>> For such a broad question you can only get a likewise broad answer: "it
>> depends, RTM".
>>     
>
> '  I'd interpret the broad question as "having read a cross section of
> manuals, I find that there are a wide range of situations that apply.
> It may be that there are best practices, frowned on procedures, most
> common means of implementation and more which I in my necessarily
> incomplete knowledge may have failed to pick up on
>
> So, can anyone skilled in the art and experienced in the ways of the
> beasts offer some generic advice through what is obviously a bit of a
> minefield? If you know no more on the subject than I do you may wish
> to wait with me for pearls of enlightenment from the masters. '
>   
The Vdd pin on the programing port needs to be connected to the pics Vdd 
pin so the IO buffers in the programmer run off the same voltage as the pic.

Depending on the particular programmer and the configuration bits of the 
pic (pics with MCLR disabled often require a programmer to do Vpp before 
Vdd to get them in program mode.) it may be able to or even require to 
power the target circuit during programming. Depening on the details of 
the target circuit and the programmer this may mean the pics Vdd needs 
to be isolated from the rest of the circuit due to either voltage 
incompatibilites. Most pics designed to run on 5V but able to run on 
lower voltages (e.g. 18LF series) need damn near 5V to do a bulk erase. 
Some programmers can automatically avoid bulk erase if the voltage is 
too low, others can't and need the chips to be running on a full 5V to 
work with them. Bulk erase is also needed to remove code protection and 
similar bits.

Some really crappy programmer designs (mostly those that abuse serial 
ports) will even put the target ground at a voltage other than mains 
ground. Some target circuits may also keep thier circuit ground at a 
voltage that is neither mains ground nor programming. Both of these 
situations are potential obstacles to ICSP.

P.S. be very careful when using a programmer originaly designed for 5V 
pics on 3.3V pics. The 12v on Vpp used for programing mode entry on the 
5V pics is harmful to 3.3v pics.
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