• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 82 subscribers
  • Views 742 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMS320F28035: how to select CL1 and CL2 for external crystal?

Aki Li
Genius 9855 points
Part Number: TMS320F28035


As shown in F2803x datasheet, the typical specifications for CL1 and CL2 are 12pF for 20MHz.

There is a note that "CL1 and CL2 are the total capacitance of the circuit board and components excluding the IC and crystal. The value is usually approximately twice the value of the crystal's load capacitance."

So, does it mean that the crystal's load capacitance should be about 6pF? How much should the value of CL1 and CL2 be?

Customer wants to use a crystal with load capacitance CL of 8~12pF. Is it a suitable one?

  • 0
    TI__Expert 8105 points

    Hi Aki,

    I am in the process of confirming the answer to your question, will get back to you first thing tomorrow.

    Regards,
    Peter

  • 0
    TI__Mastermind 28275 points

    Aki,

    CL1 and CL2 are in series so provided CL1=CL2, the effective load capacitance is (CL1)/2. So yes, you should target a crystal with approximately 6pF load capacitance requirement. If the CL requirement of the crystal is not matched closely with CL1 and CL2, you will see a ppm shift in the crystal frequency.

  • 0 in reply to Frank Ankapong
    TI__Genius 9855 points

    Hi Frank,

    Thank you. So, how much is the capacitor value of CL1 and CL2 we should use in this case? 6pF?  

  • 0 in reply to Aki Li
    TI__Mastermind 28275 points

    Aki,

    No, you can't use CL1/CL2 of 6pF which would require a crystal with a CL spec of 3pF. Besides, CL1/CL2 of 6pF is below our minimum CL1/CL2 of 12pF. You need to meet the load capacitance requirement of our embedded electrical oscillator and also that of the crystal manufacturer. I'll give you 3 examples assuming a 20MHz crystal is used.

    Example 1:

    ---------------

    i. Crystal CL spec = 15pF.

    ii. You use CL1/CL2 = 30pF because CL1 and CL2 are in series so (CL1)/2 = 15pF.

    With this example, you have met the crystal CL spec but you are violating our CL1/CL2 spec of 12pF. You are outside our specification by 18pF.

    Example 2:

    ---------------

    i. Crystal CL spec = 15pF

    ii. You use CL1/CL2 = 12pF.

    With this example, you have met our CL1/CL2 spec of 12pF but are violating the crystal CL spec of 15pF. You are off by 9pF because of 15pF - (12pF)/2.

    Example 3:

    ---------------

    i. Crystal CL spec = 6pF

    ii. You use CL1/CL2 = 12pF.

    With this example, you have met both the crystal CL spec of 6pF and our CL1/CL2 spec of 12pF.

    Let us know if this is still not clear.

  • 0 in reply to Frank Ankapong
    TI__Genius 9855 points

    Frank, 

    Thank you for the detailed explanation! It is clear now.

  • 0 in reply to Aki Li
    TI__Genius 9855 points

    Frank,

    One more question about the ppm shift. If not matched,will it be a stable shift, or a dynamic shift during operation?

  • 0 in reply to Aki Li
    TI__Mastermind 28275 points

    Aki,

    The ppm shift will be a stable shift but the crystal manufacturer should know best. When a crystal manufacturer specifies their ppm, that ppm spec is usually coupled with a specific load capacitance or range of load capacitances.

    From what i know, ppm shift with load capacitance is a very small variation. However, if you are using a crystal, you were probably looking for the smallest ppm possible. If the ppm shift is an area of concern, you should ask the crystal manufacturer to provide you a ppm vs load capacitance plot.