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PWM frequency accuracy on TMS320F28335

Albert WOROU
Prodigy 140 points
Other Parts Discussed in Thread: TMS320F28335

I'm using the TMS320F28335 in a motor control application. I have a requirement to generate a 10kHz +/- 10Hz PWM. I know I can program the ePWM module for a nominal 10kHz pwm. How do I garanty that I meet the +/-10Hz? What datasheet parameter or combination of parameters do I take into account to calculate/estimate my generated PWM's frequency accuracy?

Best regards,

Albert.

  • 0
    TI__Guru** 118800 points

    10Hz is a much longer time than 10Khz (1000 times more longer to be exact), so the requirement makes no sense.

    Generating a 10Hz signal, at +/- 10KHz at least makes logical sense.

    Considering you have a 6.67ns clock, and can control a PWM edge to that duty cycle resolution, controlling to +/-100us (10kHz) is not an issue.

  • 0 in reply to ChrisClearman
    Prodigy 140 points

    Hi ChrisClearman,

    Thanks for taking the time to look into my post.

    My interpretation of the 10kHz+/-10Hz is a frequency fitting in the range (10000-10) = 9990Hz and (10000+10) = 10010Hz....(Considering 10kHz to be another way of writing 10000Hz)... Is there anything wrong with my interpretation or notation?

    No to go back to my initial question, what I want to know is whether there is any other source of error that should be taken into account other than the main clock that gets multiplied from 33.3330MHz to 150MHz by the PLL and then divided down to 10kHz through prescalers and period registers... With a 100ppm clock total error and working through my multiplication and division chain I end up with a +/-1Hz error on the 10kHz PWM frequency generated. Is there any other error source I need to take into account for the calculation? Which datasheet parameter should I consider?

    Best regards,

    Albert.

  • 0 in reply to Albert WOROU
    TI__Expert 5410 points

    Albert,

    the XTAL clock is one error (as you have already calculated). The other error is clock jitter. But this is in the order of 100s of picoseconds. I/O voltage variation can also affect clock edge jitter, but again, this factor is small.

    Cheers,

    Alex T.