I am trying to create a countdown timer using the PIC18F46K20 and display the time on the OLED. I have set up TIMER0 to create a delay of 1 second ( T0CON = 0b00000001 ) so the variabe intCLKsecs should increase every second.

    Timer0_Init();          //Initialise timer
T0CONbits.TMR0ON = 1;  // start timer

while(INTCONbits.TMR0IF = 1) {

/* Check for clock edges and and increment countSecs by one on every falling edge. The timer is set up to output pulses that are around 1 sec apart. */ INTCONbits.TMR0IF = 0; // Clear the flag

    korf_intCLKsecs++; //Should increase by one every second on every falling edge 
    korf_InitSecs = 1800; //Initialised time for a single korfball half
    net_timeLeft = net_InitSecs-net_intCLKsecs; //Time remaining in the half


I pass the the [timeLeft] to a function that formats it and displays it like a clock i.e 8:56 Here is the time dicplay function.

int displayTime(int currentTime)
    int secs;
    int mins;
    char Buffer[60];
    secs =  currentTime % 60;
    mins = (currentTime / 60) % 60;

    sprintf(Buffer, "\n\t\t %d : %d\0",mins,secs);

I do apologize if i have made any mistakes in the way i am posting, I am very new to both forums and PICKit programming. Can someone out there please help me figure out how to get a correct value into my intCLKsecs variable by counting the CLK pulses on every falling edge? What do i write in MPLAB IDE to make sure that [intCLKsecs] increases on every falling edge?

  • \$\begingroup\$ What is your actual clock frequency? \$\endgroup\$ – Matt Young Apr 18 '14 at 15:26
  • \$\begingroup\$ I don't know. How do i find out? \$\endgroup\$ – DixonLamont Apr 18 '14 at 15:53
  • \$\begingroup\$ Timer0 runs off the internal instruction clock. At Fosc = 1MHz, the instruction clock is 250kHz. \$\endgroup\$ – DixonLamont Apr 18 '14 at 16:02
  • \$\begingroup\$ "while(INTCONbits.TMR0IF = 1)" is a classic C-language programming trap. You must code "==" when comparing and "=" when assigning a value. My preference is to omit the comparison when comparing to true (i.e. not 0): \$\endgroup\$ – aja Apr 19 '14 at 7:16
  • \$\begingroup\$ The timer will overflow in 65536 counts. At the instruction clock rate of 250kHz, the timer overflow will occur every 65536 x (4 / 250,000) = 1.048 seconds. This is a time in the range i with the prescaler assigned to Timer0. It runs directly off the instruction clock. I want to increase a variable [count] by 1 everytime the timer overflows. Can you suggest how i can write that in MPLAB C18 please? I assume interrupts will be involved. \$\endgroup\$ – DixonLamont Apr 19 '14 at 12:08

Timer 0 set up for a "delay" of 1 second sounds implausible unless you are using a very slow clock.

They way I usually create slow clock ticks (like 1 second) is to use timer 2 with its built-in period register to make a 1 ms clock tick. That is still "slow" relative to how fast the PIC can run. If the PIC is running at 10 MHz instruction rate, then the 1 ms interrupt will occur once every 10,000 insruction, which means it takes a tiny fraction of the processor's overal time. Then you count multiple 1 ms periods in firmware to make various other clock ticks.

For example, you might count 100 of those to make a 100 ms clock tick, then 10 of those to make a 1 second clock tick. Often several different internal clock tick rates are useful. If you truly only need 1 second, then set up timer 2 to the largest sub-multiple of 1 second worth of instruction cycles it can manage, then count in firmware from there.

  • \$\begingroup\$ I think i asked the wrong question. All i need to know is how to count internal clock edges for which ever timer module that gives me a number that will increase every 1 second or close to 1 second. Can you help? \$\endgroup\$ – DixonLamont Apr 18 '14 at 17:09
  • \$\begingroup\$ @Dixon: I can't understand what you are asking. "ever timer module that gives me a number that will increase every 1 second" is gibberish. \$\endgroup\$ – Olin Lathrop Apr 18 '14 at 17:28
  • \$\begingroup\$ Say if i wanted to count the pulses from the internal clock of the PICKit. How would i do it?. OR Imagine i have a square wave generator and i want to count how many cycles have passed \$\endgroup\$ – DixonLamont Apr 18 '14 at 18:00

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