Marc wrote:

> Hey, you have a PIC already in your circuit.  Use a PIC port pin
> instead of a 555!

  Beware the 555 I say!  The "bipolar" version doesn't pull at all well
to the positive supply, while the CMOS version has poor current sourcing
capability - while it can sink 100mA, it will only source 10mA.
http://www-s.ti.com/sc/psheets/slfs043c/slfs043c.pdf  This suggests it
is not as symmetrical as might be thought.

  I therefore agree - you are likely to get better conversion
performance from a PIC pin.

  If you must add something, there's a lot to be said for using the
purpose-designed 7660 (or 7662 for over 6V).
--
  Cheers,
        Paul B.