Xiaofan Chen wrote:
> I think a lot of the design has a 10nF (I've even seen 100nF)
> capacitors on the MCLR/Vpp pin for EMC reasons. I think you should put
> it there.

But why?  What purpose does it serve?  A simple 20Kohm pullup to Vdd is just
fine.

Also think about the current requirements.  The Vpp rise time is often
specified to be 1uS maximum in the programming specs.  With 100nF cap that
would require 1.3A.  It's not just a issue of programmer drive capability,
but also such large currents flowing in the target, ground offsets
especially in the programming cable, inductive coupling, etc.

And what if the programmer is actually faster?  I measured the USBProg on
this circuit and its Vpp rise time was about 12V in 200nS.  That's a 600mA
pulse.  Obviously it can do it, but do you really want a 600mA pulse flying
around?

> As ProProg is positioned as a professional level programmer, I expect
> it to have better driving capability on Vpp pin as well.

Actually it does handle 10nF OK, but I also don't see the need to push it in
this case.  The main problem in this case appears to have been a firmware
bug, and also possibly lack of filtering on PGC and PGD at the target.  I
didn't take the 47pF caps off of PGC and PGD and go back to the ICD2 cable
to see if it still works.  It might, or might *most* of the time, or not at
all.  I've seen this issue cause problems in enough cases that I think it's
important to always address it.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist