> What do you mean by a "voltage doubler"?  Is this something
> that would work off of the 3.3 v supply and able to produce
> 25 mA?

Yes. 25mA isn't a big ask. You'd use 1 or 2 pins (eg the PWM
module would be a convenient AC source that wouldn't affect
code execution) directly or with a small FET (BS170 ?) switch
and a simple diode-capacitor doubler. Varying the duty cycle
will vary the voltage out of the "doubler", bearing in mind diode
losses. If Schottkys were used throughout, including one to
isolate the 3V3 source, I'm sure this is a workable solution. You
say the unit is battery-powered, so presumably this 3V3 is possibly
higher (3V6 or more ?) and a series isolation diode will be of little
consequence

David Van Horn wrote :

> The idea of mucking about with VCC while running, is rather scary

It might take a little care, but hardly boogieman category is it ? After
all, the PIC is within Vcc limits, there's no reset, etc. You could even
ramp up/ramp down the voltage, but I don't see that being necessary.
With a suitable low-leakage (essential for battery apps) reservoir cap,
perhaps a small series resistor as well to limit charge current, it seems
to me that Vcc could be varied quite painlessly

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist