External timer with higher frequency (MHz)

Hi,

If I understand your code logic correctly, you increment g_event by 1 when your Timer_A counts to 1. This means for 2 clock edges of TACLK you increment g_event by 1. You use Timer_B to create a 1 second timer. At the end of 1 second you multiply g_event by 2 to directly get the frequency.

A possible issue with your code could be that the TimerA interrupt you receive are a bit too quick for the CPU. For a 20KHz clock, you should receive the interrupt every 100us (50us period X 2). When you are running the CPU at almost 1MHz(slightly more than that), this means you can execute less than 100 instructions before the next interrupt occurs. I am saying less than 100 instructions because all MSP430 instructions are not single cycle. Plus you are in LPM0, and it will take a few microseconds before the CPU starts running at 1MHz. Please note that you are using "long" variables arithmetic, which may take up quite a few CPU cycles.

One possible problem could be the TIMERB1_VECTOR. You are doing a "long" addition and then an "int" division to perform the "%" operation. If you are in that ISR and do not complete it within 100us then you will miss an interrupt and two events. Can you check how much cycles does your TIMERB1_VECTOR ISR take ? If it is more than 100 or so then you have a problem.

To solve this problem, you can increase the value of TACCR0 so that TIMERA0_VECTOR interrupts occur after longer durations. For example we can set TACCR0 to 99 so that the interruption interval becomes 50us X 100 = 5millisecond.  At the end of 1 second we take the value of g_event, which increments by 1 on 100 events and the current value of Timer_A from TAR register. Next we clear both g_event & TAR register.

See if this code helps:

TACCR0 = 99;    // Counter counts up to 99...


#pragma vector=TIMERA0_VECTOR
__interrupt void Timer_A (void)
{
  
    g_event++;    // Now g_event++ means 100 clock cycles...
}

#pragma vector=TIMERB0_VECTOR
__interrupt void Timer_B0(void)
{
    static long sqNo = 1;
    static long preMsec;
    long diffMsec;

    if(sqNo == 17) {
        diffMsec = g_msecCount - preMsec;
        g_secCount++;       // every 1 sec
        LED_TOGGLE;
        sqNo = 1;
      
        preMsec = g_msecCount;
       
    list[g_idx++] = g_event * 100 + TAR;    // Apart from g_event we also take current Timer_A value from TAR register...

    TAR = 0;    // Clear the counter...

   g_event = 0; // Clear the events as well...

        if(g_idx > 10) {
            _NOP();
            return;
        }
    } 

    sqNo++;
      
}

You will notice that you can increase the value of TACCR0 even further, to say 999 so that you get an event every 50millisecond. If you increase it sufficiently you will actually not even need g_event for frequencies up to 60Khz or so. If you set TACCR0 to 65000 then you will have to provide 65Khz signal for the counter to reach 65000 in 1 second.

Regards,

Praval