> > Does anyone know how I might go about making a PIC into a frequency shifted
keying modem?  I plan
> > to use it to make some form of pager but I don't really know how to start..
>
> FSK generation won't be a problem, but the detection will be
> a pain in the butt on a PIC.
>
> There is a application note on Scenix's web site for a complete
> 1200 baud FSK modem. You can get the info from :
> ftp://ftp.scenix.com/apps/fsk.src

Well I downloaded the file and it contains TX code only.

But I am sure it is no problem to implement FSK receive on a PIC, too.  After al
l it's just 1200
bps. HAMCOM type modems contain a 1-bit A->D and 1 bit math does not require MUL
 or DIV
instructions. Although HAMCOM type receivers are not perfect regarding their S/N
 requirements
they are used a lot in amateur radio and people can live with it.

If you want to see it proven with code (PIC/SX/AVR), hire me :-)




Regarding pagers: if they are POCSAG you don't need to demodulate the FSK yourse
lf, as the data
directly modulates the UHF carrier. Any FM receiver does the demodulation, so yo
u only
need a comparator (or preferably a treshold device with hysteresis) and a DPLL t
o recover
the bit clock.