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CDCI6214: Max CL requirement

Giacomo Gasparini
Prodigy 110 points
Part Number: CDCI6214

Hi,

in datasheet there is the following requirement:

CL Crystal load capacitance Using on-chip load capacitance. A supported crystal is within. 5 - 8 pF

We are using a CL(quartz)=12pF (pin-pin value) f=24MHz quartz (Abracon ABM8AIG series).

In order to know if it fits I need to know if the CL requirement from datasheet is "pin to GND"  (in this case CL(quartz)/2 is in the range) or "pin-pin" (in this case CL(quartz)/2 is outside range).

Thank you.

Giacomo.

  • 0
    TI__Mastermind 18070 points

    Hi Giacomo,

    The load cap is the equivalent capacitance (not single ended capacitance), so I think it's "pin-to-pin" by your definition.

    However, this line doesn't mean much. It just provides a conservative range where external load cap is not needed (only internal load cap is used). In fact, the internal load cap can be programmed from 3pF to 9.1pF, where the pin / parasitic capacitance is 3pF. In other words, you can still use a crystal with 12pF required load cap, just need to add some external load cap. 

    For example, if you set the internal load cap to 3pF (this means that the internal load cap is disabled, but all the parasitic capacitance adds up to 3pF), and assuming the PCB stray capacitance is 2pF, then you'll need no more than (12 - 2 - 3) * 2 = 14 pF on both ends of the XTAL. You may want to have it lower than 14pF, because then you can increase the programmable load cap if the frequency turns out to be too high.

    Regards,
    Hao