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TMS320F28379D: Dual Axis Drive : Why is CMPA configured in shadow load mode and CMPB is not?

Arun Kumar
Expert 1030 points
Part Number: TMS320F28379D

Tool/software:

Hello experts,

In the dual axis drive project,Inside the PWM initialation function `HAL_setupMotorPWMs()`:

 Could you please tell me why is shadow load mode is enabled for Counter compare A only? Why is it not enabled for Counter compare B?

Is it not required as CPMA and CMPB are configured as complementary from the action qualifiers?

  • 0
    TI__Mastermind 42896 points

    Hi,

    The dead-band Submodule is configured such that PWMA and PWMB are complimentary outputs. So you'd just use CMPA in the SW.

    Best,

    Kevin

  • 0 in reply to Kevin Allen18
    Expert 1030 points

    Thanks for your reply Kevin.

    I am trying to configure a ePWM module to act as a brake chopper which is used to dessipates excess energy via a resistor during regenerative torque.

    CMPA and CMPB are independently configured.CMPA is used for Motor 1 and CMPB is used for Motor 2.

    In this case , Do you recomend enabling shadow load mode ?

    This is my initializer function:

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    // Sets up the PWMs (Pulse Width Modulators) for Brake chopper
    void HAL_setupBrCpPWMs(HAL_MTR_Handle handle)
    {
    HAL_MTR_Obj *obj = (HAL_MTR_Obj*) handle;
    //Configure clock
    EPWM_setClockPrescaler(obj->pwmHandle[3],
    EPWM_CLOCK_DIVIDER_1,
    EPWM_HSCLOCK_DIVIDER_2);
    EPWM_setTimeBaseCounterMode(obj->pwmHandle[3], EPWM_COUNTER_MODE_UP);
    //Set period(Switching frequency : 5 KHz)
    EPWM_setTimeBasePeriod(obj->pwmHandle[3], 10000U);
    EPWM_setPhaseShift(obj->pwmHandle[3], 0U);
    //Enable compare shadow load mode
    EPWM_setCounterCompareShadowLoadMode(obj->pwmHandle[3],
    EPWM_COUNTER_COMPARE_A,
    EPWM_COMP_LOAD_ON_CNTR_ZERO);
    EPWM_setCounterCompareShadowLoadMode(obj->pwmHandle[3],
    EPWM_COUNTER_COMPARE_B,
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Thanks,

    AK

  • 0 in reply to Arun Kumar
    TI__Mastermind 42896 points

    Hi AK,

    Arun Kumar said:
    CMPA and CMPB are independently configured.CMPA is used for Motor 1 and CMPB is used for Motor 2.

    Ok, you're also using ePWMx channel A for motor 1 and Channel B for motor 2 then?

    Arun Kumar said:
    In this case , Do you recomend enabling shadow load mode ?

    Shadow load mode will load the shadow CMPA/B register to the active register on CNTR_ZERO event, so ePWM CMPA/B updates are synchronized. You cna use this mode, unless you need immediate CMPx updates for your regenerative torque application.

    Best,

    Kevin

  • 0 in reply to Kevin Allen18
    Expert 1030 points
    Kevin Allen18 said:
    Ok, you're also using ePWMx channel A for motor 1 and Channel B for motor 2 then?

    Yes correct.

    Kevin Allen18 said:
    Shadow load mode will load the shadow CMPA/B register to the active register on CNTR_ZERO event, so ePWM CMPA/B updates are synchronized. You cna use this mode, unless you need immediate CMPx updates for your regenerative torque application.

    So if we take an example of shadow loading,

    If the time period(TBPRD) is 10000, and lets say at 

    TBCTR of 1000 , i write CMPA to 500, then at

    TBCTR of 5000 , i write CMPA to 800, then at

    TBCTR of 9000 , i write CMPA to 100, and then when it the counter rolls  over to zero

    TBCTR of 0 , CMPA value of 100 will be taken in?(and the previous values of 800, 500 will be ignored?)

  • 0 in reply to Arun Kumar
    TI__Mastermind 42896 points

    Hi AK,

    Yes, the behavior you described is correct. The newest register value you write will move from shadow to active register at the configured load event (TBPRS, ZERO, etc).

    Best,

    Kevin