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CC2640: capacitor values for the crystals / oscillators

Vincenzo Pizzolante
Mastermind 25316 points
Part Number: CC2640

hi,

a customer of mine is considering using the crystals:

Y1 -  Q0.032768-JTX310-12.5pF-20-T1  (32KHz)
C17, C18 22pF MLCC

Y2 Q24.0-JXS32-8-10/30-FU-LF  (24MHz)
C22 and C23 are 15 pF MLCC 

he also selected the capacitor C17 ... C23 as for the value above:

could you help me understand if these values are ok and/or how to calculate them?

thanks a lot in advance

KR

Vincenzo

  • 0
    TI__Guru** 106705 points

    Hi Vincenzo,

    You can read about how crystal load capacitors are dimensioned here: www.ti.com/.../swra372

    As the parasitic capacitance in the design plays in you will ultimately have to fine tune the values by measuring the actual frequency. For the 32 kHz I recommend outputting the clock on an IO (explained in the TRM, SWCU117, chapter 11) and measuring the frequency with a frequency counter. For the 24 MHz crystal I recommend using SmartRF Studio to output an RF carrier and measuring the frequency offset with a spectrum analyzer.

    Note that the 24 MHz crystal oscillator has built-in, adjustable, load capacitors so external caps are not required. The value of the internal load cap is set in CCFG as explained here: processors.wiki.ti.com/.../CC26xx_Tips_and_Tricks . Furthermore it is impossible to determine the crystal CL value from the part number of the 24 MHz crystal.

    The inital values for the load caps for the 32 kHz crystal look ok.

    Regards,
    Fredrik