Question

I’m trying to understand this code:

#include <pic.h>
#include  "delay.h"

#define brisi_flag timer_flag=0

unsigned char timer;
unsigned char impuls;
unsigned char ton_koji_ne_svira;
static bit disable @ (unsigned)&PORTA*8+4;
static bit izlaz @ (unsigned)&PORTA*8+1;
static bit izlaz_inv @ (unsigned)&PORTA*8+0;
static unsigned char stanja @ 0x2e;
static bit stanje_izlaza @ (unsigned)&stanja*8+1;
static bit  timer_flag @ (unsigned)&INTCON*8+2; 

void
DelayMs_2(unsigned char cnt)
{
    unsigned char   i;
    do { asm("CLRWDT");
      if(ton_1)
         return;
        i = 4;
        do {
            DelayUs(250);
        } while(--i);
    } while(--cnt);
}

void
DelayMs(unsigned char cnt)
{
    unsigned char   i;
    do { asm("CLRWDT");
        i = 4;
        do {
            DelayUs(250);
        } while(--i);
    } while(--cnt);
}

void ton_1_2(unsigned char onaj_drugi_ton)
{
     izlaz=0;
     izlaz_inv=1;
     stanje_izlaza=0;
     TMR0=timer; 
ton_1_2_start:
     brisi_flag;
     disable=0;
ton_1_2_sviranje:
     while(timer_flag==0)
          {
        if((TMR0-timer)>=impuls)
             {izlaz=0;
              izlaz_inv=0;
             }
          }
     brisi_flag;
     TMR0=timer;
     if(stanje_izlaza==0)
        izlaz_inv=1;
     else   
        izlaz=1;
     stanje_izlaza=stanje_izlaza+1;   
     if((PORTA&0x0c)==onaj_drugi_ton)
        return;
     asm("CLRWDT");
     goto ton_1_2_sviranje;                 
}

void main()
{
    CMCON=0x07;//portA su normalni ulazi (za 16F628A)
    TRISA=0x0c;
    TRISB=0xff;
     disable=1;
    OPTION=0x81;   //WDT na 18 msec, preset TMR0 sa 4   

    izlaz=0;
    izlaz_inv=0;

    timer=107;
        impuls=0x1d;

    ton_1_2(ton_koji_ne_svira);
}

This program generates 800 Hz sound on speaker. I don’t understand how to calculate frequency (it must be something with variables impuls and timer ). This code is for PIC16F628A, it has external oscillator set at 4Mhz.

Answer 1

Here is how I understand the code.

The timer starts at 107. As the timer interrupt triggers on overflow the period is 256-107 = 149 ticks. During each period, the outputs are turned off after IMPULS=29 ticks. Hence you have a 29/149=19% duty cycle. In addition, the pulses alternate between two pins.

OPTION=0x81 sets a 1:4 prescaler for the timer. Hence if the clock runs at 4MHz (period 0.25µs), the timer ticks at 1MHz (period 1µs). Thus a period of 149 ticks should equal 149µs and correspond to a frequency of 6711Hz. Adding the time it takes to start the timer each time the actual frequency is slightly lower.

Are you sure the frequency is 800Hz ? Did you measure it ? Maybe the processor is really running at 500Hz, or there is a global 1:8 prescaler somewhere.