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AM3352 -- RTC accuracy w/ external crystal

Rob Rodrigues
Mastermind 19780 points
Other Parts Discussed in Thread: AM3352

Hi Team,

I have an application where the precision of the RTC in AM3352 needs to be within +/- 5 ppm.

 

The TRM indicates that RTC has accuracy to 32-kHz per hour which translates to roughly 8.5 ppb.  The question is, will a high precision external crystal reduce that, or is the 335x's RTC limited to 8.5 ppb accuracy?

Is this accuracy achieved with the internal oscillator?  Or does this require an external 32kHz crystal to a certain spec?  (Like 32kHz +/- 5 ppm)

 

Regards,

  • 0
    TI__Guru**** 393215 points

    Hi Rob,

    The internal oscillator is not accurate. For best accuracy an external 32.768kHz crystal/oscillator should be used. Specs are given in sections 6.2.2.3 and 6.2.2.4 of the AM335X Datasheet, Rev. G.

  • 0
    Genius 3685 points

    If you want to have +/- 5ppm, a 32Khz crystal will not be sufficient. You will have to use an external oscillator which is specified with this tolerance. Remember you will have tolerances from temperature changes and from aging! And there will be small tolerances between each produced device.

    5ppm is a very tight requirement. Maybe it's better to install a ntp client on this device and get time information from a server.

  • 0 in reply to Wolfgang Muees1
    TI__Guru 55375 points

    Rob,

    Can you point me to the location in the TRM where you found the 32kHz per hour reference you mentioned in your original post?

    I think you may have misinterpreted the description of crystal compensation function provided in the RTC. The "32-kHz period accuracy per hour" reference in this section is not a statement describing the accuracy of the crystal clock source. This statement is describing the resolution of the RTC compensation function.

    When the RTC is being sourced from OSC1, the accuracy of the RTC is a function of the clock source connected to OSC1. OSC1 can be conneccted to a LVCMOS clock source or connected to a crystal circuit. The AM335x device does not effect accuracy of a LVCMOS clock source, but may effect the accuracy of a crystal circuit. This occurs becasue the accuracy of a crystal circuit is a function of loading applied by the crystal circuit. Since the oscillator in AM335x (OSC1) applies some paracitic loading to the crystal circuit it can effect the resonate frequency of the crystal.

    Crystals are cut to be resonate at the specified frequency with a specific capacitive load.  The actual oscillating frequency of the crystal will be pulled from the specified frequency if the capacitive load applied by the crystal circuit is not the value defined in the crystal data sheet. So it is very important to tightly control capacitive loading when trying to use a crystal circuit to achive a very accurate time base for the RTC.  So the overall accuracy of the RTC will be a function of the crystal you purchase and how tightly you control the capacitive loading of the crystal circuit.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Mastermind 19780 points

    Hi Paul,

     

    I did, in fact, misinterpret the compensation function as the internal oscillator accuracy which lead to the 8 ppb calculation.  I was suspicious as soon as I saw the accuracy was on par with some very tight references like rubidium!

     

    Thank you for explanation of the interaction between the crystal and the AM335x devices.

     

    Regards,