• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 507 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.

LF crystal and HF crystal questions

Amit Benjamin
Expert 6890 points
Other Parts Discussed in Thread: TPS65920, OMAP3515

Hi Team,

need some help with the below questions:

I have some questions related to the 32KHz crystal and HFCLKIIN oscillator requirements for the TPS65920 and for the OMAP3515/30.

32KHz crystal

 

  1. According to the crystal requirements from the TPS65920 datasheet it needs crystal shunt capacitance of max 1pF. This requirement is hard to  achieve (most of the crystals have typical 1pF shunt capacitance (1.5pF max). My question is if this requirement (1pF max) is essential or I can use crystal with typical 1pF shunt capacitance (1.5pF max)?
  2. Load capacitance: I don't understand the demand on the datasheet. At the crystals datasheet this parameter is 12.5pF max (or 9pF max) is it enough?
  3. At the TPS65920 datasheet table 11-4 (attached to this mail) there are some requirements that I don't understand and no crystal datasheet has such characteristics (pin to pin capacitance, input voltage, internal capacitor on each input, parallel input capacitance). Can you tell me how can I handle such requirements?

 

 

HFCLKIN : we want to use 19.2Mhz oscillator.

At the OMAP3515/30 datasheet there is a 3.6ns max requirement for the rise/fall time. At the TPS65920 datasheet the requirement is 5ns max.

In our design the OMAP will get the fast clock from the PMIC (that supply 2.6ns max rise/fall time at is output (according to table 11-12 at the TPS65920 datasheet).

My question is if I can use 5ns oscillator for the PMIC and rely that the PMIC will provide the OMAP with 2.6ns rise/fall time?

 

 

  • 0
    TI__Genius 15100 points

    Hi Amit,

    1. The table shows crystal characteristics. In your post above you have mentioned 1pF, it is the same value as shown in the table.

    2. If the crystal datasheet shows 12.5pF then you can use the formula for the final load capacitance value. Using the formula you will have 14pF, you can use standard capacitance close to this value.

    3. These parameters are for the device side.

    TPS65920/30/50 simply pass the clock from input to output (if it is sine input then the slicer will give a square output). The internal circuitry does not change any properties on the HFCLKIN. So, you can concentrate on the OMAP requirements.

     

    Hope this helps.

     

  • 0 in reply to Gandhar Dighe
    TI__Expert 6890 points

    Hi Gandhar,

    1. question was if it possible to use 1pF  typical and 1.5pF max instead of 1pF max? customer says it is hard to find these1Pf MAX.
    2. OK
    3. what do you mean by “device side”? does it have nothing to do with the crystal choosing?
    4. HFCLKIN – means that the customer need to choose a 3.6ns max requirement for the rise/fall time, right?

     

    Thanks

    Amit

  • 0 in reply to Amit Benjamin
    TI__Genius 15100 points

    Here is a typical crystal for this purpose - FC-135 http://www.epsontoyocom.co.jp/english/product/Crystal/set01/fc135/index.html

    The table is a mix of external crystal and some internal parameters (like the internal capacitance, parallel input capacitance).

    For HFCLKIN - yes, you are correct.