CCS/TMS320C6748: How to obtain stable 10us interrupt

Thank you Sabin, it looks very useful. I'll have a try.

Hi Sahin,

        I have tested this approach. Most of the interruptions are timely.  Sometimes, however, it is still unstable.

The method provided by Graham has skipped the HWI DISPATCHER, I can't understand what causes the time jitter.

Anything wrong with the time test method?

Or, the debugger has an impact on testing?

Regards,

Dor

Hi Dor,

Are you disabling interrupts at any point in your application? This could be affecting your time cycle. 

It would be good to calculate the overhead of getting the timestamp, and subtract that from your calculation too.

Can you describe the behavior you're observing in more detail? Is there a pattern? How far off from the expected value?

Which version of SYSBIOS are you using?

Regards,
Sahin

Hi Sahin,

         I didn't disable the interrupt anywhere. The SYSBIOS version is 6.37.03.30. 

time_diff = TSCL - time_stamp;       // Tick count
time_stamp = TSCL;

time_diff_d = (double)time_diff / 456;  // Convert to us

if (time_diff_d < time_diff_min)       // Capture minimum
{
       time_diff_min = time_diff_d;
}
else if (time_diff_d > time_diff_max)      // Capture maximum
{
       time_diff_max = time_diff_d;
}

         Most of the time the "time_diff_d" is 10.  The minimum is 8, and the maximum is 12

By the way,  I didn't create any task. There is only one timer interrupt in this project.

If add the NDK and NSP, it's even worse.

Regards,

Dor

Would you be able to share more of your code?

Do you have any printfs in your code? These typically take a long time and may be impacting your timer. 

What does your ISR look like?

What frequency are you running the DSP?

Regards,
Sahin

I used CCS5.5 to create a typical SYSBIOS project with main.c. No other special configuration in app.cfg.
//---------------------------------------------------------------
// Here is the timer initialization code.

//---------------------------------------------------------------

Timer_Params timerParams;
Timer_Handle myTimer;
Error_Block eb;

Error_init(&eb);
Timer_Params_init(&timerParams);

timerParams.period = 10;
timerParams.periodType = Timer_PeriodType_MICROSECS;
timerParams.startMode = Timer_StartMode_AUTO;
timerParams.runMode = Timer_RunMode_CONTINUOUS;
timerParams.intNum = 4;

myTimer = Timer_create(1, TimerIsr, &timerParams, &eb);
if (myTimer == NULL)
{
    System_abort("Timer create failed");
}

//---------------------------------------------------------------

// Interrupt handler

//---------------------------------------------------------------

Void TimerIsr(UArg arg)
{
    time_diff = TSCL - time_stamp;    // Now ticks - previous
    time_stamp = TSCL;               // For next time

    time_diff_d = (double)time_diff / 456;   // Convert

    if (time_diff_d < time_diff_min)    // Get max and min value.
    {
        time_diff_min = time_diff_d;
    }
    else if (time_diff_d > time_diff_max)
    {
        time_diff_max = time_diff_d;
    }

    if (init < 65535)   // Delay
    {
        init++;
        time_diff_min = 10000;
        time_diff_max = 0;
    }
}

There are 2 tasks in this project. The code is as follows.

Void taskFxn(UArg a0, UArg a1)
{
    while(1)
    {
        Task_sleep(10);
    }
}

Here, the result.

The freqency is 456 Mhz. No printf or other BIG functions.

Regards,

Dor

Hello,

I don't see anything wrong with the code you provided. It should be firing an interrupt every 10us as specified. The only thing I can think of is if you're using breakpoints, or the calculations in the ISR are affecting the timer period. 

It may be worth trying to only toggle a GPIO in the ISR and probing the pin with a scope to verify the period.

Can you try with this sample code below and let me know if you are observing the same behavior?

/*
 *  ======== main.c ========
 */

#include <xdc/std.h>
#include <ti/sysbios/hal/Timer.h>
#include <xdc/runtime/Error.h>
#include <xdc/runtime/System.h>
#include <c6x.h>
#include <ti/sysbios/BIOS.h>
#include <ti/sysbios/knl/Task.h>

#include <time.h>
clock_t t_start, t_stop, t_diff;

Void TimerIsr(UArg arg)
{
    t_stop = _itoll(TSCH, TSCL);
    t_diff = t_stop - t_start;
    t_start = _itoll(TSCH, TSCL);

    System_printf("Time diff in cycles: %d\n", t_diff);

}
/*
 *  ======== taskFxn ========
 */

Void taskFxn(UArg a0, UArg a1)
{
    Error_Block eb;
    Timer_Params timerParams;
    Timer_Handle myTimer;

    Error_init(&eb);
    Timer_Params_init(&timerParams);

    timerParams.period = 10;
    timerParams.periodType = Timer_PeriodType_MICROSECS;
    timerParams.startMode = Timer_StartMode_AUTO;
    timerParams.runMode = Timer_RunMode_CONTINUOUS;

    myTimer = Timer_create(1, TimerIsr, &timerParams, &eb);
    if (myTimer == NULL)
    {
        System_abort("Timer create failed");
    }

    /* Initialize timer for clock */
    TSCL = 0, TSCH = 0;

    t_start = _itoll(TSCH, TSCL);

    while (1);
}

/*
 *  ======== main ========
 */
Int main()
{
    Task_Handle task;
    Error_Block eb;

    Error_init(&eb);
    task = Task_create(taskFxn, NULL, &eb);
    if (task == NULL)
    {
        System_printf("Task_create() failed!\n");
        BIOS_exit(0);
    }

    BIOS_start(); /* does not return */
    return (0);
}

Regards,
Sahin