Hi,
    I'm trying to program a PIC16C71 to generate a given frequency at the
output. I'm using the 71's timer overflow interrupt to generate the pulse.
I've been doing some tests using the C2C plus 4.004e compiler's wizard. But
everytime I put the PIC on I get a different frequency, about 1 time on 3 I
get the correct one, but most of the time the frequency is too high. The
4Mhz crystal of the put is constant so it does not cause the problem.

Here's how the PIC should work :
I'm using the internal clock of the PIC with a prescaler 2.
On timer overflow interrupt a function is called (tmrHandler) switch portB 1
to high or to low if it's already high.

I did some calculation and figured I should get a 976.56 Hertz signal at the
output :

(1/(1/(4 000 000 / 4) * prescaler * 256))/2
(256 is the overflow count)
(the result is divided by 2 because the output is toggled On and Off each
time
 the function is called)
(1/(1/1000000 * 2 * 256))/2 = 976.56 Hertz
or
1000000/2/256/2 = 976.56 Hertz

When I turn the PIC on sometimes I get 978 Hertz but sometimes I get 7.5KHz,
4xx Hz, 8.xKHz etc...

I really do not understand why I get this result, I'm using a new 9volt
battery to drive the circuit (with a 78x05 regulator)
I tried to add an 1000uF capacitor at the regulator without any results.

Could my PIC be damaged ? Something is wrong with my code ?

Thanks in advance !


Here's the C2C testing code  :
----------------------------------------------------------------------------
----------------------------------------------------------------------------
-------
//This file was generated by the C2G Code Generator 1.1


//If necessary insert your global variables declaration here
char toggle = 1;



//TMR0 Overflow handler
void tmrHandler( void )
{
if (toggle)
  {
   output_low_port_b (1);
   toggle = 0;
  }
  else
  {
   output_high_port_b (1);
   toggle = 1;
  }
}


//This is the interrupt handler
void interrupt( void )
{
    //Insert your code which should run
    //before any interrupt processing here


    //Interrupt processing
    //TMR0 Overflow processing
    if( INTCON & 4 )
    {
        clear_bit( INTCON, 2 );
        tmrHandler();
    }
    //Insert your code which should run
    //after any interrupt processing here


}

//This is the main function which is called
//after processor power-up or reset
void main( void )
{
    //Hardware Initialization
    set_bit( STATUS, RP0 ); //Select the Register bank 1
    set_tris_a( 1 ); //Configure the Port A
    set_tris_b( 0 ); //Configure the Port B
    OPTION_REG = 128; //Configure the OPTION register
    clear_bit( STATUS, RP0 ); //Select the Register bank 0
    INTCON = 160; //Configure the INTCON register

    //Don't forget to insert your code here!!!
    //(As there is no "return" from the main code
    //an endless loop may be a good idea)


}
----------------------------------------------------------------------------
---------------------------------

Here's the compiled code :
----------------------------------------------------------------------------
---------------------------------
; This file was generated by C2C-plus compiler version 4.00.4e

    include "p16C71.inc"
    ;Variables *****************************************
__int_save_cont_W               equ 0x0c
__int_save_cont_STATUS          equ 0x0d
__int_save_cont_FSR             equ 0x0e
__int_save_cont_PCLATH          equ 0x0f
_toggle                         equ 0x10
    ORG 0
    goto start__code

    ORG 4
_interrupt
_interrupt__code
    movwf __int_save_cont_W
    swapf __int_save_cont_W, F
    swapf STATUS, W
    movwf __int_save_cont_STATUS
    swapf FSR, W
    movwf __int_save_cont_FSR
    swapf PCLATH, W
    movwf __int_save_cont_PCLATH
    btfss INTCON, 2
    goto label_0002
    bcf INTCON, D'2'
    call _tmrHandler
label_0002
    swapf __int_save_cont_PCLATH, W
    movwf PCLATH
    swapf __int_save_cont_FSR, W
    movwf FSR
    swapf __int_save_cont_STATUS, W
    movwf STATUS
    swapf __int_save_cont_W, W
    retfie
_interrupt__end

start__code
    movlw D'1'
    movwf _toggle
_main__code
    bsf STATUS, RP0
    movlw D'1'
    movwf TRISA
    movlw D'0'
    movwf TRISB
    movlw D'128'
    movwf OPTION_REG
    bcf STATUS, RP0
    movlw D'160'
    movwf INTCON
_main__end
_tmrHandler__code
_tmrHandler
    movf _toggle, W
    btfsc STATUS, Z
    goto label_0000
    bcf PORTB, 1
    clrf _toggle
    goto label_0001
label_0000
    bsf PORTB, 1
    movlw D'1'
    movwf _toggle
label_0001
    return
_tmrHandler__end
    END
----------------------------------------------------------------------------
---------------------------------

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