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SOLVED: Timer interrupt slow

 
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JackB



Joined: 04 Mar 2016
Posts: 32
Location: Netherlands

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SOLVED: Timer interrupt slow
PostPosted: Sat Mar 19, 2016 6:49 pm     Reply with quote
I use a PIC18F67K90 at 20MHz.
It seems my 1mSec timer interrupt, is about 2 times too slow.
The LED should flash at 1 Hz rate, but it is 1 second on and 1 second off.
The other two receive interrupts are not triggered much.
What should I check to find out why the timer interrupt is slowing down?

main.c:

Code:

#include "project.h"

#define DMX_SIZE        128
#define DMX_BREAK_COUNT 8

uint16 msecs;
uint16 heartbeat_count;

uint8 dmx_in[DMX_SIZE];
uint16 dmx_index;
uint16 dmx_value;
uint16 dmx_break_time;
uint8 dmx_skip_bytes;
uint8 dmx_signal_detected;
uint16 dmx_address;

... (some more variables)

void main()
{
    setup_timer_2(T2_DIV_BY_4, 249, 10);
    enable_interrupts(INT_TIMER2);
    enable_interrupts(INT_RDA);     // UART1 gives INT_RDA
    enable_interrupts(INT_RDA2);    // UART2 gives INT_RDA2
    enable_interrupts(GLOBAL);

    while(1)
    {
    }
}

#INT_TIMER2
void timer2_isr()
{
    // Timer interrupt every 1 mSec
    if(--heartbeat_count==0) {
        output_toggle(PIN_C1);      // Heartbeat LED
        heartbeat_count = 500;            // Toggle every 500 mS
    }
    if (dmx_break_time < 1000)
        dmx_break_time++;           // Increase measured break time by 1 mSec
    if (dmx_break_time > 100)
        dmx_signal_detected = 0;    // Flag indicates presence of DMX signal
    msecs++;       
}

#INT_RDA2
void usb_isr2() {
    // Is this receiver interrupt on the STD stream? Does not trigger?
    std_byte = (int8)fgetc(USB_STREAM);             // Read RX register
    if (std_byte == 0x0d)
        input_string_fetching[input_index] = (char) '\0';
    else
        input_string_fetching[input_index] = (char) std_byte;
    input_index++;
    if (input_index > INPUT_STRING_SIZE-1)
        input_index = INPUT_STRING_SIZE-1;
}

#INT_RDA
void dmx_isr() {
    // Is this receiver interrupt on the DMX stream? Does not trigger?
    int8 dmx_byte = (int8)fgetc(DMX_STREAM);             // Read RX register
    if (dmx_break_time > DMX_BREAK_COUNT) {      // MARK time after Slot > 15mS, reset index
        dmx_index = 0;
        dmx_skip_bytes = 2;
        dmx_signal_detected = 1;
    }
    if (dmx_skip_bytes > 0) {
        dmx_skip_bytes--;
    }
    else {
        dmx_in[dmx_index++] = dmx_byte;
        if (dmx_index == DMX_SIZE)
           dmx_index = DMX_SIZE-1;           // Ensure this cannot overflow.
    }
    dmx_break_time = 0;             // Character received: reset break time
}



project.h:

Code:

#ifndef PROJECT_H
#define   PROJECT_H

#include <18F67K90.h>
#include "types.h"

#define BOARD_CURRENT_DRIVER  1
#define BOARD_VOLTAGE_DRIVER  1-BOARD_CURRENT_DRIVER

#case

#device ADC=12

#fuses MCLR,HSH,NOPLLEN

#use delay(crystal=20mhz)

#use rs232(UART1, BAUD=250000,BITS=9,PARITY=N,STOP=1,ERRORS,stream=DMX_STREAM, LONG_DATA)

#use rs232(uart2, baud=9600, stream=USB_STREAM)

#use i2c(Master,Fast,sda=PIN_C4,scl=PIN_C3)

enum errors { err_range, err_too_far, err_too_many };

#endif   /* PROJECT_H */


Last edited by JackB on Sun Mar 20, 2016 5:05 am; edited 1 time in total
jeremiah



Joined: 20 Jul 2010
Posts: 1322

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PostPosted: Sat Mar 19, 2016 7:30 pm     Reply with quote
The parameters you have have chosen make the pin toggle every second

Fosc = 20MHz
Fcy = 20/4 = 5MHz
Prescaler of 4x: 5/4 = 1.25 MHz
Postscaler of 10x: 1.25/10 = 125kHz = 8us per tick

250 ticks per period => 8us * 250 = 2ms per timer period

500 counts per toggle => 2*500 = 1 second per toggle (1 second on and 1 second off)
JackB



Joined: 04 Mar 2016
Posts: 32
Location: Netherlands

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PostPosted: Sun Mar 20, 2016 5:04 am     Reply with quote
Hi Jeremiah,

thank you very much! That fixed the problem!
I'm still learning with ccs c.
It works as expected, and I have added your comments in the code for future reference.
This is how I understood it:

// The parameters to make the pin toggle every 500ms are
// Fosc = 20MHz
// Fcy = 20/4 = 5MHz
// Prescaler of 4x: 5/4 = 1.25 MHz
// Postscaler of 5x: 1.25/5 = 250kHz = 4us per tick
// (249+1) ticks per period => 4us * 250 = 1ms per timer period
// 500 counts per toggle => 1*500 = 500ms per toggle (500ms on and 500ms off)
setup_timer_2(T2_DIV_BY_4, 249, 5);
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