FSK demodulator in firmware

[HenrikOlsson]

Thanks Richard, appreciate the effort but (always a but)....

Problem with the ZCD and my signal is that the bus idles at 35VDC for "long" periods of time and swings +0/-6V from there, not +/-3V around 35VDC.

If I'm not reading the sim setup wrong (which I might) you have a +/-3V signal sitting on a DC-bias of 32V when, in reality, it is a +0/-6V signal sitting at that 32VDC bias (or 35 but it doesn't matter).

Try keeping that 32VDC bias but have the signal only swing down from there, and then make bursts, 25 cycles followed by 5 seconds of silence. What happens then?

Also, and I'm not sure this matters but I think so, it doesn't look like Protues is simulating the true behaviour of the ZCD-pin. If it did you wouldn't be able to measure a signal there (which looks to be Ch. C on the scope) because as long as you have the correct (enough) series impedance the ZCD-peripheral will drive the input pin to a voltage of zero. Weather the signal is above or below GND is determined by if the internal current source or current sink is active.

/Henrik.

[richard]

i'm guessing the signal is impressed on to the power line in a "balanced" phantom cailho style. i don't know how to simulate that or how to extract it properly either in any simple way with proteus .

it doesn't look like Protues is simulating the true behaviour of the ZCD-pin

entirely possible, i never fully trust that it is fully realistic. it does however look like the zcd would work with an unbalanced ac signal provided the offset and magnitude are adjusted suitably.
zcd is NOT particularly useful for your project since it cannot drive a timer gate directly or be a capture source

[HenrikOlsson]

zcd is NOT particularly useful for your project since it cannot drive a timer gate directly or be a capture source

Oh, but it can. I guess it might depend on the specific device but on the 18F57Q43 I happened to use it certainly could.

[richard]

more fully.
zcd is NOT particularly useful for this project since it cannot drive a timer gate directly or be a capture source for any of the pic chips that my crusty old version of proteus can simulate

[NickMu]

Hi Henrik,
If having access to the clean modulating sine wave signal is your goal I believe that an old school series circuit made of one capacitor and the primary winding of an high impedance, audio insulation transformer should do the trick.
The capacitor will eliminate the DC component and transformer will pass only the sine wave. At the secondary winding of the transformer you should have, at scale, a copy of your desired signal to process and monitor as you wish.
Some minor phase shift might be expected but everything should be affected equally.

Regards,
Nick

[amgen]

could work good, input coupling cap, output would swing about 0, then ZCD could work...