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ADS1271: spi MISO voltage too low

Teijo Kauppi
Prodigy 20 points
Part Number: ADS1271

Hi,

I have problem with SPI data transfer. Spi is used with external ads1271.

ADC data ready voltage level seems OK ~1,8 V (Vdd) but the MISO line shows only about half of this ~0,9 V. This seems to be too low for data transfer. MISO line is direct line to CPU.

CPU provides master clock at 27MHz and SPIclk is 27 MHz from same source. MOSI is not in use.

 

Are there any ideas that could explain this behaviour?

  • 0
    TI__Mastermind 36130 points

    Hello Teijo,

    The ADS1271 digital outputs, /DRDY and DOUT are always driven low or high (GND or DVDD), including when the device is in power-down mode.

    1.  Do you have some other device connected to the MISO line, or is it possible the MCU MISO line has been accidentally configured as an OUTPUT?

    2.  Are you measuring the MISO line with an oscilloscope?  If it is switching high and low, then a DMM would read an average voltage that is lower than DVDD.

    3.  Do you have a pull-up or pull-down resistor on the MISO line, or some other load?  The output can only drive about 5mA.

    4.  Does this same issue show up on another board?  The device could possibly be damaged.

    Regards,
    Keith Nicholas
    Precision ADC Applications