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CC2640: Crystal Oscillator and Crystal Selection for the CC2640

Sujata Dane
Intellectual 590 points
Part Number: CC2640


Hi all,

With reference to http://www.ti.com/lit/an/swra495e/swra495e.pdf

I have following questions. These all are basic understanding doubts. Could somebody please solve these.

1. For 24MHz crystal oscillator what is CL_INT means? Does it mean input/output capacitance of TI chip CC2640.

2.Document says the default setting of the internal capacitance is 9 pF, is it with/without external capacitors.

3.Can we use internal capacitor array when external loading capacitors are used? If not then what is the value of CL_INT (is it 2pf)

4. Could you please explain relation between CL_INT and CL (load capacitance)

5.To set the CL to 9pF without using external capacitors i.e. using the internal capacitor array what is the value of Cdelta

Thanks a lot in advance.

Cheers,

Sujata

  • 0
    TI__Guru** 106705 points
    Hi Sujata,

    1. CL_INT is the variable internal load capacitor array
    2. That is the default setting for the internal capacitor array
    3. There is no point in using external load capacitors
    4. CL_INT is internal load capacitors and CL is external caps
    5. The default setting is 9 pF, so if this is the value you need, no changes are required.

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    Intellectual 590 points

    Hi Fredrik,

    Thank you for your response. Still two things are not clear to me.

    1. With respect to question 4 and equation 14 of TI document

    i.e. CL_INT=9+Cdelta, in this 9 indicates which parameters? Does 9 corresponds to CL (acco. to Table 1,  CL is 9pf for 24MHz crystal).

    2. Regarding question 5, does the default setting is 9 for CL or is it for CL_INT

    Thank you very much.

    Cheers,

    Sujata

  • 0 in reply to Sujata Dane
    TI__Guru** 106705 points
    Hi Sujata,

    Table 1 only refers to the specific crystal used in TI reference designs. Other crystals, with other CL values can be used.

    In equation 4, "9" is the default setting for the internal capacitors, including parasitic capacitance on the board, pins, etc. Cdelta is the offset from the default value which can be positive or negative. You apply the delta value in CCFG according to table 3.

    In other words, if you choose a 24 MHz crystal with CL = 6 pF, you have to set the CCFG delta value to either -18 or -19.

    Cheers,
    Fredrik