frequency = [number of interrupts per second]

ticks = 0;

time = 0;   // Time in seconds



void UpdateClock()

{

    // Add one each time the procedure is called

    ticks++;



    if(ticks == frequency)

    {

        // Add one second to your system-time

        // Flush the tick-counter 

        time++:

        ticks = 0;

    }

}



void ChangeFrequency(new_freq)

{

    DisableInterrupts();



    // Convert the ticks that were received until now

    // freqA = ticksA/s -> s = ticksA/freqA

    // freqB = ticksB/s -> s = ticksB/freqB

    // ticksA/freqA = ticksB/freqB

    // ticksB = (ticksA/freqA)*freqB



    ticks = (ticks/frequency)*new_freq;    



    // Example:

    // freqA  = 10hz, freqB = 20hz

    // ticksA = 3

    // -> ticksB = (3/10)*20 = 6



    frequency = new_freq;



    Enable_Interrupts();

}