Frecuency Detection

[Bruce]

Measuring a frequency is pretty straight forward, and can be done on just about
any PIC with a spare input pin.

Here are a few ideas.

1. Use the COUNT command.
2. Use TIMER1.
3. Use the hardware capture module.

And here's a simple test program using all 3 options. Tested on a 16F627A.

    @ DEVICE MCLR_OFF, INTRC_OSC, WDT_OFF, LVP_OFF, BOD_ON, PWRT_ON, PROTECT_OFF
    DEFINE OSC 4
    
    T1 VAR WORD
    T2 VAR WORD
    TRISB = %00001000 ' RB3 = input

    T1CON = 0
    TMR1L=0
    TMR1H=0
    
'    GOTO Test3      ' using COUNT
'    GOTO Test2      ' using capture
'    GOTO Test1      ' using TMR1 with pin state watch
    
Test1:
    WHILE PORTB.3=1  ' noise filter
    WEND
    WHILE PORTB.3=0  ' wait for low to high edge
    WEND
    T1CON=1          ' start TMR1 on rising edge
    WHILE PORTB.3=1  ' wait for high to low edge
    WEND
    WHILE PORTB.3=0  ' wait for 2nd rising edge (end of period)
    WEND
    T1CON=0          ' stop TMR1
    
   ' Record the period as T1. 
    T1.HighByte = TMR1H
    T1.LowByte = TMR1L
    HSEROUT ["Period = ",DEC T1,13,10]
    PAUSE 10000
    TMR1L=0
    TMR1H=0
    GOTO Test1
'    Serial from PIC ' Shown on O-Scope    
'    Period = 3143   ' 315 Hz
'    Period = 2403   ' 412 Hz
'    Period = 2113   ' 468 Hz
'    Period = 1643   ' 603 Hz

Test2:
    'Configure capture for rising edge.
    CCP1CON = %00000101 ' Capture mode, capture on rising edge
    'Setup & start Timer1.
    T1CON = 1           ' TMR1 prescale=1, clock=Fosc/4, TMR1=on.

Loop:
    'Wait for the capture flag bit to be set.
    WHILE PIR1.2 = 0
    WEND
    
    'Record the first capture value as T1. 
    T1.HighByte = CCPR1H
    T1.LowByte = CCPR1L
    
    'Clear the capture flag. 
    PIR1.2 = 0
    
    'Wait for the capture flag to be set a 2nd time.
    WHILE PIR1.2 = 0
    WEND
    
    'Record the 2nd capture value as T2.
    T2.HighByte = CCPR1H 
    T2.LowByte = CCPR1L
    
    'Clear the capture flag
    PIR1.2 = 0

    'T2-T1 = the period in uS.
    HSEROUT ["Period = ",DEC T2-T1,13,10]
    PAUSE 1000
    GOTO Loop    
'    Serial from PIC ' Shown on O-Scope
'    Period = 3149   ' Freq = 315 Hz
'    Period = 2410   ' Freq = 412 Hz
'    Period = 2120   ' Freq = 468 Hz
'    Period = 1648   ' Freq = 603 Hz

Test3:
    COUNT PORTB.3,20,T1  'Count low to high transitions for 20mS
    HSEROUT ["Count = ",DEC T1,13,10]
    PAUSE 10000
    GOTO Test3
'    Serial from PIC ' Shown on O-Scope    
'    Count = 6       ' 315 Hz
'    Count = 8       ' 412 Hz
'    Count = 9       ' 468 Hz
'    Count = 12      ' 603 Hz
' Resolution with COUNT isn't that great, but it works.
' 20mS/count value = the approximate period.

    END

An 8-pin PIC was used for the frequency generator.

Capture could be used with the capture flag generating an interrupt if
you prefer not to wait for state changes.

I'm sure there are several more ways to do this, but this should get you
started.