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TMS570LS3137: and TMS570LS0332: Oscillator input impedance

Felix Risch25
Intellectual 1110 points
Part Number: TMS570LS3137
Other Parts Discussed in Thread: TMS570LS0332

Hi team,

my customer wants to supply the TMS570LS3137 and TMS570LS0332 with the same clock source. The 16MHz Clock is able to drive loads with: R>10k and C<15pF. The Clock is in between both Microcontrollers with a distance of 1.5cm each.

The customer wants to know the input impedance of the OSCIN Pin to calculate if the clock source can drive both oscillator input pins.

Thank you in advance and kind regards,

Felix

  • 0
    TI__Guru 59540 points
    Hello Felix,

    As far as I am aware, this hasn't been done before.

    In regard to the OSCIN Input Impedance, this is not a value that we have characterized as well so we wont necessarily have data on it. I will check with our Clock expert that has a deep understanding of the device clocks including the OSC circuit to get their inputs and get back to you.
  • 0 in reply to Chuck Davenport
    TI__Intellectual 1110 points
    Hi Chuck,

    thank you for your reply. Did you get the chance to contact your Clock expert? My customer needs an answer as soon as possbile.

    Thank you in advance and kind regards,
    Felix
  • 0 in reply to Felix Risch25
    TI__Guru 59540 points

    Hello Felix,

    Below is the response from our OSC circuit expert and general clock expert.

    "            I assume that this is a canned 3.3V oscillator which drives a square wave and leaves OSCOUT open. 

                The input impedance of OSCIN is very high; I do not know what the input capacitance is, but it is pad capacitance only. The intentional capacitance of the crystals are typically in the 5 – 20pF range, so a pad capacitance in the range of 15pF would dominate the intentional capacitance. The pad capacitance should be much smaller (and more or less process independent so that a simple test will reliably tell whether driving both together will work).

                 I would not hesitate to do this. Waveforms on canned oscillators are typically square waves. The oscillator works with a sine wave for a crystal (e.g. a strongly degraded square wave), so this is another check – how square is the waveform?"

    Please let me know if this helps or if additional information is needed. As noted previously and in the response from the expert, we do not have characterization data in order to specify a value on the input impedance but can make the general statement that it is very high.