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AM335x OSC0 Crystal Circuit Load Capacitance

Daisuke Maeda
Guru 20205 points

Our customer uses the crystal oscillator specifying 10pF load capacitance (CL).

The capacitance of C1 and C2 matched with the crystal circuit by the crystal manufacturer is 10pF. This is beyond the recommended value in the datasheet. Is that allowed?

Best regards,

Daisuke

 

  • 0
    TI__Guru 62845 points
    If the crystal was designed for a load capacitance CL of 10pF, then the value of C1 and C2 would need to be larger than 10pF. The formula for calculating CL is [(C1*C2)/(C1+C2)] + Cshunt, where Cshunt is the crystal shunt capacitance (C0) specified by the crystal manufacturer plus any mutual capacitance (Cpkg + CPCB) seen across the AM335x XTALIN and XTALOUT signals. If you assume Cshunt is very small, capacitors C1 and C2 should be 20pF to load the crystal with 10pF.
  • 0 in reply to peaves
    Guru 20205 points

    Hi Paul,

    Thank you for your reply.

    If Cshunt is 5pF, the value of C1 and C2 can be 10pF. Is that not allowed?

    Should the value of C1 and C2 not be smaller than 12pF? Should Cshunt not be larger than 4pF if the crystal is designed for a load capacitance CL of 10pF?

    Best regards,

    Daisuke

     

  • 0 in reply to Daisuke Maeda
    TI__Guru 62845 points
    If Cshunt is 5pF, the value of C1 and C2 can be 10pF. Is that not allowed? The min value for C1 and C2 is 12pF if Cshunt is less than or equal to 5pF and 18pF is Cshunt is greater than 5pF. So this is not possible when CL must be 10pF.

    Should the value of C1 and C2 not be smaller than 12pF? Yes, when Cshunt is less than or equal to 5pF.

    Should Cshunt not be larger than 4pF if the crystal is designed for a load capacitance CL of 10pF? Yes, this is correct.

    Regards,
    Paul
  • 0 in reply to peaves
    Guru 20205 points

    Hi Paul,

    Thank you for your reply.

    Even if the requirements are met for the crystal of 24MHz, should the requirements of C1 and C2 capacitance and ESR be met for the OSC0? If it is not met, what issue can there be?

    Best regards,

    Daisuke

     

  • 0 in reply to Daisuke Maeda
    TI__Guru 62845 points
    Yes.

    I'm not a oscillator expert, but know a few potential problems. The crystal circuit may not start oscillation. If it starts, the frequency of oscillation may be pulled a few PPM from the expected frequency. The crystal could be over-driven which can cause premature failure. There may be other issues.

    Regards,
    Paul
  • 0 in reply to peaves
    Guru 20205 points

    Hi Paul,

    Thank you for your reply.

    Best regards,

    Daisuke