16f882 TMR1 as frequency counter problem

[dope40]

Hi,

I am trying to make a frequency counter using a pic 16f882 .
The idea is using TMR1 to measure the pulse period and use it to get the frequency.

I am using a 16 MHz crystal so that TMR1 increases every 250ns ( FOSC/4 and 1:1 Prescale Value )

Here is the code :

#include <xc.h>
// CONFIG1
#pragma config FOSC = EXTRC_CLKOUT// Oscillator Selection bits (RC oscillator: CLKOUT function on RA6/OSC2/CLKOUT pin, RC on RA7/OSC1/CLKIN)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled and can be enabled by SWDTEN bit of the WDTCON register)
#pragma config PWRTE = OFF      // Power-up Timer Enable bit (PWRT disabled)
#pragma config MCLRE = ON       // RE3/MCLR pin function select bit (RE3/MCLR pin function is MCLR)
#pragma config CP = OFF         // Code Protection bit (Program memory code protection is disabled)
#pragma config CPD = OFF        // Data Code Protection bit (Data memory code protection is disabled)
#pragma config BOREN = ON       // Brown Out Reset Selection bits (BOR enabled)
#pragma config IESO = ON        // Internal External Switchover bit (Internal/External Switchover mode is enabled)
#pragma config FCMEN = ON       // Fail-Safe Clock Monitor Enabled bit (Fail-Safe Clock Monitor is enabled)
#pragma config LVP = OFF        // Low Voltage Programming Enable bit (RB3 pin has digital I/O, HV on MCLR must be used for programming)

// CONFIG2
#pragma config BOR4V = BOR40V   // Brown-out Reset Selection bit (Brown-out Reset set to 4.0V)
#pragma config WRT = OFF        // Flash Program Memory Self Write Enable bits (Write protection off)					

#define _XTAL_FREQ 16000000
#include <stdio.h>
#include <stdlib.h>
int aa, bb, state ;
unsigned long overflow, total ;


void interrupt tc_int()
{
  if (T1CONbits.TMR1ON == 0) {
    T1CONbits.TMR1ON = 1 ;  // start timer
    INTCONbits.INTF = 0 ; // clear flag
  }   
  
  else if (T1CONbits.TMR1ON == 1) {
      T1CONbits.TMR1ON = 0 ; // disable TMR1
      total =(TMR1H<<8)|(TMR1L);
      INTCONbits.INTE = 0 ; // disable interupt
      TMR1H=0;
      TMR1L=0;
      // total =(4000000/total);
      INTCONbits.INTE = 1; //Enable RB0/INT external Interrupt
      INTCONbits.INTF = 0; // clear the interrupt flag
  }

}   
void Init(void){
    TRISA = 0xFF;   //set all digital I/O to inputs
    TRISB = 0xFF;
    TRISC = 0xFF;
    ANSEL = 0x00;   //disable all analog ports
    ANSELH = 0x00;
    
    T1CON=0X00;
    TMR1H=0;
    TMR1L=0; // intialization of timer one prescalar and internal clock
    INTCONbits.GIE = 1; //Enable Global Interrupt
    INTCONbits.INTE = 1; //Enable RB0/INT external Interrupt
    OPTION_REGbits.INTEDG = 0; //Interrupt on rising edge
}
void main(void)
{
    Init() ;

    while(1)
    {
     
    }

}

I am feeding 100 Hz in RB0 pin. Based on that i am expecting the value of Total to be 40000 , but i get around 19000 ( using PICKit 3 as debugger , also my freq. gen is not ideal so i am expecting some fluctuation . Still i get half of what i am expecting ).

The interrupt is on the rising edge.

Does anyone know what i am doing wrong ?

P.S .... Also i failed to mention , for the purposes i need it the duty cycle is high ( ~ 90 % ) , but still i think that shouldn't be a problem , right ?

 

[KlausST]

Hi,

The precise way is to use "capture" mode to get exact timestamps.

On one rising edge you get a timestamp (start)
On the next rising edge you get the next timestamp (stop)

The frequency is calculated by: f = 4000000 / (timestamp(stop) - timestamp(start))

Don't reset the counter value.
The result is correct as long as the input frequency is higher than: 4000000/65536

Klaus