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TMS320F280021: External Oscillator Specs Question

Dominik Lorenz25
Prodigy 140 points
Part Number: TMS320F280021
Other Parts Discussed in Thread: LAUNCHXL-F280025C

Hello Ti,

quick question bc I'm just choosing an external oscillator for my application featuring the TMS320F280021:

In the Datasheet under point 7.11.3.4.1 ("Crystal Oscillator Parameters") it is stated that CL1, CL2 Load capacitance of the crystal must be between 12pf and 24pF. Here I wonder if or why you specify the CL1 and CL2 (assuming that they refer to the 2 Capacitors in Figure 7-13) instead of the capacitance parameter of the crystal itself. In my understanding, between the 2 capacitors connected to the cystal and the load capcitance of the crystal lies a factor 2 (see e.g. https://www.crystek.com/documents/appnotes/PierceGateLoadCap.pdf). That would mean that if CL1 and CL2 is between 12pF and 24pF, the load capacitance of the Crystal would have to be between 6pF and 12pF.

Below, in Table 7-3, it is furthermore stated that CL1=CL2=12 or 24pF, indicating that you really mean the external capacitors with that range of 12pF-24pF.

Please tell me: does the Parameter of the Crystal have to be between 12pF and 24pF or do the external Capactors connected to it have to be between 12pF and 24pF. From my understanding, there is roughly a factor 2 between those definitions, but feel free to correct me on this.

Cheers, 

Dominik

  • 0
    TI__Mastermind 28275 points

    Hi Dominik,

    Your understanding is correct. CL1=CL2 = 12 to 24pF. From the perspective of the crystal, CL1 and CL2 are in series so the effective load capacitance seen by the crystal is CL1 divided by 2 provided CL1==CL2. The CL specified by the crystal manufacturer is usually the effective load capacitance and not the capacitance on each branch so you need to do the conversion. We specify the CL1 and CL2 separately because at the end of the day, you are installing individual capacitors and not an effective capacitor.

    We will add a note to clarify this in the next datasheet update. Let us know if this is still not clear.

  • 0 in reply to Frank Ankapong
    Prodigy 140 points

    Thank you for the clarfication and sorry for my late response.

    I want to mention that in the design of the Lauchpad for this Processor family (LAUNCHXL-F280025C), you used the ECS-200-18-30B-AGN-TR as the crystal that has a CL of 18pF for the CPU with CL1=CL2=15pF. As you confirmed my understanding with the conversion, I assume that the CL of the Crystal is technically not correctly chosen, altough its real-life effect is apparently small.

     

  • 0 in reply to Dominik Lorenz25
    TI__Mastermind 28275 points

    Hi Dominik,

    Your observation is correct. The crystal being used on the launch pad doesn't quite match the load cap. Nevertheless, load cap effect on the crystal is mostly "frequency pulling" where the frequency ppm will vary from the nominal. Usually this ppm variation is very small and mostly insignificant. I have forwarded your feedback to our EVM team.