I'm trying to program a PIC16LF871 in circuit and can't seem to get
it to work no matter what I try. It always fails, sometimes on
verify, sometimes it says the config bits weren't programmed, etc.
Trying to program just the config bits (or just part of memory) it
always tells me that the chip is code protected.

I have programmed 16F877's in the same circuit without trouble
(though that was a year ago - this project is recently unshelved).
Does anyone have a clue why one should give me trouble and the other
wouldn't? Any suggestions on what I might try?

Here's a bunch more detail about what's going on and what I've tried:

Setup
-----

I'm using a PicStart+ with a short homemade cable, MPLAB running
under Win98 under VirtualPC on a Mac. On the board, I have the usual
5 lines (MCLR, Vdd, Vss, RB6, RB7) completely isolated from the rest
of the circuit (shorting jumpers hook them up again when I want to
run). This scheme has worked for me with no trouble before, on a
couple different boards, with 16F84, 16F877, 16C774.

Things I've tried:
------------------

When I first had trouble and went poking around, I found Microchip's
note about RB3 and LVP (and then read much of the voluminous
discussion about it in the PICList archives :-). I assumed that was
my problem, so I temporarily tied RB3 to Vss on the board though 10K,
just for programming. It didn't seem to change anything.

I was distressed to discover that the MPLAB/PicStart+ combo doesn't
let you know if there's no PIC in the programmer (?!?!). And if there
isn't, then the symptoms are just like what I'm seeing. This of
course made me think that I just had a connection problem, but I
double checked my connections all the way from the programmer to the
PIC pins, and everything seems fine.

I upgraded the PicStart+ firmware from 2.01 to 2.30, just in case.
(getting desperate now...almost ready to try burning candles and
chanting...)

What now?
---------

I'm honestly not sure what to try next, which is why I'm writing here.

One thought I had: I'm only connecting one set of Vdd/Vss pins to the
programmer, the others are still connected to the circuit. Maybe I
should isolate the others, too? Didn't seem to matter on the 16F877,
but I'm getting desperate.

The boss is willing to spring for a Promate II and ICSP socket, if
that will make a difference, but I'm not sure why a little extra
drive capability would help.

Anybody have any other ideas that I should try? Any thoughts on this
are MUCH appreciated.

Dave Johnson

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