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TMS570LS1114: TMS570LS11 xtal oscillator

Tamas Fenyvesi
Prodigy 180 points
Part Number: TMS570LS1114


TMS570LS1114 datasheet encourages to send a sample to crystal vendors for validation. On the other hand, it says nothing on e.g. recommended crystal load capacitance.

Could you, please, tell me transconductane “gm” min/typ values of the oscillator of this microcontroller? This would help in sizing the oscillator for worst case tolerances – which is not the case with measuring a sample piece. On the other hand, "gm" should be known or easy to characterize for TI, I'd guess. (You must know section 3.4 of ST Micro’s AN2867 about xtal oscillators. I'd like to use those equations, but this input parameter is missing.)

Thanks

  • 0
    TI__Guru 59540 points
    Hello Tamas,

    I will check with our oscillator expert on this.
  • 0 in reply to Chuck Davenport
    TI__Guru 59540 points
    Hello Tamas,

    I have discussed this with our oscillator circuit expert and they have confirmed that we do not have the characterization of 'gm' available and there are no plans for any additional effort in this area. Therefore, I recommend that you work with your crystal manufacturer to have the characterization completed as noted in the datasheet. This is the methodology that we have recommended over the prior 20+ years and have not ran into any issues with the effectiveness of this practice. the crystal manufacturers are generally capable of this activity and it is a normal practice for them.
  • 0 in reply to Chuck Davenport
    Prodigy 180 points

    Hi Chuck,

    OK, I send one or two pieces to them, and they measure it. What about tolerances (chip side)?

    BR,

    Tamas

  • 0 in reply to Tamas Fenyvesi
    TI__Guru 59540 points
    Hello Tamas,

    Can you explain what you are looking for about the tolerances on the chip side? There are some variations introduced by the PLL in the form of Jitter but over time these average out and are not generally considered significant even when looked at with respect to cycle to cycle accuracy. Our on-chip LPOs have ranges as specified in the datasheet and the OSCIN circuit is not a significant source of error as it is predominantly a pass through element. In most cases, our customers use standard automotive grade crystals with a accuracy measured in ppm. I recall that they usually are specified as 3-5 ppm. Any related tolerances/specifications of the clock signals will be found in the datasheet. If there is something not present, we have not characterized it and will not be able to provide more detailed information.
  • 0 in reply to Chuck Davenport
    Prodigy 180 points

    Hi Chuck,

    I thought of a more basic thing than PLL jitter or crystal pull. An oscillator has certain requirements on crystal and capacitor selection for safe start and oscillation. The suggested (20+ years old) process, according to the datasheet, is to send some sample to crystal manufacturers, and they'll do the job. This job is to determine two parameters for crystal type selections: OSCIN/OSCOUT pin capacitances, and the the crystal type range in terms of load capacitance.

    Pin capacitances should be obvious. Anyway, neither the datasheet or the IBIS files include sufficient data on that. (IBIS files writes something, but it seems to me unrealistic. It seems to be only package capacitance rather than total pin capacitance together with silicon).

    On the other hand, crystal type (load capacitance) range that the oscillator can drive is connected to transconductance (gm) of the oscillator's inverter. (There's a simple equation between gm and suitable load capacitances.) Transconductance usually has a rather high tolerance.

    That's why I disprefer to characterize these chips based one or two samples pieces. We don't know, where those one or two pieces are between min/typ/max values.

    But TI certainly can obtain some min/typ values based on either many pieces' experience or simulation data.

    Really not a big deal: please, provide some realistic pin capacitance, and preferably some info on either transconductance (which is very much a silicon side parameter) or applicable crystal capacitances.

    BR

    Tamas