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32kHz Stability; CC2541

John Pelochino
Prodigy 125 points
Other Parts Discussed in Thread: CC2541

We are having some 32kHz stability issues on our implementation with the CC2541. Our circuit is heavily based on the CC2541 keyfob circuit, so we're not doing anything out of the ordinary. 

(1) Have others experiences stability issues?

(2) Can we you an oscillator instead of a crystal? I believe I read no, but can't track down where I read that, so wanted to see if anyone else has tried this -> an oscillator will be more stable than using a crystal.

We have played with the crystal part number and the load capacitors, but the stability hasn't been improved. When I blow air (from my lungs) on the crystal, the oscillation gets crazy and the code crashes (crashes when using low-power modes -> same behavior seen in normal power mode, but obviously doesn't crash because system isn't using the 32khz). My thoughts are that this is due to heating from my warm breath causing the resonant frequency to change quickly, but this is just speculation. I am not seeing this same behavior on the EM board. Any thoughts on this would be very much appreciated.

Thanks - John

  • 0
    Guru 22270 points

    Hello John,

    Your blowing on the circuit board and its crazy response brought back some old memories.  I had that same problem with a thermocouple circuit I designed.  At fist I thought it was somehow related to my bad breath affecting the Type J thermocouple, but then I realized it was happening when the air reached the circuit board and not the T/C.  Turns out a good washing of the circuit board to remove the solder flux residue fixed the problem.

    Clean your board thoroughly and then check the oscillator again.

    This thread may also be helpful. http://e2e.ti.com/support/low_power_rf/f/538/t/258034.aspx

    Thanks,

  • 0 in reply to Greenja
    Prodigy 125 points

    Thanks for the response!

    Unfortunately, a board cleaning didn't help the instability. Was a great suggestion. I think our board house did a pretty good job already. 

    And thanks for sending over the other thread as well. Was helpful to read through because I think we might be in a similar boat with crystal and cap selection.

    We have ordered some alternative crystals and some additional load capacitors to test out to see if it makes any difference. I am going to try supplying a stable signal from an oscillator to see if the source of the instability is elsewhere (although I'm pretty convinced based on watching the oscilloscope and blowing on the crystal).