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TMS320F280039C: PWM skip when TBPHS value is less than CMPA

Luiz Ramelan1
Prodigy 115 points
Part Number: TMS320F280039C

Hi Champ,

I would like to seek your advice regarding an issue I am currently facing. My application utilizes two PWM modules, PWM1 and PWM2. Phase is enabled on PWM2, with PWM1 as SYNCIN (master).

The problem arises when the phase is slowly descending near the CMPA value. I have observed that PWM2 skips for one cycle. Based on some posts I found here, the suggested solution is to handle the phase update by changing the CMPA value to be less than TBPHS in one cycle and then changing back the CMPA to the normal value in the next cycle.

However, this method(*) does not resolve the issue in my case. Are there any specific settings or handlers that I need to add?

I have attached my code here for your reference.

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void PWM_init(void){
EPwm1Regs.TBPRD = pwm_prd;
EPwm1Regs.TBPHS.bit.TBPHS = 0; // PWM-1 serves as reference
EPwm1Regs.CMPA.bit.CMPA = pwm_cmpa;
EPwm1Regs.CMPB.bit.CMPB = 0;
EPwm1Regs.TBCTL.bit.CLKDIV = TB_DIV1;
EPwm1Regs.TBCTL.bit.HSPCLKDIV = TB_DIV1; //change to sysclk
EPwm1Regs.TBCTL.bit.CTRMODE = TB_COUNT_UPDOWN; // Symmetrical mode
EPwm1Regs.TBCTL.bit.PRDLD = TB_SHADOW;
EPwm1Regs.EPWMSYNCOUTEN.bit.ZEROEN = 1;
EPwm1Regs.EPWMSYNCINSEL.bit.SEL = SYNC_IN_SRC_DISABLE_ALL;
EPwm1Regs.CMPCTL.bit.SHDWAMODE = CC_SHADOW;
EPwm1Regs.CMPCTL.bit.SHDWBMODE = CC_SHADOW;
EPwm1Regs.CMPCTL.bit.LOADAMODE = CC_CTR_ZERO;
EPwm1Regs.CMPCTL.bit.LOADBMODE = CC_CTR_ZERO;
EPwm1Regs.AQCTLA.bit.CAD = AQ_SET; // Active high
EPwm1Regs.AQCTLA.bit.CAU = AQ_CLEAR;
EPwm1Regs.AQCTLB.bit.CAD = AQ_CLEAR; // Active LOW
EPwm1Regs.AQCTLB.bit.CAU = AQ_SET;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

E2E references(*)

https://e2e.ti.com/support/microcontrollers/c2000-microcontrollers-group/c2000/f/c2000-microcontrollers-forum/1163045/tms320f28069-epwm-miss-cycle-while-tbphs-change?tisearch=e2e-sitesearch&keymatch=tbphs%2525252525252520cmpa%2525252525252520pwm%2525252525252520missed#

https://e2e.ti.com/support/microcontrollers/c2000-microcontrollers-group/c2000/f/c2000-microcontrollers-forum/1161072/tms320f28033-when-tbphs-load-value-is-less-than-cmpa-value-the-below-figure-will-the-aq-action-be-lost

  • 0
    TI__Genius 9835 points

    Hello Luiz,

    Yes the general idea is to modify the compare values such that when we modify the timebase counter value at syncin event with the PHS values the compare event is not missed.

    Based on the code you posted here, EPWM2 is set at count down CMPA value and cleared at count-up CMPA value. Can you tell which compare event is missed set or clear in your case ? Since you are using Active -high complementary CMPB values wont matter.


    In your case there is possibility that both set or clear events are missed depending on your phs value and loading time. For example  in up count is phs is set to value higher then cmpa it could miss clear/set-low event if timebase counter for epwm2 is less than cmpa when syncronization happens. Same for set-high at down count cmpa value but that you have already accounted for in PWMUpdate() function.

    Sequence for the phase_update and PWMUpdate would also matter. Also if you can share the snapshot where the compare event is missed on sync-in based phase loading ?

    Thanks.

  • 0 in reply to Prarthan Bhatt
    Prodigy 115 points

    Hi Prarthan,

    Thank you for your swift response.

    Prarthan Bhatt said:
    Yes the general idea is to modify the compare values such that when we modify the timebase counter value at syncin event with the PHS values the compare event is not missed.

    Understand.

    Prarthan Bhatt said:
    ased on the code you posted here, EPWM2 is set at count down CMPA value and cleared at count-up CMPA value. Can you tell which compare event is missed set or clear in your case ?
    The missed event happens when PHS is changed from 340 to 339. CMPA default value is 340, while the default PRD is 681.
    The scope capture reveals that the missed event at PWM2_CTR count-down, where PWM2A is expected to be cleared, and PWM2B is expected to be set.
    Prarthan Bhatt said:
    Sequence for the phase_update and PWMUpdate would also matter
    Phase_update is called every 10ms, it won't update PHS immediately. PHS will be updated in PWMUpdate, which is executed in adc interrupt. ADC interrupt is triggered by the PWM1 PRD event.
    Given that the missed event happens when PHS changes from 340 to 339. I am using the following approach:
    1. PHS will not be loaded immediately if target (pwm_phs <= CMPA) and previous (pwm_phs_prev >= CMPA).
    2. PWM values are then updated in three sequences:
      1. First, reduce CMPA value to be lower than target phase. CMPA = pwm_phs - 1.
      2. Second, set PHS to target phase. TBPHS = pwm_phs.
      3. Finally, set back CMPA to normal values. CMPA = (EPwm2Regs.TBPRD >> 1);
    This procedure requires three interrupts to update the PHS. While this approach helps avoid most PWM skip events, approximately 30% of skip events still occur.

