have faster output drivers.  The noise coupled from PGD to PGC looks like
extra clocks at the target chip, which cause the programmer and target to
get out of sync and communication to break down.  Unfortunately, the whole
effect can happen within the propagation time from the target chip to the
programmer and back (I estimate about 7nS), so there is absolutely nothing
that can be done at the programmer end to fix this.

My solution is to add a 22pF cap to ground on each of the PGC and PGD lines
on the target board.  I also add a 100ohm resistor in the PGD line between
the target chip and the cap.  The resistor and cap on the PGD line form a
low pass filter that removes attenuates high frequencies (from the sudden
edges) going onto the PGD line when driven by the target.  The cap on the
PGC line makes it less susceptible to coupled noise.

In practise, just a 22pF cap on PGD right at the target PIC fixes the
problem most of the time.  I've never seen it fail with a 22pF cap on both
lines, but I add the resistor to target boards just to make sure.  I've been
doing this for over a year now, and all such target boards can be reliably
programmed and debugged using the ICSP interface.


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