Measuring audio phase shift through a circuit with a PIC

[HankMcSpank]

Need a sanity check here.

I'd like to measure the phase shift through an audio circuit at different test frequencies, therefore to measure this with a PIC.

In principle it's fairly simple - ie time a an audio signal for one complete cycle, store the result, immediately start timing how long until the output signal gets to the same point (ie use a comparator for both signals). Do some maths with the resulting times...this will yield the phase shift in degrees.

I've setup something along these lines.

(the input to my audio circuit is fed into the PIC comparator1, the output of my audio circuit feeds comparator2. Also timer1 is mapped to comparator 1, an timer2 is mapped to comparator 2 from a 'clock counting' perspective)

++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++
enable comparator1 interrupts

when comparator1 interrupts, start timer1 running

when comparator1 interrupts again, do the following....

stop timer1 running & store the 'count' (this gives a clock pulse 'count' for the arriving frequency, but since I'm only after phase shift...which is a time based measurement, I don't calculate the frequency)

clear timer2
start timer2
enable comparator2 interrupts

Ok, now when comparator2 interrupts, stop timer2 & store the count


My problem is that I can't seem to get the count of timer2 up in count value (no matter what I do with the prescalers) .

For example I know the phase of the input vs output for my circuit 180 degrees out of phase (I can see this on my scope - it's a simple inverting opamp setup to test this all out), but I'm getting readings like this...

comp1=2434 comp2=151
comp1=2432 comp2=151
comp1=2431 comp2=151
comp1=2431 comp2=151
comp1=2433 comp2=151
comp1=2432 comp2=151
comp1=2433 comp2=151
comp1=2432 comp2=151


Now for a signal that is 180 degrees out phase, I reckon comparator 2 should interrupt 'in or around' half the time that timer1 counted.

Is my methodology wrong, is is my code wrong?

(the prescalers prob aren't right below, but my issue is that no matter what I set the prescalers to, the second timer2 count (ie the inverted signal) never get's anythere near 50% of timer1)

@ __CONFIG _FCMEN_OFF & _INTRC_OSC_NOCLKOUT & _WDT_OFF & _MCLRE_OFF & _CP_OFF & _IESO_OFF & _BOR_OFF & _PWRTE_OFF
'***********************************************************************************************

'16F690 Pin Connections....
' PIN# NAME     USE & CONNECTION
'  1   Vdd      +5VDC power supply   
'  7   RC3      C12IN3- (external comparator1 input)
'  14  RC2      C12IN3- (external comparator2 input)     
'  20  Vss      Ground
'*************************************************************************************
DEFINE  OSC 8          ' set Oscillator at 8Mhz.
DEFINE  NO_CLRWDT 1 
  
DEFINE HSER_SPBRG 25   'HSEROUT Stuff.
DEFINE HSER_TXSTA 24h  'HSEROUT Stuff.
DEFINE HSER_CLROERR 1  'HSEROUT Stuff.

txsta = %10100100       'setup the tx register
RCSTA.7 = 1             ' Enable RB7 for TX USART
INTCON.0 = 0            ' clears the RABIF Flag (to 0), COULD be 1 on reset (unique to F690)
ANSEL      = 0    'disable AtoD.
ANSELH     = 0    'disable AtoD.

'Turn on & Set up Comparator 1
CM1CON0 = %10100111    'Comparator1 On
VRCON = %0000001       'turn on internal reference


'Turn on & Set up Comparator 2
CM2CON0 = %10100110   'turn on comparator 2.

Comp1Time          var  word    ' used to amalgamate TMR1 High & Low Bytes.
Frequency       var  word    'used to convert the count to frequency.
Comp2Time  VAR WORD
PHASE           var   word

          
INCLUDE "DT_INTS-14.bas"     ' Base Interrupt System  PO90OOO9
INCLUDE "ReEnterPBP.bas"     ' Include if using PBP interrupts

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   CMP1_INT,  _Comp1_Int,   PBP,  yes
        INT_Handler   CMP2_INT,  _Comp2_Int,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
T1CON = $10               ; 

T1CON.0=0 'stop tmr1
TMR1H = 0 'Set the high part of the timer1 value to 0
TMR1L = 0 'Set the low part of the timer1 value to 0

@ INT_ENABLE  CMP1_INT     ; enable Comparator 1 interrupts

t2con = %00000000010 ' tmr2 prescaler 16
TMR2 = 0 'Set the high part of the timer2 value to 0

Comp1Time = 0 'clear down Comp1Time, prior to starting.
comp2Time = 0  'clear down Comp2Time, prior to starting

'Main body of Code*********************************************************************************************
Main:
        HSEROUT ["comp1=", dec Comp1Time,9, "comp2=", dec comp2Time,13, 10]
        pause 10
        goto Main
        end
   
'Comparator1 Interrupt Handler+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++        
Comp1_Int:

        if T1CON.0= 0 then 'if timer1 is not running then...
        TMR1H = 0 'Set the high part of the timer value to 0
        TMR1L = 0 'Set the low part of the timer value to 0 
        T1CON.0= 1  'start timer
        else        'therefore if it is running, stop the timer & calculate the number of 'clock' counts between comparator interrupts....
        T1CON.0= 0  'stop tmr1
        TMR2 = 0    'clear tmr2
        T2CON.2= 1  'start tmr2
@ INT_ENABLE  CMP2_INT  
        Comp1Time.Lowbyte = TMR1L 'puts the timer's low byte in Comp1Time's lower 8 bits 
        Comp1Time.Highbyte = TMR1H 'puts the timer's high byte in Comp1Time's upper 8 bits

        endif 
@ INT_RETURN

'Comparator2 Interrupt Handler+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++  
Comp2_Int:
@ INT_DISABLE  CMP2_INT 
        T2CON.2= 0 'stop timer2
        Comp2Time = tmr2
@ INT_RETURN