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TDC7200: Trigger width during Start

user6411853
Prodigy 20 points
Part Number: TDC7200
Other Parts Discussed in Thread: TDC1000

Good morning.

I want to make TOF measurements on a pulse train, so that the first pulse should be treated as START and the rest as STOPs.

To ensure good timming, I was thinking of using a multiplexer and use the TRIGGER output as select bit. This way, when the device is ready to measure and TRIGGER is set high, the train is at the START pin. After the first (START) pulse, TRIGGER is set low, multiplexer changes its output pin and the rest of pulses in the train would be at the STOP pin.

I'm aware of the minimum 5ns from TRIGGER rising edge to the START rising edge. But I cannot find any information referring the time from START falling edge at its end and the TRIGGER falling edge (when TDC resets the pin). How much time would it be? Or could TDC reset TRIGGER before START pulse finishes? In that case, part of the START pulse will be seen at the STOP pin, and I don't want this at all.

  • 0
    TI__Mastermind 27780 points

    user6411853,

    The TRIGGER pin only responds to a low-to-high transition to begin the time-of-flight measurement. Toggling the trigger high-to-low at any point does not affect the START pulse in any manner. The TRIGGER can also remain high throughout the entire time-of-flight measurement, and will not affect either the START or STOP pulses.

  • 0 in reply to Akeem Whitehead
    Prodigy 20 points

    Thank you for your quick answer. But I think you didn't understand. I'd not externally toggle the trigger. The trigger is toggled internally by the TDC7200. The datasheet specifies:

    "8.4.6 Measurement Sequence"

    "3. After the start new measurement bit START_MEAS has been set in the CONFIG1 register, the TDC7200
    generates a trigger signal on the TRIGG pin..." (low to high)

    "5. After receiving a START, the TDC resets the TRIGG pin..." (high to low)

    My question is: when is the TRIGGER reset (high to low)? Yes, after receiving a START. But it could mean:

    1) AFTER the START rising edge and BEFORE the START falling edge

    2) AFTER the START rising edge and AFTER the START falling edge

    So, what is the TRIGGER reset delay from the START rising edge??

    This is the only picture in the datasheet where you can see what I mean (8.4.4 Multi-Cycle Averaging), but no timing information is provided.

  • 0 in reply to user6411853
    TI__Mastermind 27780 points

    user6411853,

    I initially failed to clarify that the TRIGGER pin response I was referring to was on the TDC1000. The TDC1000 is the AFE device the TDC7200 was made for and intended to be paired with. Because the TDC1000's TRIGGER pin ignores the high-to-low transition of the TRIGGER signal from the TDC7200, the timing spec you have requested was never characterized. Unfortunately, there is no plan for our validation team to characterize this at the moment, so you will need to characterize the TDC7200's high-to-low timing spec with regard to the START edges on your own.