PWM Frequency Accuracy

Fulano de Tal

Expert 2090 points

I have F28069 running at 90MHz (I did change the PLL DIV to change from 80MHz default to 90MHz).

I have setup my PWMs (up-down count mode) for 16kHz.

I have setup TBPRD to be 2813 (2*2813/90MHz = 15997Hz) ... this is close enough for me, and I don't need to use HRPWM.

However, when measured on a scope, I am getting something slightly off, I am measuring 16.1kHz when I look at the PWMs. So I have to increase TBPRD to something like 2830 to measure close to 16.0kHz.

This is important because in my control computations (PID, etc.), I use the step time of 1/16kHz (62.5us) in my calculations and causes everything to be slightly off.

What is the source of this inaccuracy? I am thinking it may be SYSCLK, is there any way to verify and check what my SYSCLK is actually running at? How can I account for this correctly?

over 11 years ago

0 Gautam Iyer over 11 years ago

Guru 192875 points

Hi Fulano,

I have F28069 running at 90MHz (I did change the PLL DIV to change from 80MHz default to 90MHz).

You didn't mention about the board you're using; is it a TI dev board or a custom one?

Regards,

Gautam

0 Fulano de Tal over 11 years ago in reply to Gautam Iyer

Expert 2090 points

This is the exp kit dev board, not yet on a custom pcb.

0 Gautam Iyer over 11 years ago in reply to Fulano de Tal

Guru 192875 points

What is the source of this inaccuracy? I am thinking it may be SYSCLK, is there any way to verify and check what my SYSCLK is actually running at? How can I account for this correctly?

As you've mentioned above, SYSCLK is the responsible hardware for peripheral clock variations. Measuring SYSCLK woudn't be possible. Seems like CLK tolerances might be playing a major role here.

Regards,

Gautam

0 Mitja Nemec over 11 years ago in reply to Gautam Iyer

Genius 5885 points

You are probably using internal 10 MHz oscillator which is not really accurate.

0 Fulano de Tal over 11 years ago in reply to Mitja Nemec

Expert 2090 points

Yes, I am using internal oscillator but I expect for 16kHz PWM it should be more accurate. I could understand if clock tolerances came into play if I tried to output a 1MHz PWM but not something low frequency like 16kHz.

0 Brett Larimore over 11 years ago in reply to Mitja Nemec

TI__Guru 57255 points

Fulano de Tal,

Like you (and others) I would suspect SYSCLK tolerances to be your root cause.

You can bring SYSCLKOUT to a pin (GPIO18) by setting up the GPIO mux configuration in the right way. Note that depending on register configuration this may be some divided version of SYSCLK. By default, I believe that it will be SYSCLK/4.

If you plan to use the internal oscillator there are some tactics to improve tolerance such as the below appnote:
http://www.ti.com/lit/sprabi7
http://www.ti.com/lit/sprab84

Since you are using an Experimenter's Kit, you could also experiment with using the (highly-precise) crystal on the controlCARD.

Thank you,
Brett

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PWM Frequency Accuracy