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ADS8688: Program register in daisy chain connection and feature select register

Alejo Cani
Prodigy 10 points
Part Number: ADS8688


Hi, I am a bit confused by the way the program register works in daisy chain mode. If I have a daisy chain of three devices, how does the following works?

  1. If I write to the program register to select the input range I send the ADDR + WRITE + RANGE. I expect to receive the configuration readback (RANGE) after this, on the SDO line. Should I receive three daisy chained readbacks (one for each ADC)? So SDO line = RANGE_ADC1 + RANGE_ADC2 + RANGE_ADC3? I only seem to be getting response from the first ADC in the daisy chain and 0 after that.
  2. If I am expected to get three daisy chained readbacks on the program register, are these readbacks 8bits size each or 16bits (8bits info + 8bits trash/discard) size each? 
  3. How does the Feature Select ID works in daisy chain mode? Again, I send ADDR + WRITE + FEATURE. But if I send DEV[1:0] = 0b01 and SDO[2:0] = 0b001 what is this doing? Is this telling the ADC 1 in the daisy chain to output the a conversion with the specified format? If so, how does the ADC determines that it is ADC 1 or X?
  4. Joining points all previous points; if I read the Feature Select Register, ADDR + READ, what response should I expect? three daisy readbacks, of 8 bits each, where the DEV[1:0] is, 0b00, 0b01, and 0b10 for each of these readbacks?

Thank you

  • +1
    TI__Mastermind 46715 points

    Hi Alejo,

    1. You should be able to get the register data from all three ADCs if you follow the data sheet in terms of connection and timing. Can you please share your schematic and the digital timing (/CS,SCLK,SDI and SDO3) for register reading?

    2. All program registers are 8-bit format, so the data for readback from each ADC is 8-bit by default. The readback operation will stop after the 24th SCLK by default.

    3. If you follow the daisy-chain in the data sheet, the command with data is sent to all ADCs at the same time and no need to select ADC device because the SDI pins for all ADCs are shorted together. The first ADC is the ADC device whose DAISY pin is shorted to the DGND ground, and this configuration determines the data from the this first ADC will be the last data which is shifted out on the the SDO line of last ADC device. The last ADC in the daisy-chain is the device whose SDO pin is connected to the SDI (MISO) pin of your host controller.

    The device ID is just used to help your controller to identify which device the data belongs to. To configure each ADC for an unique ID in daisy-chain mode, you have will have to use additional resource on your host controller to control either /CS or SDI pin of each ADC.

    4. Three 8-bit register data are expected to get if the SDO format is 000.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 10 points

    Hi Dale, thank you for your reply. I am not able to share the schematic but I think you have responded to my doubts. So to conclude, if I have three daisy ADCs, and I try to read the Program Register, I should be able to get three 8-bit data in sequence.

    Thanks,

    Alejandro