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I have been trying to set up Timer 1 on my pic32mx320f128h based development board (Uno32 by Digilent). The clock speed is supposed to be 80MHz which should mean each clock cycle takes 12.5ns. a 500Hz signal should take 2,000,000ns. By that logic this is 160,000 clock cycles. I decided to use prescalar of 8 so that the 16bit timer 1 does not roll over. This should mean the timer hits a value of 20,000 for a 500Hz frequency.

However on my oscilloscope I measure a much lower frequency of only 1.57Hz

If i change the prescalar to 1, the speed increaes as expected to 1.57x8 =12.56Hz. However if I then lower PR1 by half to 9999, the speed does not exactly double resulting in only 25.2Hz

I must be missing something here, can anyone guide me on what I might try?

I have tried adjusting the priority levels, and using non multi vectored interrupts to no avail.I have also tried using timer 2 with similar speed issues.

#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <plib.h>

#define TICKS_PER_SECOND 80000000 //80MHz

#define PIN2_TRIS TRISDbits.TRISD8
#define PIN2_BIT PORTDbits.RD8
#define PIN2_LAT LATDbits.LATD8

#define PIN3_TRIS TRISDbits.TRISD0
#define PIN3_BIT PORTDbits.RD0
#define PIN3_LAT LATDbits.LATD0

#define PINA0_TRIS TRISBbits.TRISB2
#define PINA0_BIT PORTBbits.RB2
#define PINA0_LAT LATBbits.LATB2

int8_t pwmActive = 0;


void __ISR(_TIMER_1_VECTOR, IPL5SOFT) Timer1ISR(void){

    PIN3_LAT = ~PIN3_LAT;

    IFS0bits.T1IF = 0; //set timer 1 int flag back to off
}

int main(int argc, char** argv) {



//    PIN2_TRIS = 1;
//    PINA0_TRIS = 1;
//    
//    PIN3_LAT = 0;


    INTDisableInterrupts();
    T1CONbits.TCKPS = 1; //prescale 8
    T1CONbits.TCS = 0; //80MHz internal source
    PR1 = 19999; //500Hz period
    TMR1 = 0; //start timer at 0
    T1CONbits.ON = 1; //start the timer

    IPC1bits.T1IP = 5; //int priority 5
    IPC1bits.T1IS = 0; //sub int priority 0
    IFS0bits.T1IF = 0; //set timer 1 int flag off
    IEC0bits.T1IE = 1; //enable interrupt for timer 1
    INTEnableSystemMultiVectoredInt();


    PIN3_TRIS = 0;

    while(1){

    }

    return (EXIT_SUCCESS);
}

I expect to get a 500Hz square wave toggled on pin 3 but instead the frequency is much much lower at 1.57Hz

Patrick Ayers
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    This seems to be specific to your specific operating environment and implementation, as you don't seem to be using any of the standard C or POSIX timer interfaces. Nevertheless, I'm suspicious that you may have a misconception about the timer resolution. No timer interface I've dealt with defines timer ticks to correspond to single clock cycles, and some do not even promise to provide single-tick resolution. – John Bollinger Jun 12 '19 at 17:05
  • I am using MPLAB X and a PICKit 3 to program the microcontroller. I had sourced the information to write this code from here https://www.youtube.com/watch?v=bu6TTZHnMPY – Patrick Ayers Jun 12 '19 at 17:25
  • show you config Bits and the oszillator setting. Is the PIC rreally running at 80MHz? I guess not. With PR = 20000 you should measure 250Hz and not 500 Hz. – Mike Jun 13 '19 at 06:01
  • @Mike 80MHz is the upper bound for SYSCLK on this PIC32 – bigwillydos Jun 14 '19 at 17:55
  • of course the PIC32 could run at 80 MHz but I guess in would not in your application. Therefore I ask for the config Bits. – Mike Jun 17 '19 at 05:29

0 Answers0