I have a circuit in which I can successfully in-circuit verify
a pre-programmed PIC16C73A, but when I erase and re-insert that
same processor in the same circuit, I can't program.  I can
repeat the process, including programming the processor with the
same device that adapts to the board for in-circuit programming,
so I know the programmer and processor are both good.

I've done this by building a socket adapter to go from the Pro Mate II
to a header on my circuit board.  This same socket adapter has all
of the pins cut except for 1, 19, 20, 27, and 28.  I can coincidentally
plug a processor directly into the socket adapter and successfully
program it.  However, when the header plug that is soldered onto this
same socket adapter is plugged into my circuit, I can read or verify
a pre-programmed processor but I can't program it.

On my circuit board, pins 27 and 28 go nowhere except to the processor.
Pin 1 going to the processor has a 15k pullup, however I tried
in-circuit programming with this pullup removed and it still failed,
indicating that pin 1 could also "go nowhere except to the processor"
and the problem would not be fixed.

Meanwhile, pins 19 and 20 go to the power supply of not only the
processor, but numerous other circuit components.  Nevertheless,
an oscilloscope shows the Vdd properly varying between 5 volts for
programming, 3 volts for low-verifying, and 6 volts for high-verifying.
(This is what the Pro Mate II does, verified with a scope and a
processor successfully programmed directly in the Pro Mate II socket.)

So, have any guesses what my problem may be?

Thanks very much.
Helmut Forren
helmut@atlanta.com


built a socket adapter to go from the Pro Mate II to a header
on my circuit board.)