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TMS570LS0432: Resonator Selection

Alex Terry
Intellectual 625 points
Part Number: TMS570LS0432
Other Parts Discussed in Thread: BQ76PL455A-Q1

Hi,

Im currently producing a board using the TMS570 to control a series of BQ76PL455A-Q1 IC's, is there a recommended resonator to use with this TMS570 as I cant find any recommended values of resonance, capacitance etc. in the datasheet?

Thanks,

Alex

  • 0
    TI__Guru**** 196816 points

    Hello Alex,

    We don't have recommendation for the resonator. The maximum frequency of oscillator is limited to 20MHz.

    TI strongly encourages each customer to submit samples of the device to the resonator/crystal

    vendors for validation. The vendors are equipped to determine what load capacitors will best tune

    their resonator/crystal to the microcontroller device for optimum start-up and operation over

    temperature/voltage extremes

  • 0 in reply to QJ Wang
    Intellectual 625 points
    Thanks QJ,

    do you know what the internal capacitance of the MCU? as i need to calculate which capacitors to use with a resonator as unfortunately I cannot afford to produce a board to submit to the manufacturer as a test.

    Thanks,
    Alex
  • 0 in reply to Alex Terry
    TI__Guru**** 196816 points

    Hello Alex,

    Correct operation of the crystal oscillator is dependent on the values of the two external capacitors, C1 and C2. These capacitors together with any parasitic capacitance in the PCB and the crystal terminals compose the total load capacitance seen by the crystal. The optimum load capacitance for the crystal, CL, is given in the crystal datasheet and C1 and C2 should be matched to this value according to the following formula:

    CL = (C1*C2)/(C1+C2)

  • 0 in reply to QJ Wang
    Intellectual 625 points
    Thanks QJ,
    helpful as always, have a good day!
  • 0 in reply to QJ Wang
    Prodigy 180 points
    Hi,

    Just a bit more precisely, C1 and C2 in that formula include pins' internal capacitance. Alex asked about these pin capacitances. FYI: chip internal capacitances are usually higher (4pF-6pF on other microcontrollers) than PCB stray capacitance. In other words they'll dominate over PCB design.
    The datasheet doesn't say anything on these capacitance values.

    Neither it says anything about recommended load capacitance (CL) range. We know the capacitance value that a given crystal requires. But how much load capacitance can TMS570's oscillator drive?

    Any relevant info on TMS570 oscillators besides there are two capacitors in series?
    Thanks,
    Tamas
  • 0 in reply to Tamas Fenyvesi
    TI__Guru**** 196816 points
    Hi Tamas,

    I am sorry we don't have those data.