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AMC7836: Can the AMC7836's SPI be serially daisy chained?

Nicholas Gromet
Prodigy 20 points
Part Number: AMC7836


My question is, does the AMC7836’s SPI protocol allow me to chain the AMC7836’s serially like chaining together shift registers?

 

For example, each AMC7836 SPI interface has 24 binary bits (6 hex numbers). I would like to chain three AMC7836’s serially so that a SPI command to all three looks like the table below, which has 18 hex numbers (6 per AMC7836).

AMC7836 #1

AMC7836 #2

AMC7836 #3

Hex 1A

Hex 1B

Hex 1C

Hex 1D

Hex 1E

Hex 1F

Hex 2A

Hex 2B

Hex 2C

Hex 2D

Hex 2E

Hex 2F

Hex 3A

Hex 3B

Hex 3C

Hex 3D

Hex 3E

Hex 3F

 

I would like to chain the three AMC7836’s serially in this manner, so that I can write another 18 hex numbers to them, which pushes out the previous 18 hex numbers allowing me to read the previous command. Below is an example:

 

current state

AMC7836 #1

AMC7836 #2

AMC7836 #3

Hex 1A old

Hex 1B old

Hex 1C old

Hex 1D old

Hex 1E old

Hex 1F old

Hex 2A old

Hex 2B old

Hex 2C old

Hex 2D old

Hex 2E old

Hex 2F old

Hex 3A old

Hex 3B old

Hex 3C old

Hex 3D old

Hex 3E old

Hex 3F old

Transition

AMC7836 #1

AMC7836 #2

AMC7836 #3

prior hex back to PC via SDO →

in-process command from PC to AMC7836 SDI →

Hex 3D new

Hex 3E new

Hex 3F new

Hex 1A old

Hex 1B old

Hex 1C old

Hex 1D old

Hex 1E old

Hex 1F old

Hex 2A old

Hex 2B old

Hex 2C old

Hex 2D old

Hex 2E old

Hex 2F old

Hex 3A old

Hex 3B old

Hex 3C old

Hex 3D old

Hex 3E old

Hex 3F old

New state

AMC7836 #1

AMC7836 #2

AMC7836 #3

Hex 1A new

Hex 1B new

Hex 1C new

Hex 1D new

Hex 1E new

Hex 1F new

Hex 2A new

Hex 2B new

Hex 2C new

Hex 2D new

Hex 2E new

Hex 2F new

Hex 3A new

Hex 3B new

Hex 3C new

Hex 3D new

Hex 3E new

Hex 3F new

 

Does the AMC7836 support this?

Thank you.

  • 0
    TI__Guru 55003 points

    Hi Nicholas,

    The AMC7836 does not support daisy chaining for two reasons:

    1. The readback data is in the same frame as the read command, so read back would not work.  This might not be important in your system.

    2. The SDO line is high-z when the device is being addressed and the register is being sent (the first two bytes of the three byte frame).  It does not "echo" the previous clocked in values.  

    I believe you will need a dedicated CS line for each AMC.

    Thanks,
    Paul

  • 0 in reply to Paul_Frost
    TI__Intellectual 1925 points

    Paul, a follow-up to this. If you really need to use 3 devices to drive all the different PAs in the system, is there a better device or different scheme you would recommend with this device family that allows some sort of sync between the devices? (Other than separating CS and individually writing to each device).

  • 0 in reply to Kannan Sockalingam
    TI__Guru 55003 points

    There are some synchronization features already, such as the ADCTRIG input, which can trigger the ADCs to convert at the same time.  You could also have them all share a CS, MOSI, and SCLK line, while have different MISO pins.  You can address them at the same time, if they are get the same data.  You could do a combination of these methods, such as having discrete CS and MISO lines for each device, then you can chose to address them one at a time with separate CS frames, or bring all the lines down at the same time and 'broadcast' a command.

    Thanks,

    Paul