>I'm designing a small board that will
>plug into an ISA slot on a PC. ...  I want to be able to program the
>PIC while
>it's in the slot.
>Should be easy enough - the interfacing is trivial, but the problem
>is Vpp. This has to be 12-14 volts ... But ... there will
>be at least one diode drop or transistor drop (or in my design, both!)
> ... This brings the 12V below the required Vpp.

Hi Conor,

the solution is to experiment.  Firstly I assume that this design is not
for a commercial device (if it is then of course you must be sure to
adhere rigidly to the spec's, and you can ignore the rest of this
message).  It is important to note that the Vpp voltage is not actually
used to power the programming process, it is simply the presence of a
voltage at this level that sets the '84 into program mode.  The actual
programming voltage for the EEPROM is internally generated.

Bearing this is mind just what voltage will trigger the device into
program mode?  Is 11.4V enough, or is your PC's 12V line actually 12.6V?
 Try it and see.

I know this thread is now two days old, and so is ancient history as far
as the original problem is concerned.  However, if you like I will have
a go back at home, and try programming an '84 with different voltage
levels on the ^MCLR pin, and let you know how I get on.

As I said above however, if this is a commercial design you will want to
ensure that you meet the specifications exactly, and you must also be
able to vary Vdd to perform verification to Microchip's specification.

Andy (the other one)
--
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Andrew M Errington                               Tel: +44 1524 593678
Microcomputer Consultant                         Fax: +44 1524 844011
The Computer Centre                  Mobile (Orange):     0976 243931
Lancaster University                      a.errington@lancaster.ac.uk
Lancaster LA1 4YW     http://www.lancs.ac.uk/people/cpaame/cpaame.htm
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 sandwiches on Boxing Day" - Vladimir Illich Ulyanov 1920