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TM4C129XNCZAD: RTC drift over the time period

soumya bhat
Prodigy 60 points
Part Number: TM4C129XNCZAD

Hi,

We are using external crystal CM200C from citizen in our schematics which has a +/-20ppm  drift in frequency.

Can anyone please tell me how to calculate the drift in time with +/-20ppm. 

And also how to mitigate the drift in time.

  • 0
    TI__Guru**** 176826 points
    Hi,
    I think you will get the best answer from the crystal manufacturer. However, I find this forum that discusses the same topic.
    forums.parallax.com/.../timing-drift-how-do-you-read-ppm
  • 0 in reply to Charles Tsai
    Prodigy 60 points

    Thank you Charles.

    Is there any possibility of monitoring external oscillator frequency and adjust the drift in RTC time. However I found from the microcontroller datasheet that there is something called ' trim register' which is used to compensate for the drift in RTC.

  • 0 in reply to soumya bhat
    TI__Guru**** 176826 points
    Hi Soumya,
    The datasheet has good description on how to use the RTC trim value. Please refer to the 'RTC Trim' sub-section in the Hibernate Module chapter in the datasheet.
    Below post also answer how to adjust the trim values. Please take a look.
    e2e.ti.com/.../1337224
  • 0 in reply to Charles Tsai
    Guru 117855 points

    Unstated is the accuracy which you demand from this RTC implementation.       (i.e.  'What qualifies as, 'Good for Gov't Work?')

    While the stated 'RTC Trim' method (referenced) provides an 'adjustment capability' - you may further consider:

    • how does temperature - especially change of temperature (notably range) - impact that adjustment?
    • how does the MCU powering battery's 'change of voltage' - impact that adjustment?
    • how does (both) MCU & crystal (& xtal support component) 'aging' - impact that adjustment?

    You may note that when 'superb' frequency stability is demanded - extremely tight temperature & component specifications - are demanded.     (so much so - that often the components are entombed w/in a highly temperature regulated miniature 'oven' - which also provides great physical robustness for the well chosen components.)

    It is suspected that you may monitor the relative impact of those 3 main 'effects' listed (above) - and employ those findings to 'enhance' the MCU's 'RTC Trim' correction value...     (the MCU's method appears to be somewhat 'blind' to the cumulative impact of those 3 (predictable) 'Trim Challenges.')

    Sometimes - more precise and/or Real-Time  'aware'  devices - are able to regularly, 'Pass their superior Time Data' to your MCU - which nicely augments such 'MCU RTC Trim.'