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Coldfire V1 Reference Trim Accuracy
Robert R. Feb 3, 2016 at 01:15 PM (13:15 hours)
Staff: Johnny N.

  • I was wondering if you could tell more more information about how the internal reference frequency is trimmed and what the expected accuracy is.  I am trimming the internal reference to 31.25KHz on a Freescale MCF51JM device.  I would like to know so I can determine if I should implement a trim algorithm in my code to fine tune the accuracy of the reference.  Thank you,




    Comments

    • Robert,

      Our developers have determined from testing that we are able to generate a value that will trim the Internal Reference Clock to within +/- 0.2% of your desired frequency.

      The following FAQ has a sample assembly program that can be used to test the accuracy:
      https://www.pemicro.com/faqs/faq_view.cfm?id=123

      Regards,
      Johnny
      P&E Engineering Support

  • Greetings,

    Great question. We cannot tell you exactly how we calculate the trim because that is proprietary. However, our tools are within one fine trim value. As you may know, there is a TRIM register and another fine TRIM (FTRIM) bit. The Fine trim is like a half step to the next TRIM value.

    For example, say the value of $3D in TRIM and $0 in FTRIM is the exact values you need. Our tools will be within 1 fine trim of this value. So we may put in the value $3D and $1, or $3C and $1. Does that make sense to you? Hopefully that is enough information for you to determine how to best to take into account the trim accuracy. Depending on how you multiply and divide within the clock generator will determine how different the frequencies could be.


    Takao Yamada

  • Thank you very much.  I was not asking for your algorithm, just wanted to be sure you did not put a value in the trim that "should" result in a 31.25KHz frequency. Thank you also for the assembly link.  I can use that to verify the trim during test and possibly power up to be sure the flash value is not corrupt.

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