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    void PWMUpdate(void)
    {
    if (skip_pwm_flag == 1) {
    EPwm2Regs.TBPHS.bit.TBPHS = pwm_phs;
    pwm_phs_prev = EPwm2Regs.TBPHS.bit.TBPHS;
    skip_pwm_flag++;
    } else if (skip_pwm_flag == 2) {
    EPwm2Regs.CMPA.bit.CMPA = (EPwm2Regs.TBPRD >> 1);
    skip_pwm_flag = 0;
    }
    if (update_request == 1) {
    update_request = 0;
    if (pwm_phs_prev >= EPwm2Regs.CMPA.bit.CMPA && pwm_phs <= EPwm2Regs.CMPA.bit.CMPA) {
    EPwm2Regs.CMPA.bit.CMPA = pwm_phs - 1;
    skip_pwm_flag = 1;
    } else {
    EPwm2Regs.TBPHS.bit.TBPHS = pwm_phs;
    pwm_phs_prev = EPwm2Regs.TBPHS.bit.TBPHS;
    }
    }
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Prarthan Bhatt said:
    For example  in up count is phs is set to value higher then cmpa it could miss clear/set-low event if timebase counter for epwm2 is less than cmpa when syncronization happens.
    Just to confirm, the synchronization here refers to PWM1 CTR = ZERO, right? Based on the PWM settings, as follows
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    EPwm1Regs.EPWMSYNCOUTEN.bit.ZEROEN = 1;
    EPwm1Regs.EPWMSYNCINSEL.bit.SEL = SYNC_IN_SRC_DISABLE_ALL;
    EPwm2Regs.EPWMSYNCINSEL.bit.SEL = SYNC_IN_SRC_SYNCOUT_EPWM1;
    EPwm2Regs.CMPCTL.bit.SHDWAMODE = CC_IMMEDIATE;
    EPwm2Regs.CMPCTL.bit.SHDWBMODE = CC_IMMEDIATE;
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    is there any additional setting required? apart from the attached setting.

    Best regards,
    Luiz

  • 0
    TI__Genius 9835 points
    Luiz Ramelan1 said:
    • First, reduce CMPA value to be lower than target phase. CMPA = pwm_phs - 1.
    • Second, set PHS to target phase. TBPHS = pwm_phs.

    If CMPA is updated before phase is written it should be okay. At zero sync-in event phase is loaded automatically how are you delaying the phase load event?

    Main part is sequence timing for counter zero sync-in event and CMPA value update. Can you describe more on that?

  • 0 in reply to Prarthan Bhatt
    Prodigy 115 points

    Hi Prarthan,

    Luiz Ramelan1 said:
    • First, reduce CMPA value to be lower than target phase. CMPA = pwm_phs - 1.
    • Second, set PHS to target phase. TBPHS = pwm_phs.
    • Finally, set back CMPA to normal values. CMPA = (EPwm2Regs.TBPRD >> 1);

    The sequence is executed in the ADC interrupt which is triggered by the PWM1 PRD event.

    After the update request is received, in the next interrupt CMPA is updated. Then PHS is updated only in the next following interrupt.

  • +1 in reply to Luiz Ramelan1
    TI__Genius 9835 points

    2nd event change phs how are you providing synchronization event. phs is loaded to counter only on synchronization event.

     

    Luiz Ramelan1 said:
    After the update request is received, in the next interrupt CMPA is updated. Then PHS is updated only in the next following interrupt.

    When does these 3 event interrupts occur ? at pwm1 prd event.

    Luiz Ramelan1 said:
    The sequence is executed in the ADC interrupt which is triggered by the PWM1 PRD event

    Do you mean to say the three interrupts occur within ADC interrupt ?

    Thanks,
    Prarthan.

  • 0 in reply to Prarthan Bhatt
    Prodigy 115 points

    Hi Prarthan,


    Prarthan Bhatt said:
    When does these 3 event interrupts occur ? at pwm1 prd event.

    Yes,  at pwm prd event as shown by the pic.

    Prarthan Bhatt said:
    Do you mean to say the three interrupts occur within ADC interrupt

    No, one interrupt per A/D Conversion. but there's an interval (1x interrupt) between setting the CMPA and PHS. They are not loaded at the same time.

    I did not see the pwm skip, when setting the interval to 2x interrupt between setting the CMPA and PHS. I guess 1x interrupt is not enough based on my config.


    Thank you for your support,

    Luiz