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Measuring the exact period through eCAP

Kummi
Mastermind 6220 points


Hi,

We are using eCAP module in F28069 to measure a frequency of 50Hz.
But we are not getting the exact frequency 50Hz, but we get 49.5Hz
and in case of 60Hz we get 59.4Hz.. the eCAP7's count is around 8000 more
counts than the expected count. (CPU runs at 60MHz)

I would like to know is it possible to measure the exact period through eCAP?
and do we always need to add correction values to the eCAP counts in order to get the exact count?

I couldn't find details about this in the eCAP reference guide below.
www.ti.com/.../spru807b.pdf

Best Regards
Kummi

  • 0
    Expert 1500 points

    Are you getting stable value continuous, though it is 8000 count more?

    Or are you getting a count +/- 8000? 

    Case 1 could be result of crystal/ main clock inaccuracy.

    Case 2: Check the input signal, does it have a clean rising edge? Using a schmitt trigger can improve the waveform.

  • 0 in reply to Joy Philipz
    TI__Guru 57255 points

    I agree with Joy on both notes. 

    Just one additional comment: if you're using the F28069 controlCARD for your evaluation, it has a very precise crystal populated on it.  However, it is not used in most of our software - instead the less precise/accurate internal oscillator is used.  A few tweaks in software would allow you to use the crystal instead and experiment with how it effects your results.


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Mastermind 6220 points
    Hi Joy, Brett,

    Thank you for the information.
    I tried on the F2806x LaunchPad and got the right output.
    As per your suggestions something might be wrong the custom board
    will cross check the board.

    Best Regards
    Kummi