Adolfo Cobo Garcia wrote:

>  I need to program a 16c84 part soldered to the PCB, and I've built two
> homemade programmers: a "ludipipo" for the serial port and a "topic" for
> the parallel port.
>  None of them are able to program the pic in the circuit, but they work
> fine with a "off-circuit" PIC.

What circuitry is connected to RB6 and RB7?  As explained in the
hardware.txt file that came with the TOPIC info this programmer can
only drive a load of around 20-30kohms when the PIC is in-circuit
(based on the assumption that the parallel port uses open-collector
drivers).  Also how do you ensure /MCLR is raised to VPP?  With both
LUDIPIPO and TOPIC you will have problems if you don't isolate the
normal connection to VDD using a diode (the TOPIC schematic shows
the sort of thing).  You asked whether it is really necessary to stop the
oscillator while programming in-circuit.  Using the TOPIC software
I've not found this to be necessary using either an RC (4k7, 22pF) or
an XT (4MHz) oscillator (which are the only configurations I've
actually tested).  My guess is your problems are due to lack of drive
to RB6/RB7, poor control of /MCLR or using a excessively long cable to
connect the programmer to your board.  Have you looked at the programmer
described in Microchip Application note AN589 (or one of the clones)
as this has active drive of RB6/RB7?

David
--
http://www.man.ac.uk/~mbhstdj