In message  <01IAG1GW2WLC8WWUQX@ccvax.ucd.ie> PICLIST@MITVMA.MIT.EDU writes:
> Hi guys [ok,ok and gals, and plas :-) ],
>
> Just a quick question - I'm designing a small board that will
> plug into an ISA slot on a PC. It's a simple enough project except
> for one minor detail - 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, right? But when you use the
> 12V available from the ISA bus you get a problem - there will
> be at least one diode drop or transistor drop (or in my design, both!)
> in the circuit in order to switch the Vpp in and at the same time
> not blow your 5V line. This brings the 12V below the required Vpp.
> It's a pain to use an external power supply and you have to be a
> lot more careful about gating...
>
Conor,


From experience, the PCs 12V rail is not very useful. The PC to PC
variation is too high.

I a previous job designing PCMCIA FLASH card readers for we derived
Vpp from the Vcc using a switcher IC. Both Maxim and LT both do
chips specific to programming flash memory which would work well
in your application.


Regards,



Mike Watson