• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 84 subscribers
  • Views 574 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMS320F28075: Using PWMxA&B with different frequency.

Alex Lifshitz
Intellectual 560 points

Part Number: TMS320F28075

Dear support,

I have 2 peripherals connected to the my TMS320F28075 to the same PWM (PWM11).

One to the A (gpio20).

One to the B (gpio21).

I need to configure them to different frequency (200Hz and 1000Hz). 

Is it possible? Both of them share the same TBPRD register...

By playing with the CMPA/B registers, I can set different duty cycle to A and B, but the frequency is common for both of them.

Thanks in advance,

Alex.

  • 0
    TI__Intellectual 1360 points

    Due to US holidays, you can expect a response by 3'rd Dec.

  • 0 in reply to Srinivasa Chakravarthy
    TI__Guru 52080 points

    Well obviously the easiest way would be to use two different ePWM modules (ePWM11 and ePWM10 for example).

    But for your specific setup, using one ePWM module's output A and B. You have to set the PWM frequency to 1000Hz (set the TBPRD accordingly) and then do the math to generate 200Hz signal every 5 PWM periods. This requires you to set up the ePWM interrupt and change your CMPx and action qualifiers when the interrupts occur.

    Nima

  • 0 in reply to Nima Eskandari
    Intellectual 560 points

    Thanks a lot for your answer. 

  • 0 in reply to Nima Eskandari
    Guru 56278 points

    Hi Nima,

    Typically it is ill advised to change any modules period while it is PWM'ing a device or at all as you described for a second option. How is the ePWM not prone to the same destructive mayhem that can occur to others by doing what you advised? 

    Seemingly your first answer of two generators would be more prudent and far less prone to electrical errors during duty cycle updates. 

  • 0 in reply to Genatco
    TI__Guru 52080 points

    On my answer for the second part, the PWM period is not changing (TBPRD). It is just selected to match the higher frequency PWM and then mathematically you can create lower frequency PWM waves, by selecting different actions every X number of PWM periods.

  • 0 in reply to Nima Eskandari
    Guru 56278 points

    Nima Eskandari said:
    by selecting different actions every X number of PWM periods.

    I still fail to see how the frequency can be made lower on PWM-B if the period has not been changed at the generator level. Other TI ePWM modules do not work well or even allow B side generator to have a different period load value than PWM- A, ignores the change or locks. Same issues occur for CMP-B for duty cycle updates, yet no such errata was ever listed. The match value loaded into CMP-B can not be significantly > than CMP-A or mayhem occurs.

    So the action qualifier allows independent generator frequency changes after the period initial setting without any mayhem? Changing CMP-B value only alters the duty cycle in other TI ePWM modules and not the period.

    How can AQ of CMP-B modify generator B period for 200Hz without a new count load value input as generator B period? Seeminly the AQ can only change the duty cycle, not the frequency (period).

  • 0 in reply to Genatco
    TI__Guru 52080 points

    Okay here is what I'm saying you can do:

    TBPRD = 1000

Output A:

CTR=ZERO, Clear the output LOW

CTR=CMPA, Set the output HIGH

Now you have your high frequency signal



Output B:

software_register_keeping_track_of_how_many_periods = 0;

__interrupt EPWM1()

{

	software_register_keeping_track_of_how_many_periods ++;

	if (software_register_keeping_track_of_how_many_periods == 10)

	{

		software_register_keeping_track_of_how_many_periods = 0;

		//Update Actions

		ZERO :

		CMPB : 

		PRD :

	}

	else if (if (software_register_keeping_track_of_how_many_periods == SOME_OTHER_NUMBER_BETWEEN_0_AND_10)

	{

		//Update Actions

		ZERO :

		CMPB : 

		PRD :
	}

	else

	{

		//Update Actions

		ZERO :

		CMPB : 

		PRD :

	}

}

    This gives you technically 10 times lower frequency.