Crystals troubleshooting

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  • So, I'm having issues with my crystal not working..

    Here's the actual schematic and crystal:

    XRCGB26M000F3M00R0: Crystal 26.0000MHz, 30ppm, 6pF, 150 Ohm

    I'm pretty sure that I used the exact same model (on crystal and caps) that I found in a reference design for the micro (cc430f5137).

    My oscilloscope does not see any frequency for the crystal.

    A couple of questions:

    1. Can one measure from ground to one of the outputs, or will the probe need to be between the xtal in/out?
    2. When the datasheet says floating, would connecting it to ground inhibit the crystal?
  • I'd connect the probe between GND and xout (xout is amplified by the CPU so won't matter as much). If the crystal is on the edge of working adding a probe might kill it - but then if it's running well enough you'd be fine.

    I'd say that yes, connecting to ground might cause problems. I haven't seen that before, but I'd guess it isn't supposed to be grounded.

    Also, my impression was that the capacitance needed for the crystal is the roughly the load capacitance minus the board capacitance of maybe 2pF. 27pF looks way too high, and it seems you'd be better using something nearer 4pF.

    It's why I gave up with the Pico and used a resonator that has the caps inside it :)

    It's a total black art (at least to me) but often you can try different values and see how strong/well shaped the oscillations look on the scope.

  • Right now I have zero oscillations :( It's just flat.

    Do you think I could use another Espruino as the clock somehow? I really need to find some way of hacking this together just to check that my RF balun works. Hehe. Nothing much going on with the radio as it's not clocked...

    So, you're recommending that I change to an oscillator?
    Maybe this:Ā­.pdf

    How would one wire to XIN and XOUT if using an oscillator (or external MCU / freq gen - if this is even possible)?

    Pulling my hair out!

  • I'm not sure about 'oscillators' - what I used was a resonator, which included the load caps (but nothing else). Having said that, it shouldn't be that tricky to get what you have going as-is.

    You might be able to poke some settings to enable the STM32's clock output on Espruino (but it's only 8Mhz) - you'd have to look at the Datasheet.

    But this isn't connected to an STM32 at all though... it's a TI part?

    You know you probably need firmware on it that specifically enables the oscillator? The one on the STM32 doesn't do anything until the Espruino firmware turns it on.

  • Thanks @Gordon
    I'll spend time on this tomorrow and get it ticking. It's TI CC430 indeed. The only connection it has to the Espruino is on Serial1. The crystal is used for the RF part (CC1101 equiv) of the mCU.

    It's programmed with a slightly modified Dash7 stack. I'll try to use the vanilla one just to be sure. It is configured for an external crystal, at least I'm told. Probably a good idea to double check this as well.

    Hope to get it resolved tomorrow :)

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Crystals troubleshooting

Posted by Avatar for alexanderbrevig @alexanderbrevig