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ADS8332 Daisy Chain Method

Scott Kerstein
Expert 1395 points
Other Parts Discussed in Thread: ADS8332

Customer only has one CS available.  From Figure 46 in the spec, it looks like the ADS8332 requires multiple chip selects and has a dedicated input pin for the the chained data input on the cascaded part.  He needs to connect two parts for 6 channels.

He would like to use one CS and connect SDI to the SDO pin instead of Figure 46..  Both he and I are both curious to understand why CS1 is used for Converter #1, and CS2 is used for Converter #2 and #3.  Wouldn't there be some propagation delays to deal with?  What's the advantage of using this approach over vs. other supplier that use just on CS and SDO -> SDI.
  • 0
    TI__Intellectual 1165 points

    Hi Scott,

    I see what you mean, this seems confusing.  Let me do some quick research on this and get back to you.

    Best Regards,

    Marc

  • 0
    TI__Intellectual 1165 points

    Hi Scott,

    You are correct that the ADS8332 requires multiple chip selects and has a dedicated input (configurable) on the cascaded part(s).  Referring to Figure 46, one reason you need two different chip selects to implement this is because the micro's SDO drives all three converters SDI's --- which is used to program the Configuration Register for each converter.  This in turn is used to configure the setting on the EOC/INT/CDI pin, which in the daisy chained configuration requires the first device to be configured differently (Program Device #1: CFR_D5 = ‘1’) as compared to all of the following daisy chained devices (Program Devices #2 and #3: CFR_D5 = ‘0’).  This would not be possible to do with one chip select driving all 3 converters (or 2, as is your case) from the micro because they would all be programmed identically.

    From a high-level, I suppose we can think of this as being two completely separate serial interfaces, one being a control interface used to program the device (SDI), the other being a "data" interface used to shift data through the daisy chain (CDI).  When using the control interface you want to use CS1 and CS2/3 separately.  But, when using  the "data" interface, all 3 will toggle together.

    Regarding dealing with propagation delays, are you thinking of using GPIO's or some other method of controlling the CS and is that why you are concerned with prop delay?   The reason I ask is assuming the three CS's are on the same serial interface on the uP, they should fire at about the same time and therefore I'm assuming the prop delay in this configuration should not be all that different as compared to a single CS driving all 3 converters.

    Can you tell me which other device you are looking at? Although there are several ways to implement this, I'm not sure how it could be done with just one CS and using only SDO --> SDI.  I'm imagining that there are probably some other control pins required.

    I'm looking forward to hearing back so I can further assist you.

    Best Regards,

    Marc

  • 0 in reply to Marc Royer
    TI__Guru 71526 points

    This is a very helpful explanation - thanks. 

    -Leonard