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TDC7200: SPI MISO issues

José Caeiro
Prodigy 90 points
Part Number: TDC7200
Other Parts Discussed in Thread: TDC1000, , TDC7201-ZAX-EVM

Hi,

I've been working on a design which uses TDC1000 and TDC7200.

I've had no issues with using SPI to configure TDC1000, but the TDC7200 has been misbehaving.

The device seems to be quite reluctant in sharing the SPI bus with other devices. Even connecting a Data Analyzer to the SPI bus, renders it non functional.

I've debugged to issue to the MISO line (DOUT for TDC7200). My design uses two TDC1000´s and two TDC7200´s. While the TDC1000´s have no issues coexisting, the moment I connect the 2nd TDC7200, both stop working.

I'm using a 33 Ohm resistor near each DOUT pin.

Why? Workaround? Should I use a Tri-state buffers on each of the TDC7200 MISO lines?

Best Regards,

José Caeiro

  • 0
    TI__Mastermind 31655 points
    Hi José,

    So just to clarify, the TDC7200 operates properly even on the same SPI bus as two TDC1000s, but once the second TDC7200 connects, the TDC7200s both fail and the TDC1000s continue to work? Will writes to the TDC7200 work properly if both are connected? Can you try writing with both connected, then disconnecting one and reading back again to see if the write went through properly?

    I will say that this is not usual behavior, and an external tristate buffer should not be necessary to communicate properly with the TDC7200 even if there is more than one on the same bus. For reference, the TDC7201-ZAX-EVM has its two DOUT pins shorted, and there is no series resistance in the line to the master. You may want to try shorting the 33ohm resistors as well.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 90 points
    Hi Scott,

    That is correct. I've debugged the problem to the MISO line on the second TDC7200. If it is not connected, the first TDC7200 works without issues. If I disconnect the MISO line on the first TDC7200, the second works without issues as well. MOSI and SCLK (both shorted) have no influence.

    If both are connected, my measurements on the first pair return the following error:
    TDC7200 COARSE_CNTR_OVF_INT: Coarse overflow detected, running measurement will be stopped immediately

    I've had many issues with writes to the TDC7200, which I've realized were being caused by the data analyzer I was using to debug the SPI protocol. Even connecting or disconnecting the oscilloscope after powering the devices, causes strange behavior...

    I'm using an ATMega1284P as the MCU.

    Both SPI CS lines on the TDC7200's have pull-up resistors (10k Ohms).

    Meanwhile I've tried to pass both DOUT lines through a multiplexer and both TDC7200 seem to be working without issues.
    They just don't like to be shorted. Even the COMPIN signal on the oscilloscope seems to degrade.

    Best Regards,
    José Caeiro
  • 0 in reply to José Caeiro
    Prodigy 90 points

    Hi Scott,

    I've figured out what the problem was. 

    For some reason, after a while, the MCU port controlling the CS line would come low, thus making both devices fight for the SPI DOUT.

    Changed my software to handle this, and is working.

    Many thanks,

    José Caeiro