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AM335x OSC1 LVCMOS Reference Clock Requirements

Matteo Geromin
Prodigy 200 points

Hello,


I want to use an oscillator instead of a crystal circuit for the RTC_CLOCK in an AM335x custom design, but I see that the LVCMOS oscillator rise and fall time requirements are very small (5ns max). Table 4-7 of the AM335x datasheet (SPRS717F)

I can't find an 32.768kHz oscillator with that characteristics. The best oscilllator I found has a rise-fall time of 10ns max.

Are these constraints right or is there a mistake as with the unit of the frequency in the same table?

Best regards

Matteo

  • 0
    TI__Guru**** 393215 points

    Hi Matteo,

    I will ask the factory team to check this, but you may need to insert a buffer (e.g. Schmitt trigger) to form the clock fronts.

  • 0 in reply to Biser Gatchev-XID
    TI__Guru 55265 points

    The value of 5 ns was used for this parameter because the fast slew rate makes it very difficult for external noise sources to couple into the reference clock sourcing AM335x.

    When using OSC1 with a 32.768 kHz crystal circuit the input signal will be mostly sinusoidal with slow slew rates. So it is possible for the AM335x to operate with slow slew rate clock sources, but a slow slew rate reference clock is more susceptible to clock glitches when exposed to high noise environments.

    A rise/fall of 5 ns has been proven to be robust.  There is a reasonable chance the oscillator with the 10 ns rise/fall output will not have any problems, but you should consider testing your product for noise immunity by exposing it to worst case operating conditions before releasing the final product to production.

    Regards,
    Paul