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AM335x crystal issue

Hello, I've recently received a batch of prototype boards whose design I based off of the ICEv2 board and I am having trouble getting the 24MHz crystal to oscillate.  It will run for just a second or two when I apply power to the board and then it stops.  I've been playing with load capacitors, series resistance and feedback resistance with no luck.  Even tried a different crystal.  This is my first crack at a design with a TI processor, are there any gotchas that I may be overlooking?  To my understanding the crystal should fire right up when power is applied to the chip.

A couple of other notes, the NRESETIN_OUT output pin stays low, I am assuming that this is due to the crystal not running and not the other way around.  I also just found that I've pulled the RTC_PWRONRSTn pin up to 3.3V but the datasheet says it should be pulled up to 1.8V.  I hope I haven't damaged the chip with this as I haven't been able to find what the damage threshold is for that pin in the datasheet.

  • John Turner said:
     I also just found that I've pulled the RTC_PWRONRSTn pin up to 3.3V but the datasheet says it should be pulled up to 1.8V.  I hope I haven't damaged the chip with this as I haven't been able to find what the damage threshold is for that pin in the datasheet.

    RTC_PWRONRSTn pin is a 1.8V pin. If it has been pulled to 3.3V the device is definitely damaged. AM335x I/O pins are not fail-safe. See parameter "Steady state max voltage at all IO pins" in section 5.1 from the AM335x Datasheet, and Note 8 below the table.

  • Thank you for the response.  I modified a new board so that signal is pulled up to 1.8V rather than 3.3V.  Unfortunately still no oscillations from my crystal.  Any other thoughts as to what might be happening?

  • It is very unusual for the oscillator to stop after it starts. If either power supply has a problem, this could cause stop the oscillator to stop. Check the VDDS_OSC and VDD_CORE power supplies and see if anything is happening on these supplies when oscillation stops.

    How are you checking the oscillation? If you are using a scope probe that applies a DC offset to the crystal circuit it may cause this type of problem. Normally a low impedance scope probe will never allow the oscillator to start. Do you have the scope probe connected when applying power and see this effect? If so, what happens if you connect it later?

    Regards,
    Paul