martarse

why don't you just use a single chip FSK modem?

I doubt you'll get an "acceptable" sine at a precise frequency and timing out of a 16F84,

The part count and effort for filtering will be much more than with a FSK modem.

(I have tried to build a switching StepUp / StepDown DC-DC converter based on a PIC and had to realise soon that it doesn't make sense. There are single chip solutions available that do a better job at a lower price)

regards