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Original question:

AM3358: Frequency tolerance and frequency stability of 24Mhz Crystal

AM3358: Frequency accuracy

Kanae
Guru 10225 points

Part Number: AM3358

Tool/software:

Hi Support Team,

My customer decided to switch to custom board oscillators after referencing the following thread
e2e.ti.com/.../

The oscillator is to be changed to the following experienced oscillator.
download.epsondevice.com/.../SG-210STF_ja.pdf

So, I have a question.
The following thread says below, but depending on how it is used,
will it require a clock with a higher precision than the “±50ppm” specification?

e2e.ti.com/.../am3358-frequency-tolerance-and-frequency- stability-of-24mhz-crystal/3841749?

The accuracy of the AM335x reference clock source should be based on system level requirements.
For example, you may need a 30 PPM reference clock
if an AM335x timer is being used to operate something that needs this level of accuracy.

Where is the required accuracy listed in the technical documentation?
Also, please let us know if there are any other design considerations we should keep in mind.

Best Regards,
Kanae

  • 0
    TI__Guru 63605 points

    Crystals are defined to have an initial frequency accuracy at room temperature. They should also provide another parameter that defines frequency deviation due to temperature change relative to room temperature. They typically provide another parameter that defines a frequency change due to aging.

    You must combine all three contributors of frequency error to determine the maximum frequency deviation expected across operating conditions for the life of the product.

    I have included a couple of links to crystal datasheets that you can use as examples.

    ABM11W-25.0000MHZ-8-D1X-T3: https://abracon.com/Resonators/ABM11W.pdf

    ABM10W-25.0000MHZ-8-D1X-T3: https://abracon.com/Resonators/ABM10W.pdf

    Keep in mind the crystal doesn't dissipate much power, so there is no significant self-heating. The operating temperature of the crystal is typically the same temperature as the PCB board. Therefore, a crystal rated for 85 degrees Celsius may be good enough for most applications.

    Regards,
    Paul

  • 0 in reply to peaves
    Guru 10225 points

    Hi Paul,

    Thank you for your reply.
    The datasheet states the following, but there is a note in 1),
    which means that the maximum value is not necessarily 50ppm, correct?


    (1) Initial accuracy, temperature drift, and aging effects should be combined
         when evaluating a reference clock for this requirement.

    I will share the above answer with our customer.

    Best Regards,
    Kanae

  • 0 in reply to Kanae
    TI__Guru 63605 points

    The maximum frequency error allowed by the AM335x device is 50PPM and you must include all of the contributors when selecting a crystal. In most cases you will find it is necessary to select a 20PPM or 30PPM crystal so the remaining 30PPM or 20PPM allows enough margin for temperature variation and aging.

    Regards,
    Paul 

  • 0 in reply to peaves
    Guru 10225 points

    Hi Paul,

    Thank you for your support.

    Please reply to the additional confirmation from our customer.

    Q1. We will put the following oscillator on board this time.
     Do we still need to select 30ppm product?
     https://download.epsondevice.com/td/pdf/brief/SG-210STF_ja.pdf

     If it is LVCMOS, can it be said that 50ppm products are acceptable
     because they are more stable than crystal?
     The oscillator we have already been using is a 50ppm product,
     so we are confirming this.

    Q2: What is the reason for requiring high accuracy like 30ppm or 50ppm,
     such as being aware of RTC clock deviations?

    Best Regards,
    Kanae

  • 0 in reply to Kanae
    TI__Guru 63605 points

    The oscillator manufacture would need to answer the first question. Does the PPM error defined in the oscillator datasheet include initial accuracy, temperature variation, and aging?

    Note: Some oscillators only implement a crystal circuit, oscillator, and output buffer while other oscillators may also include a PLL. We do not recommend using an oscillator with a PLL output because the PLLs inside AM335x were not evaluated with a PLL based clock source that may have more jitter that a basic oscillator that doesn't use a PLL. We expect a low jitter clock source like what you get from a basic crystal circuit and oscillator without a PLL.

    The 50PPM requirement comes from the RMII and RGMII Ethernet standards.

    Regards,
    Paul