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TDC7200: START and STOP pins connected to the same wire

Agnelo Silva
Prodigy 230 points
Part Number: TDC7200
Other Parts Discussed in Thread: TDC1000,

I would like to know if I can use TDC7200 to time-measure the period between the rising and falling of a pulse. I mean, assume that this signal is properly conditioned and sent to the both START and STOP pins. Also, I define "rising" config for START, "falling" for STOP, and a 1 STOP pulse. Would it work?

  • 0
    TI__Mastermind 31655 points
    Hello,

    It is not recommended to have free-running inputs directly connected to the START and STOP pins. This can violate the timing requirements of the TDC7200 and result in dropped pulses and bad measurements. If you would like to use a single free-running signal, the best method is to use a MUX that can be controlled to allow the signal through with the correct timing.

    To answer the second question, yes, you can define a rising edge trigger for START and a falling edge trigger for STOP.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 230 points

    Hi Scott,

    thank you for your attention.

    I had not mentioned some details of my application:

    1) I want to use mode measurement #1 only (12ns to 500ns). The width of this pulse will vary from 30ns to 450ns.

    2) The mentioned single "wire" comes from a conditioning circuit which provides a very stable and well-formed 2-level pulse. 

    3) Sharp transition high-low or low-high: < 1ns.

    At the datasheet of TDC7200 we have:

    T1STARTSTOP_Min   Minimum Time between Start and Stop Signal          12 ns
    T1STOPSTOP_Min    Minimum Time between 2 Stop Signals                 67 ns
    T1STARTSTOP_Max   Maximum time bet. Start and Stop Signal            500 ns
    T1STOPSTOP_Max    Maximum time bet. Start and last Stop Signal       500 ns

    TRST, TFST Maximum rise, fall time for START, STOP signals (20%-80%)   1 ns

    Therefore, I believe my application can potentially work fine, unless I am missing something else. What do you think?

    Regards,

    Agnelo

  • 0 in reply to Agnelo Silva
    TI__Mastermind 31655 points
    Hi Agnelo,

    Yes, in this case if you have a single pulse it will work just as you expect. Just make sure that the START does not come earlier than 5ns after the TRIGGER is input to the TDC1000.

    It is only with continuous periodic signals that trouble can arise.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 230 points

    Thank you for the information. I just confirmed the results with TDC7200EVM

    I performed a test with a 100kHz pulse signal (trail/lead-time 19.5ns, 3.3V_high, 0V_low) connected to both START and STOP connectors. I varied the duty cycle of the pulse to get the time between the rising and the falling of the pulse.

    The accuracy was awesome. I configured Meas. Mode 2 with START:rising edge and STOP:falling edge. However, few errors (in fact, huge outliers) were observed (less than 1% of the time) which is not a problem for me since my application can easily identify and filter such outliers. Even an ordinary statistical filter based on mode can be efficiently employed (i.e., averaging only the readings that are close to the identified mode).

    When I tried tests with 1MHz pulse, I had problems with both Mode 1 and Mode 2. Using Mode2, it is still possible to "subtract" readings and get the expected pulse width time, but the errors happen a lot. I will try to invest more time here because my target range is 12ns..100ns, that is, Mode 1 operation only.

    Thanks.