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ADS1298: Cannot establish SPI communications

Eric Shum
Prodigy 40 points

Part Number: ADS1298

Hi there,
We are trying to baseline the Modular EVM board connected to the ADSx98 FE Rev C against our design which is almost identical to the Dev kit with a few deviations. 
At the moment we cannot establish SPI communications with the microcontroller on board. One peculiar hardware variation between the two setups is pin 55 of the ADC chip. The Dev kit pin reads 4.4V with respect to DGND however the design board reads 1.3V with respect to DGND at pin 55. Both setups have the 1uF and 0.1uF decoupling capacitors going to AVSS (-2.5V).
So how is this voltage reference produced within the ADC chip? Do you know why there is such a variation?
We also have a question on pin 31 of the ADS1298 - does it have to be connected to the digital ground?
Best regards,
ES
  • 0
    TI__Genius 11550 points

    Hi Eric,

    Welcome to the E2E forum !!!

    The VCAP3 pin (Pin #55) is the output voltage of the internal charge pump circuit. The voltage on this pin should be AVDD+1.9V (4.4V = 2.5V + 1.9V ). You will see a different voltage if there is no clock to the device as the internal charge pump requires a system clock for operation. Please verify that you are using either the internal clock oscillator (CLKSEL pin = Low) or provide the device with an external clock. 

    Yes, Pin 31 (RESV1) needs to be tie to logic low (DGND).

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 40 points

    Hi TC,

    We are using the internal clock oscillator (CLKSEL pin = Low). What can we expect if the voltages are different? No communications on the MISO line at all?

    As for the RESV1, we have it floating at the moment. Again, would this result in no data coming out of the MISO line.

    We have followed all troubleshooting steps for SPI comms, including trying to force a reset pulse. We also attempted to use the START pin to force comms on the MISO line. Both steps made no difference, we are still NOT getting data on the MISO line.

    What do you propose we do next?

    Cheers,
    ES

  • 0 in reply to Eric Shum
    TI__Genius 11550 points

    Hi Eric,

    The CLKSEL pin needs to be set to HIGH if you are using the internal clock oscillator. Please see Table 11 of the datasheet for the device clock setting. If you do not see the correct voltages on Pin 55 (VCAP3), the device either malfunctions or does not have a clock signal.

    You may see an issue with the SPI communication if the RESV1 pin is left floating. Please tie it to DGND and verify the functionality of the SPI communication.

    Thanks

    -TC