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CC2530: Crystal oscillator pin capacitance

John Man
Prodigy 100 points
Part Number: CC2530

Tool/software:

Dear sir,

Could you let me know the Crystal oscillator pin capacitance(Pin 22&23 and pin 32&33) of CC2530?

The CC2530 EVM board uses load capacitance of 12.5pF for 32.768KHz and the external capacitor connected is 15pF. To reach load capacitance of 12.5pF, there must be 10pF leftover on the oscillation pin respectively (let us ignore PCB trace parasitic capacitance at this moment).

And for 32MHz crystal oscillator, CC2530 EVM board selects load capacitance of 16pF and the external capacitor connected is 27pF. To reach load capacitance of 16pF, there must be 5pF leftover on the oscillation pin respectively (let us ignore PCB trace parasitic capacitance at this moment). 

Normally the oscillation pin capacitance of the MCU is in the range of 2~3pF, I can not imagine the Cpin can reach about 10pF. That is why I ask the pin capacitance of Pin 22&23 and pin 32&33 of CC2530.

Thanks

John

  • 0
    TI__Expert 6520 points

    Hi John,

    We don't specify the pin capacitance for this device.

    The capacitor selection is described in the data sheet as:

    So for the 32-MHz crystal you end up with 16pF = 13.5pF + C_parasitic so the assumed parasitic capacitance is 2.5pF. The capacitance is further tunable with an onboard capacitor array to further fine tune the frequency:

    For the 32kHz crystal you have 12.5pF = 7.5pF + C_parasitic so the assumed parasitic capacitance is 5pF. Admittedly this is a bit higher than I would expect, but not completely out of line. It is quite a large package with large terminals and the pins for the 32kHz crystal also support alternate functions so the capacitance is probably slightly higher than if the pins were dedicated to the clock functionality. The device was fully validated on this board with these values and I expect if you replicate the design that you will have good results. If you choose a different crystal and want to verify the frequency accuracy you can output the clock to one of the device pins and measure directly:

    Best Regards,

    Jake

  • 0 in reply to Jake Pulliam
    Prodigy 100 points

    Hi Mr. Jake,

    Thank you for your reply.

    I doubt the correctness of CL formula specified in the datasheet because all of the parasitic capacitor are in parallel with "load capacitor" connected between XOSC pin and GND. So for a CL of 16pF of 32MHz crustal oscillator connected with a 27pF capacitor, the leftover capacitance should be 5pF, not 2.5pF.

    Similar calculation for 32.768KHz as well. But for 32.768KHz, the leftover capacitance is much high as we all realized. 

    Could you let me know what the extra capacitance be added to XOSC pin if I tune the register FREQTUNE and how the extra capacitance associated with the counts [3:0]? 

    For 32MHz and 32.768KHz, I will use different setting on FREQTUNE register. And I also guess it is because this capacitor array setting that EVM board selects such a high load capacitor crystal oscillator for 32.768KHz.

    Thanks

    Have a nice Thanksgiving holiday

    John

  • 0 in reply to John Man
    TI__Expert 6520 points

    Hi John,

    The FREQTUNE register setting doesn't apply capacitance for the 32kHz crystal. 

    I don't have data on the equivalence between the register setting and the capacitance that is switched in. You can set the device to output an unmodulated CW and monitor the frequency when adjusting the register setting. This can easily be accomplished using SmartRF Studio 7.

    BR,

    Jake