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LMX2594: Reference Clock High/Low Thresholds and SYNC timing

Josh Merti
Prodigy 190 points

Part Number: LMX2594
Other Parts Discussed in Thread: LMX2694-EP

In order to use the device in Category 3 SYNC the SYNC pulse needs to arrive "away from the rising edge of the OSCin signal."  I had interpreted that to mean it needed to arrive after the hold time (2ns) once a OSCin rising edge was detected.  At what OSCin voltage level does the LMX register a rising edge?  Is my 2ns interpretation accurate?  If not, what is the required SYNC timing "away from the rising edge of the OSCin signal" and what would the backoff need to be from the next OSCin rising edge?

  • 0
    TI__Mastermind 32152 points

    Hello Josh, 
    Refer to snippet regarding min and max values for Sync pin: 

    For example the way this works is if you have a 100MHz signal a valid PSYNC window is 10ns - 2.5ns (setup time) -2ns (hold time)  = 5.5ns

    Your interpretation is correct as this should occur after the rising edge is detected. 

    Regards, 

    Vicente 

  • 0 in reply to vicente flores prado
    Prodigy 190 points

    Thank you for the reply and confirming my timing was correct.  I'm still looking for the level where the rising edge of the OSCin clock is detected.  I don't know where to reference the hold and setup times.  For example, if the input were a 100 MHz 1.5Vp-p sine wave at what voltage would the LMX determine a rising edge of the OSCin clock had occurred?

  • 0 in reply to Josh Merti
    TI__Guru** 119780 points

    Hi Josh,

    Here is the timing diagram, hope it helps. You can find this diagram in LMX2694-EP datasheet.

  • 0
    Prodigy 190 points

    The timing diagram is what I expect but there is just a vertical line in the approximate middle of the OSCIN waveform that is used to determine the times.  I'm looking for where that threshold is.  Since the reference input can be different levels at the input is it 50% of the Vp-p of OSCIN?  I don't know what threshold on the OSCIN clock to use to calculate the setup and hold times (50% of Vp-p? A specific voltage level?  % of Vrms? Vmax?).

  • 0 in reply to Josh Merti
    TI__Guru** 119780 points

    Hi Josh,

    Yes, 50% of OSCin and 50% of SYNC/SYSREFREQ.