Multiple AD5293 In SPI Bus Not Reading

Hello,

I have successfully written and read to a single AD5293 using LabVIEW. I now have three AD5293 wired into a SPI Bus format (http://upload.wikimedia.org/wikipedia/commons/f/fc/SPI_three_slaves.svg)  and can write okay as I can measure resistance and see that is changes, but cannot read back on any device. With the pull-up resistor on the SDO line at 2.2k in both single and multiple chip scenarios, the line is not at +5V initially and actually fluctuates between a high and low states depending on what value I read back. The only way I can read with multiple devices is to explicitly place each device in high impedance mode. I didn't have to do that with the single chip. Does anyone have insight as to why I need to add this extra step with three, and if there is a way I can get around it. Thanks.

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    •  Analog Employees 
    on Aug 13, 2012 8:49 AM over 8 years ago

    Hi,

    The SDO pin is not place at high impedance after SYNC is released.

    The only way to work around this, its to place the SDO line at high impedance so, any other AD5293 can get the control over the line.

    Regards,

  • Hi,

    Is the SDO line after SYNC is released just the LSB of the previous transfer? Because the LSB is clocked in last and would be shifted out through SDO being the last one to be present at SDO in order to be clocked into the next AD5293 in case of a daisy chain.

    If that was the case, one could send a NOP command with the LSB set to 1 in order to make the open drain output a high impedance one. Should be faster than putting the device in high impedance mode and release it therefrom later on.

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  • Hi,

    Is the SDO line after SYNC is released just the LSB of the previous transfer? Because the LSB is clocked in last and would be shifted out through SDO being the last one to be present at SDO in order to be clocked into the next AD5293 in case of a daisy chain.

    If that was the case, one could send a NOP command with the LSB set to 1 in order to make the open drain output a high impedance one. Should be faster than putting the device in high impedance mode and release it therefrom later on.

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