I finally succeeded in programming my 16LF871 in circuit. The problem
was apparently the OTHER set of Vdd/Vss pins, that were still
connected to the circuit (as I mentioned in the first post, I had
isolated only one set for programming, the others remained
connected). I still don't know why it worked OK for the 16F877 and
not the 'LF871 (anyone have a clue?), but as soon as I disconnected
those pins from the circuit, the 'LF871 programmed without any
problem. My thanks to Michael Rigby-Jones who replied directly to me
suggesting I try that.

Lesson learned: isolate both sets of power pins when programming
parts that have 2 sets.

(A side note, and resulting question: This board was originally
designed for the 16C774, with its 12-bit A/D converter. That chip has
separate "AVdd" and "AVss" pins, intended for clean analog power, and
those are the two that remained connected on the board. My question
is this: on parts with 2 sets of Vdd and Vss pins, is there any good
reason not to leave one set completely unconnected? They're connected
internally anyway, as a continuity test shows, so it seems that just
hooking power to one set is sufficient. True?)

Dave Johnson

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