Timers acting weirdly

Question

• PIC24FJ1024GB610 from Microchip

• Debugger Pikit On Board from Explorer 16/32 Development Board from Microchip

I wrote a program, where I tried to create a 250 ms Delay using Timer 1, and whenever the Timer 1 Interrupt Flag IFS0bits.T1IF is set, the pin connected to the LED PORTAbits.RA7 / LATAbits.LATA7 will toggle.

this is the following code:

#include <xc.h>

#pragma config FWDTEN = OFF //watchdog timer off
#pragma config FNOSC = FRC //Osc source from FRC (8MHz)
#pragma config ICS = PGD2

#define LED LATAbits.LATA7
#define TMR1_IF IFS0bits.T1IF
#define GLOBAL_INT INTCON2bits.GIE 
#define TRIS_LED TRISAbits.TRISA7

void Set_LED (void)
{
    TRISAbits.TRISA7 = 0;//output
    ANSAbits.ANSA7 = 0;//Digital Not Analogue
    LED = 0;  
}

void Set_Timer1 (void)
{
    T1CONbits.TCS = 1;//Clock Source internal from Tcy 
    T1CONbits.TCKPS = 2;//1:64
    T1CONbits.TSIDL = 0;//continues through idle mode

    /*
     Fosc = 8MHz, Fosc/2 = Fcy = 4MHz
     Tcy = 1/Fcy = 1/4 us = 0.25 us
     t = 250 ms = 250,000 us
     t = Tcy*(PRESCALAR)*PR1
     PR1 = t/(Tcy*PRESCALAR)
     PR1 = 250,000 / (0.25*64)
     PR1 = 15625
    */

    PR1 = 15625;

   T1CONbits.TON = 1;//run timer
}

int main (void)
{
    Set_LED ();//Set LEDs
    Set_Timer1 ();//Set Timers


    while(1)
    {
        while(!TMR1_IF);//stay till IFS0bits.T1IF = 0

        LED = !LED;//Toggle LED / LATAbits.LATA7

        TMR1_IF = 0; //Clear Interrupt Flag

    }

    return 0;
}

I run the program into the PIC24, and I expect the LED to blink very fast but instead, it takes (approximately) 40 sec to toggle the LED.

This confuses me, because I checked OSC Datasheet page, and the Timer 1 Datasheet page from the PIC24FJ1024GB610 Datasheet, it seems fine, and I should expect a 250 ms toggle.

Looking at the DS, this is what I deducted:

• FNOSC = FRC hence source is FRC (Fosc = 8MHz)
• Fosc gets divided by 2, insctruction frequence (4MHz)
• Register associated with DOZE is CLKDIV
• CLKDIVbits.DOZEN = 0, CPU Peripheral Clock ratio is 1:1, hence Fcy = 4MHz
• Tcy = 1/Fcy, Tcy = 1/4 us = 0.25 us

• T1CONbits.TCS = 1 Clock Source is internal from FRC Oscillator
  • Clock Source is internal Tcy which in our case is 0.25 us
  • No Gate operation
• T1CONbits.TCKPS = 0b10 Prescalar is 1:64, (each 64 Clock Source tick will increment TMR1 register by 1
• if TMR1 == PR1,T1IFwill be set. TMR1 counts till it is equal to the value in PR1

giving the information I have to make a 250 ms (250,000 us) time delay, I used to following equation:

• t = Tcy*(PRE)*(PR1)
• 250,000 = 0.25*64*(PR1)
• PR1 = 250,000/(0.25*64)
• PR1 = 15625

I thought that it is straight forward, can someone help me with my question.

Thank you in advance.

Answer 1

You are clocking the timer with SOSC, check your T1CON bits again.

TCS should be 0.