Paul Mathews wrote:

... The correct way to detect a 2kHz square wave. However, Paul
don't forget the quadrature waveform! It's possible that the incoming
signal and the square wave you're beating it against  (i.e. multiplying)
are 90 degrees out of phase. In which case the dot product is zero.

I've been looking at this fairly closely in the last couple of weeks
and I've come up with a VERY SIMPLE algorithm that will perform the
filtering Paul describes. It also takes care of the quadrature waveform
too. I originally was going to post the theory too, but it's too long
for the current PIC List temperament. Those of you who enjoy the
theoretical enlightenment then drop me a line.



If you have seen the theory then.....
Finally, all of the above can be combined to create the following
(UNTESTED) code. It takes 15 cycles/sample regardless of the path
that the code takes (recall the recent "Isochronous Code" thread).

TONE_FREQ       EQU     2000  ;Hz
SAMPLING_TIME   EQU     15    ;uS
RESOLUTION      EQU     2<<13 ;8192
DPH             EQU     SAMPLING_TIME*TONE_FREQ*RESOLUTION/1000000
Qbit_c          EQU     13-8  ;Bit 5
Qbit_s          EQU     12-8  ;Bit 4

start:
        MOVLW   NUM_OF_SAMPLES
        MOVWF   samples
samp_loop:
        MOVLW   DPH
        ADDWF   ph_lo,F       ;Low byte of phase
        SKPNC
         INCF   ph_hi,F

        BTFSS   INPUT_PORT,INPUT_BIT
         goto   low

    ;Input is high
        BTFSC   ref_wave,Qbit_c ;
         INCF   DFT,F           ;c=1,s=x,in=1
        nop
        goto    next

low:
    ;Input is low
        BTFSS   ref_wave,Qbit_c ;
         goto   $-2
        BTFSS   ref_wave,Qbit_s
         INCF   DFT_s,F         ;c=1,s=0,in=0
        nop
next
    ;Put a delay loop here if your crystal frequency>4Mhz

        DECFSZ  samples,F
          goto  samp_loop


    ;At this point the variable "DFT" has the sum of both
    ;quadratures. We can decide/experiment with thresholds
    ;to see  how well they correlate to actual signal.
    ;
    ;  Do  some more system stuff...

        goto     start


Scott

PS The theory behind this is almost identical to the
theory behind the DTMF decoding.