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TDC7200: Time1 register & Calibration Registers remain ZERO. Measurement doesn't occur.

Stewart Kushner
Prodigy 50 points
Part Number: TDC7200

Using TDC7200EVM.

Have my own working SPI interface.

After ENA transitions high, all registers at default.

Default corresponds to Mode1 ( < 500ns); No averaging; ONE Stop pulse; All Interrupts enabled.

After setting Start Bit over SPI, Trigger Pulse goes out. My Bench-Top Pattern Generator responds with a Start Pulse followed by a Stop Pulse, 450ns later.

INTB never goes low; All result registers remain 000000h. The Status Register remains 00h.

Nothing happens! (Only the Trigger returns to zero.)

My Start and Stop Pulses are 3.3V, 100ns wide, 10ns rise/fall.

WHAT IS WRONG? (no measurements!)

Thank you.

Stewart

  • 0
    TI__Mastermind 31655 points
    Hi Stewart,

    How much time do you allow between the trigger output and the start pulse? This must be at least 5ns for a correct measurement.

    Also, do you poll the Interrupt Status register at all during the measurement period? This can interfere with the measurement. It is best to wait for the INTB to signal that the measurement is done before any SPI transactions take place.

    If the above conditions are met, and there is still not INTB signal then I will have to look into this further.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 50 points

    Hello Scott,

    Thank you for your response..

    Time between trigger and start pulse is 400 ns.

    I am not polling the status register.

    I'm using Default register values as previously described.

    It seems I ONCE got a result (status, time1, calibration1&2), but that hasn't repeated.

    I'm following 8.4.6 Measurement Sequence.

    Thank you again,

    Stewart

  • 0 in reply to Stewart Kushner
    Prodigy 50 points
    Hello Scott,
    We got it working.
    I'm running SPI at 100KHz (spec indicates only maximum SPI frequency of 20MHz).
    Transmit pulse and start pulse were occurring while the SPI CS was still active, so 7200 couldn't start measurement ( apparently).
    So I delayed the start pulse by 10us to occur after SPI CS goes inactive & now getting measurements.
    It seems that to avoid delaying the start pulse so much, there aught to be a minimum SPI frequency specification;
    That way the SPI CS would be finished earlier and not conflict with the start pulse measurement.
    Thank you again.
    Stewart
  • 0 in reply to Stewart Kushner
    TI__Mastermind 31655 points
    Stewart,

    I'm glad it's working for you. Thanks for replying with the fix.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 50 points

    Hello Again.

    I'm getting measurements, but they're not accurate, especially in MODE 1.

    Furthermore, with Calibration2Count = 10 (default value) my Calibration 2 count is not10 times my Calibration 1 count, but about 5 times.

    Sample readings:

    Cal2Count:     Cal1:     Cal2:         (ratio : 1)

    10                   275        1530           5.5

    20                   272         2761         10.1

    40                   276         1809           6.5

      2                   273          303            1.1

    Except for Cal2Count=40, the Cal2 counts appear to be about half of expected value.

    Also, my SPI clock frequency is currently 4 MHz.

    Any ideas? 

    Thank you again.

    Stewart.

  • 0 in reply to Stewart Kushner
    Prodigy 50 points

    OK now, was just programming error :)

    Thank you.