Rough 'n Ready Audio Frequency extraction with a PIC

[ScaleRobotics]

I just noticed that my answer will overflow when at the higher frequencies. So for a single decimal point it should stay within limits throughout the range you gave in post 1.

b=5000
c=1000
dummy = b * c       '5,000,000 
 frequency = div32 mytime
LCDOut dec frequency/10,".",dec frequency//10," Hertz"

[HankMcSpank]

Cool Walter.

All is well (± 0.1Hz)...


154.3Hz
154.3Hz
154.3Hz
154.3Hz
154.3Hz
154.3Hz
154.2Hz
154.2Hz
154.3Hz
154.3Hz
154.3Hz
154.3Hz


So, for anyone who may find this thread via a search, to summarize here's the code to frequency detect audio signals fed into pin 7 of a 16f690 (using the PIC's internal comparator to generate interrupts, which are in turn 'timed' successively)....

@ __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 comparator input)
'10   RB7     HSEROUT Pin (fed into Pickit2's pin 4 to display data onsreen via the Pickit2's UART tool)
'19   RA0      Ext VREF for Comparator1   
' 20  Vss      Ground
'*************************************************************************************
DEFINE  OSC 4          ' set Oscillator at 4Mhz.
DEFINE  NO_CLRWDT 1   ' PBP doesn't clear WDT automatically
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.
CM2CON0 = 0   'turn off comparator 2.

'Turn on & Set up Comparator 1
CM1CON0 = %11100011    'Comparator ext op pin disabled (op of comparator avaible internally only), compare against external VREF

MyTime       var  word    ' used to amalgamate TMR1 High & Low Bytes.
Frequency    var  word    'used to convert the 'count' to frequency.

' the following is pretty much a straight lift from the compiler manual (DIV32 section)
a Var Word
b Var Word
c Var Word
dummy Var Word

'these two below will later set the dummy variable to total  5,000,000 in the interrupt handler for DIV32 to use
b = 5000   
c = 1000

MyTime = 0 'clear down Mytime, prior to starting.
          
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
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
T1CON = $11               ; Prescaler setting - this one means 2us between successive clocks

@ INT_ENABLE  CMP1_INT     ; enable Comparator 1 interrupts

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



'Main body of (dummy) Code*********************************************************************************************
Main:
        pause 10
        goto Main
        end
   
'Comparator1 Interrupt Handler+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++        
Comp1_Int:
        if T1CON.0= 0 then 'if timer1 is not running...
        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 the timer
        MyTime.Lowbyte = TMR1L 'puts the timer's low byte in MyTime's lower 8 bits 
        MyTime.Highbyte = TMR1H 'puts the timer's high byte in MyTime's upper 8 bits
        dummy = b * c       '5,000,000 
        frequency = div32 mytime ' this convertes to Hz, but with no decimal points
        HSEROUT [dec frequency/10,".",dec frequency//10,"Hz",13, 10]   .....this places a decimal point in the right place to make the reading easier on the eye.
        endif 
@ INT_RETURN

Note: for the comparator to have something to erhm 'compare' against, I've still not got the PIC's internal voltage reference to work yet, so I was feeding 1/2 VCC externally into the 16F690's RA0 Pin 19 (the bolded bit in this command sets this up.... CM1CON0 = %11100011 )

[HankMcSpank]

To get more resolution/accuracy for calculating phase, I really need to crank the OSC up to 20Mhz (ie use an ext oscillator).

Ok, the lowest note on a guitar is about 82Hz (std tuning)

Is my line of thinking correct here.?...

One full cycle @82hz takes 0.012195122 seconds

One PIC clock cycle @20Mhz is 0.000000125 seconds.

therefore the number of clock cycles that will occure when a note of 82Hz is played will be 243,902.

I'm using a 16F690, which has a 16 bit timer - a 16 bit timer will over flow at a count of 65536 - alas this number is smaller than 243,902.

therefore to be able to count 82Hz at 20Mhz, I need a tmr overflow interrupt handler to increment a variable with each tmr overflow (ie count the number of tmr overflows). therefore at 82Hz signal and 20Mhz clock a 16 bit timer will over flow 3 times.

So (you still awake at the back?), to get the true total number of PIC clock counts @ 82Hz, I need to multiply the stored tmr overflow (3) count by 65536 & then add in the existing tmr count number. chunky numbers in play for a device costing $2.

Is the PIC gonna start struggling with the above...or will it take it in its stride? (is there a better route I should wrt all of this?!!)