Measuring change of frequency with PIC

[Bruce]

With Timer1 configured for 16-bit read/write, you need to write to TRM1H
first, then write to TMR1L.

So change;
TMR1L = 0 ' clear TMR1 low
TMR1H = 0 ' clear TMR1 high

To;
TMR1H = 0 ' writes to TMR1H buffer
TMR1L = 0 ' writes TMR1H 'buffer' to TMR1H register & writes to TMR1L

[Bruce]

If RC2 is avilable you could also use the hardware capture module.

' Measuring pulse width with capture module
' Input signal connected to RC2/CCP1 pin

    DEFINE OSC 4
   
    Symbol Capture = PIR1.2 ' CCP1 capture flag
    T1 VAR WORD          ' 1st capture value
    T2 VAR WORD          ' 2nd capture value
    T3 VAR WORD          ' 3rd capture value
    LowCycle VAR WORD    ' low pulse width rsult
    HighCycle VAR WORD   ' high pulse width result
    
    TRISC.2 = 1          ' CCP1 input pin (Capture input)
    INTCON = 0           ' Interrupts off
    CCP1CON = %00000101  ' Capture mode, capture on rising edge
    T1CON = %10000001    ' TMR1 prescale=1, clock=Fosc/4, TMR1=on
 
Main:
    WHILE !Capture      ' wait for rising edge capture
    WEND
    ' Rising edge detected so record timer1 value
    T1.HighByte = CCPR1H : T1.LowByte = CCPR1L
    
    Capture = 0         ' Clear capture flag bit
    CCP1CON.0 = 0       ' Configure capture for falling edge now
    
    While !Capture      ' While here for capture on falling edge
    Wend
    ' Falling edge detected so record timer1 value again
    T2.HighByte = CCPR1H : T2.LowByte = CCPR1L
    
    Capture = 0         ' Clear capture flag bit
    CCP1CON.0 = 1       ' Configure capture for rising edge now
    
    While !Capture      ' While here for capture on falling edge
    Wend
        
    ' Falling edge detected so record timer1 value again
    T3.HighByte = CCPR1H : T3.LowByte = CCPR1L
    
    CCP1CON = 0         ' disable capture during data processing
    
    HighCycle = T2-T1   ' High pulse width = T2 - T1
    LowCycle = T3-T2    ' Low pulse width = T3 - T2
      
    HSEROUT ["High pulse width = ",DEC HighCycle,"uS",13,10]
    HSEROUT ["Low pulse width = ",DEC LowCycle,"uS",13,10]
    HSEROUT ["Period = ",DEC LowCycle+HighCycle,"uS",13,10]
    
    Capture = 0          ' Clear capture flag bit
    CCP1CON = %00000101  ' Capture mode, capture on rising edge
    
    GOTO Main

    END

If the measured frequency is too slow you can use the timer prescaler. You
want the frequency to be high enough so the timer doesn't overflow during
the measurement period.

[Aussie]

With Timer1 configured for 16-bit read/write, you need to write to TRM1H
first, then write to TMR1L.

So change;
TMR1L = 0 ' clear TMR1 low
TMR1H = 0 ' clear TMR1 high

To;
TMR1H = 0 ' writes to TMR1H buffer
TMR1L = 0 ' writes TMR1H 'buffer' to TMR1H register & writes to TMR1L

Thanks a lot Bruce,
I changed only that thing in my program and it started working .
Have never seen in the manual that such a sequence of work with the timers should be...