Question

I am trying to figure out how to create a timer for my C8051F020 MCU. The following code uses the value passed to init_Timer2() with the following formula:

65535-(0.1 / (12/2000000)=48868.

I set up the timer to count every time it executes and for every 10 counts, count one second. This is based on the above formula. 48868 when passed to init_Timer2 will produce a 0.1 second delay. It would take ten of them per second. However, when I test the timer it is a little fast. At ten seconds the timer reports 11 seconds, at 20 seconds the timer reports 22 seconds. I would like to get as close to a perfect second as I can.

Here is my code:

#include <compiler_defs.h>
#include <C8051F020_defs.h>

void init_Clock(void);
void init_Watchdog(void);
void init_Ports(void);
void init_Timer2(unsigned int counts);
void start_Timer2(void);
void timer2_ISR(void);

unsigned int timer2_Count;
unsigned int seconds;
unsigned int minutes;

int main(void)
{
    init_Clock();
    init_Watchdog();
    init_Ports();
    start_Timer2();

    P5 &= 0xFF;

    while (1);
}

//=============================================================
//Functions
//=============================================================
void init_Clock(void)
{
    OSCICN = 0x04;  //2Mhz
    //OSCICN = 0x07;    //16Mhz
}

void init_Watchdog(void)
{
    //Disable watchdog timer
    WDTCN = 0xDE;
    WDTCN = 0xAD;
}

void init_Ports(void)
{
    XBR0    = 0x00;
    XBR1    = 0x00;
    XBR2    = 0x40;

    P0      = 0x00;
    P0MDOUT = 0x00;
    P5      = 0x00; //Set P5 to 1111
    P74OUT  = 0x08; //Set P5 4 - 7 (LEDs) to push pull (Output)
}

void init_Timer2(unsigned int counts)
{
    CKCON = 0x00;   //Set all timers to system clock divided by 12

    T2CON = 0x00;   //Set timer 2 to timer mode

    RCAP2 = counts; 

    T2 = 0xFFFF;    //655535

    IE |= 0x20;     //Enable timer 2

    T2CON |= 0x04;  //Start timer 2
}

void start_Timer2(void)
{
    EA = 0;

    init_Timer2(48868);

    EA = 1;
}

void timer2_ISR(void) interrupt 5
{    
    T2CON &= ~(0x80);

        P5 ^= 0xF0;

        timer2_Count++;

        if(timer2_Count % 10 == 0)
        {
            seconds++;
        }

        if(seconds % 60 == 0  && seconds != 0)
        {
            minutes++;
        }
}

Answer 1

I cannot take full credit for answering this question. Everyone contributed. I ended up using the external clock at 22.1184Mhz. The following snippets of code are the sections of the original post code that has changed to make it work:

void init_Clock(void)
{
    OSCXCN = 0x67;  //External 22.1184Mhz

    while ( !(OSCXCN & 0x80) );

    OSCICN = 0x88;
}

and

void start_Timer2(void)
{
    EA = 0;

    //External clock
    init_Timer2(47103);

    EA = 1;
}

From my tests this is very accurate.