PIC18F timer for minutes and hours

Question

#include<p18f452.h>
void T0_init(); 
void main()
{
    TRISC=0;        // Configure PortB as output Port.
    LATC=0x01;
    T0CON=0x07;     // Prescaler= 1:256, 16-bit mode, Internal Clock
    T0_init();
    while(1)
    {
            // Initialize Timer0
            LATC=0x00;
            T0_init();      //Delay for 1 sec
            LATC=0x01;
            T0_init();     //Again delay for 1 sec
    }
}

void T0_init()
{
    int a=0;

    //while(a<2)   // {
    TMR0H=0xF0;    // Values calculated for 1 second delay with 4MHz crystal
    TMR0L=0xBD;    //4/4MHz =1us=1us*prescaler=256us=1sec/256u=0xF42-(FFFF)=F0BD
    T0CONbits.TMR0ON=1;         // Timer0 On
    while(INTCONbits.TMR0IF==0);        // Wait until TMR0IF gets flagged
    T0CONbits.TMR0ON=0;                 // Timer0 Off
    INTCONbits.TMR0IF=0;                // Clear Timer0 interrupt flag
    //   a++;
    //  }
}

It is very accurate delay and it is doing its work but I want longer delays like of hours or minutes. So , what to do for longer delays. I tried using counter which increments on every flagged, but it does not work.

Answer 1

A second is 1000000 microseconds, that is less than 256*256*256=16777216. So a 256-prescaled 16-bit timer is sufficient to count a second. But it's not enough to count a minute.

The solution is to add a for loop to count the seconds. And another one for minutes, and another one for hours, as suggested by @Andrew .

A source of uncertainty is that the overflow of the timer is checked once in the while() loop. So if long computations are performed in that loop, the precision will decrease. To overcome this, using interrupts is the way to go. Whatever is happening, as the overflow of the timer occurs, the interrupt flag is raised and the interruption function is executed within a few cycles.

Since nothing is done in the while(1) loop, i wanted to use the SLEEP() command to spare some energy. Unfortunately, the timer0 is switched off by the SLEEP() command. It is not the case of timer1, but timer1 is a 8-prescaled 16-bit timer... So a for loop is needed to count a second.

Here is a code compiled by XC8 and tested with the simulator of MPLAB X...

#include <xc.h>

#pragma config WDT=OFF


volatile unsigned char nb;
volatile unsigned char seconds;
volatile unsigned char minutes;
volatile unsigned char hours;

void interrupt high_isr (void)
{

    if(PIR1bits.TMR1IF) //interrupt flag of TIMER1
    {
        if(nb==0){
            TMR1H=0;    // Values calculated for 1 second delay with 4MHz crystal
            TMR1L=0;
            nb=1;//4/4MHz =1us=1us*prescaler=8us=1sec/8u=...
        }else{
            TMR1H=23;
            TMR1L=184;
            nb=0;
        }

        if(nb==1){
            seconds++;
            if(seconds==60){
                seconds=0;
                minutes++;
                if(minutes==60){
                    minutes=0;
                    hours++;

                }
            }
        }
        PIR1bits.TMR1IF=0;  //erase interrupt flag of TIMER1
    }
}

void interrupt low_priority low_isr (void)
{

}

void main(void) {
    //OSCCON=0x7D;//Internal oscillator, 8Mhz


    T1CONbits.RD16=1;  //timer1 is a 16-bit timer
    T1CONbits.T1CKPS1=1; //prescaler 11 -> 8-prescaled
    T1CONbits.T1CKPS0=1;
    T1CONbits.TMR1ON=1;// switch on timer1

    TMR1H=0x00; //setting timer1 : high bit first
    TMR1L=0x00;

    //   TMR0H=0;
    //   TMR0L=0;

    TRISC=0;
    LATC=0x00;

    INTCON2bits.TMR0IP=0; // low priority interuption timer0
    IPR1bits.TMR1IP=1;   // high priority interuption  timer1

    INTCONbits.TMR0IF=0;//erase interruption flap if set
    PIR1bits.TMR1IF=0; //erase interruption flag if set

    RCONbits.IPEN=1;// Interruption prioritaires

    INTCONbits.TMR0IE=0;// interuption activation on timer0 (0=no active)

    PIE1bits.TMR1IE=1;//enable interupt on timer1

    nb=1;
    seconds=0;
    minutes=0;
    hours=0;
    PORTCbits.RC0=0;
    PORTCbits.RC1=0;
    PORTCbits.RC2=0;
    INTCONbits.GIEH=1;// autorise interuptions
    INTCONbits.GIEL=1;
    // INTCONbits.GIE=1;



    while(1){
        //problem with modulo and pic...do not use %
        if(seconds-2*(seconds/2)==1){
            PORTCbits.RC0=1;
        }else{
            PORTCbits.RC0=0;
        }

        if(minutes-2*(minutes/2)==1){
            PORTCbits.RC1=1;
        }else{
            PORTCbits.RC1=0;
        }

        if(hours-2*(hours/2)==1){
            PORTCbits.RC2=1;
        }else{
            PORTCbits.RC2=0;

        }
        SLEEP();
    }
    return;
}

The duration of a "second" is subjected to caution...Playing with TMR1H,TMR1L and nb may be required.

And using an external clock source (quartz) is better : How to make Timer1 more accurate as a real time clock?

Answer 2

I'm not an experienced programmer and not sure exactly where you want to use your delays in, but I had tried to make a lengthy delay with pic18f4550 but it didnt work, i.e. for minutes and hours. But I have considered using an IC like, CD4060 or DS1307. you can follow this link for CD4060: http://dmohankumar.wordpress.com/2012/05/13/design-your-circuit-part-ii-cd-4060-timer/. For DS1307 you can check up in studentcompanion site: http://www.studentcompanion.co.za/post/Digital-Clock-using-PIC-Microcontroller-and-the-DS1307-Real-Time-Clock-XC8-Compiler/. I wish you success, thanks

Answer 3

forget the internal timer and do a busy wait routine on your own: make first a 1-second delay routine (very simple to do it accurately, take care of the call and retf execution time!), then make a 1-minute delay routine on top of the first, then make a 1-hour delay routine on top of the 1-minute... you get the idea.