Wouter van Ooijen wrote:
> IIRC worts-case USB voltage can be too low to power the
> pic-under-programming directly.

Correct, and is also something Micochip seems to have forgotten when they
designed the PICKit2.  In fact I was at a PICKit2 class at masters this
summer and the Microchip guy teaching it was actually saying that USB power
was "5 volts", which is above the 4.5V required for bulk erase.  That's just
plain wrong.  The PICKit2 may be OK for hobbyists, but it's a lot more quick
and dirty than people seem to realize.  The problem is that it does work
most of the time, so it's hard to convince people there is anything wrong
with it.

The power issues is one reason my USBProg has more parts than the PICKit2.
It actually has three switching power supplies run from two 10F204s.

One boosts the USB power up to about 8V.  This is linearly regulated to 5V
to run the control PIC.  It is also used as the source for the programmable
Vdd supply, which is linearly regulated based on a analog voltage filtered
from a PWM output of the control processor.

The second produces about 17V that the programmable Vpp is linearly
regulated from.  The same processor that controls the 17V supply also
wiggles a pin regularly to run a charge pump to make a small low current
negative voltage used as the negative supply of the opamps.  There is no
such thing as a true rail to rail opamp.  With a little negative voltage you
don't have to worry about that, and the opamp still has a little drive left
at 0V out.


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