Jesse Lackey wrote:
> So a means of switching the PIC's Vdd (and only the PIC, not everything
> else on the 3.3V bus) to 5V must be part of the design?

Your circuit doesn't have to create the 5V, just be tolerant of when the
programmer drives Vdd to 5V.

> So this means
> some pcb space must be used for any design that could ever need to be
> erased and re-programmed?  At least easily cuttable traces.  Or .1"
> header with a jumper to 5V or 3.3V.  This seems completely ridiculous.

Nothing is free, including ICSP, but you're making it sound harder than it
is.  It's part of what your circuit does, and needs to be considered as part
of the design.  This of course includes reading the specifications to
understand what voltages are required.  Often this can be addressed with
just a diode.

> I'm just real real suprised that in 2.5 years of PIC work I've never
> seen this mentioned in any microchip literature or any ICSP-howto
> design.

The required voltage levels are clearly spelled out in the programming
specs, which of course is something that should be consulted when designing
circuits for ICSP.  I also mention this in my ICSP writeup at
http://www.embedinc.com/picprg/icsp.htm.

> I'm thinking back to other < 5V designs, there have been a couple, and
> this has never come up.  But I was using a F452 and a F1320, not the
> F2320.  Maybe the other ones can erased @ 3.3V;

I don't think so.

> maybe sometimes it works
> @ 3.3V and sometimes it doesn't and I've been lucky until now.

That, or the programmer forced Vdd to 5V and you were lucky that the rest of
the circuit tolerated it.


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