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about 32MHz crystal suitable load capacitance for CC2520

darthvader7174
Prodigy 250 points
Other Parts Discussed in Thread: CC2520

hi,

I found a very small nice 32MHz crystal with 8pF load capacitance and I want to use it for my new CC2520 design.

Then I got an idea maybe this crystal may not need two external load capacitors anymore, because according to datasheet of CC2520, load capacitance = tune capacitance + parasitic capacitance + external loading capacitance, then if we assume 2pF as parasitic capacitance and 6pF as tune capacitance, then external capacitors are not needed anymore.

Is this way ok?

I know that 8pF means this crystal has higher sensitivity on environmental change, but it's 2*1.6mm small.

  • 0
    TI__Prodigy 70 points

    There are a couple of things to be aware of here:

    • The reset value of the internal tuning caps is 0pF. I.e. if the CC2520 is reset, the oscillator will stop, and won't start again untill max tune cap is programmed.
    • If you have a very low load capacitor, it's not only frequency stability that suffers. Oscillation margin could also be compromised, if the shunt capacitance becomes significant (see e.g. paper by E. Vittoz, JSSC vol 23, no 3, jun 1998, for the mathematical proof of this). I.e, if you have a low load capacitance (e.g. 6pF), along with a shunt capacitance of a few pFs, and a crystal with high ESR, the oscillator won't start.

    It would be nice to hear how it works out.

  • 0 in reply to PKR
    Prodigy 250 points

    ok, then maybe I should add these two load capacitors with each one 6.8pF or 8.2pF, just to be save.

  • 0 in reply to darthvader7174
    TI__Prodigy 70 points

    If it's an 8pF crystal, then the load caps need to be about 15pF each.

  • 0 in reply to PKR
    Prodigy 250 points

    well, we also need to add the parasitic capacitance of about 2pF to the load capacitance. So then I can put maximum 12pF for these two external capacitors, and of course this is not good for frequency tunning, so I think 8.2pF should be fine.

  • 0 in reply to darthvader7174
    Expert 1240 points

    If the design is still in the prototype stage, there is no harm in including the capacitor footprints on the board.  Whether they need to be populated or not, and with what exact value can be determined at a later time.  It is better to have the pads on the board and not need them, than to need them and not have them.

  • 0 in reply to sparkchaser
    Expert 1835 points

    Having too little load capacitance, can keep the oscillator from starting all together. Also, less capacitance can lead to frequency instability and the oscillator being affected by hand capacitance and nearby metal objects. I believe you are better with a little too much, rather than too little. After spending 3 days chasing a problem that was due to the series capacitors being too small,I can safely say..."don't".

    Ron

  • 0 in reply to ronnym
    Prodigy 250 points

    yes, I will put two 0402 pad there to be safe.

     

    Thanks, guys