At 05:32 AM 28/04/98 GMT, you wrote:
>Hello all,
>
>I have decided in my application to use half a dozen 16F84's to interface
>to a bunch of components.
>
>However, space is very limited, so I want to use the surface-mount version
>of the chip. I plan on programming them in circuit, and will provide a
>four-pin header for each chip on the board. I plan on using RB6 & RB7 only
>for programming, and so will not need to isolate them from the circuit
>(since they will only go to the pins on the header).
>
>The question is, how do I isolate Vdd? I was thinking along the lines of
>running a trace from the Vdd pin directly to the power header pin, and to
>the rest of the circuit through a diode. The diode would be mounted to
>allow current to pass into the chip from the circuit power supply, but not
>from the chip to the rest of the circuit when powered from the programmer.
>Presumably, since the voltage drop would be about 0.6 volts, the power
>running into the PIC from the 5 volt circuit supply would end up being 4.4
>volts,  which is well within the limits.
>
>Will this work? Is there a better way?
>
>Thanks in advance...
>
>Later,
>Jon
>
>--------------------------------------------------------------
>   Jon Hylands      Jon@huv.com      http://www.huv.com/jon
>
>  Project: Micro Seeker (Micro Autonomous Underwater Vehicle)
>           http://www.huv.com
>
>
I don't understand the need to isolate VDD. Is the circuit in such a way
that there be no pull ups or downs from pins being driven by the PIC,
remember that the PIC should be in a RESET type state (All pins as inputs)
during the programming cycle.

Cone to think of it, you have control of the reset line to all the PICs, if
this is the case, then you should be able to hold the PICs not bing
programmed in RESET, and place RB6 and RB7 in //.

You do indicate a 4 pin header for each chip, so from this can we assume
that the reset lines are common? If you use the above, then control and
progamming should be simple. However! A seperate header for each will speed
up development as a stand alone programmer could be used.

Hope that this helps
Dennis