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TM4C1294NCPDT: External capacitance value for crystal in TM4C1294NCPDT

Pavithra R
Intellectual 445 points
Part Number: TM4C1294NCPDT

I am using TM4C1294NCPDTI3 in my custom board design. In this XOSC0 and XOSC1 uses a 32.768KHz crystal. We are using Abracon's ABS25-32.768KHZ-T(CL = 12.5pF,C0=1.35pF)

From datasheet, C1-C2 ranges between 12-24pF.

CPKG(nominal) = 0.5pF, CPCB(nominal) = 0.5 pF, CSHUNT(max) = 4pF

CL = (C1*C2)/(C1+C2) + CPKG + CPCB

Assuming C1=C2=C

C/2 = CL - (CPKG + CPCB + C0)

= 12.5 - (0.5 + 0.5 + 1.35)

= 10.15

Therefore C1=C2 = 20.3pF. (Use 22pF)

In Development Kit,(Document spmr241), crystal used is CMR200T-32.768KDZY-UT(CL=7pF, C0 = 1.35pF)

Assuming C1=C2=C

C/2 = CL - (CPKG + CPCB +C0)

= 7 - (0.5 + 0.5 + 1.35)

= 4.65

Therefore C1=C2=9.3pF. (But 12pF is used in DK)

The calculation for the DK does not account for C0,shunt capacitance.

Q1. Is the above calculations correct?

Q2. Why 12pF is used instead of 10pF in DK?

Q3. Shall i proceed with Cl1 = C2 = 22pF for ABS25-32.768KHZ-T?

  • 0
    TI__Guru**** 190406 points

    Hi,

    Pavithra R said:
    Q1. Is the above calculations correct?

     I have some difference in terms of the calculation.

    CL = (C1*C2)/(C1+C2) + CPKG + CPCB            --------------------- (1) given in the datasheet

    Assuming C1=C2=C

    C/2 = CL - (CPKG + CPCB)                                     ----------------------(2)

    Cshunt = CPKG + CPCB + C0                               ----------------------(3) given in the datasheet

    C/2 = CL - (Cshunt - C0)                                        -----------------------(4)

    C/2 = 12.5 - (4 - 1.35)  = 9.85                                -----------------------(5)

    C = 19.7pf

    Pavithra R said:

    Q2. Why 12pF is used instead of 10pF in DK?

    I don't really know since the board was designed a long time ago and I don't have the history why on was chosen over the other. My guess is the decision may have something to do with cost. I just check Mouser and the unit price for 10pf in bulk of 4000 units is $0.047 while the 12pf only costs $0.04. 

    Pavithra R said:
    Q3. Shall i proceed with Cl1 = C2 = 22pF for ABS25-32.768KHZ-T?

    I don't see a problem.