> * From what I understand the VPP pin must be tied to 12VDC+
> during programming.

Yes (13V ish)


> How do I isolate the VDD pins from this high voltage?
I use a schottky diode in line with a 10K resistor between my 5V supply and
the MCLR/VPP pin.
The programmer is connected directly to VPP so the diode isolates the 13V
from affecting the 5V rail.

> Is it recommended to use a diode between the VDD pin and VPP pin?
I do.

> Or should I use a resistor?
Well both I guess. Resistor to current limit and diode to block.


> If a resistor is recommended, what value is recommended?
I use 10Kohm


> * Do I need to power the PIC with +5VDC while it is being
> programmed (VDD, VSS pins)?

Yes.
Make sure you power it from the same 5V and 0V supplies from the programmer.
I've had loads of grief only connecting the programming pins and using the
5V supply from my normal on-board voltage regulator. The ground imbalance or
something really screws it up however much I tried to couple the grounds
together.


> * When a PIC is in program mode (+12VDC on the VPP pin), what
> is the state of the I/O pins
All high impedance/inputs I think. You must make sure the RB6 and RB7
programming pins are not being affected by your circuit. Isolating them may
help but I normally work my circuit out to either not use them or have them
connected to high impedance inputs (driving a 74HCXXX device) so I don't
have to worry about it. It depends on your circuit.

I have no trouble with 16F84 programming but did have a lot of trouble with
my in-circuit programming of 16F87X's using the PICALL/PIC16PRO programmer.

Since then I've invested in one of the really cheap Microchip ICD
(In-Circuit Debuggers) which programs without fault and gives me some
debugging facilities as well. Best $99 I've spent.

Pete

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