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DAC60508M: SPI reserved bits

Andy Alleman
Prodigy 40 points

Part Number: DAC60508M

Hello,

I have a question on the reserved SPI bits(22:20). What happens if these bits are written to? I was trying to possibly share the CS pin with another device(MCP23S08). If I intend to write to the DAC, I can leave the bits 0, but if I try to write to the MCP23S08, I need to set bit 22 high.

Thanks for your help and time,

Andy

  • 0
    TI__Mastermind 46290 points
    Andy,

    I'm not sure I totally understand the situation.

    If these devices are sharing an SPI bus then they have separate CS signals which should hold arbitration over whom is being interacted with. In such a case if you are communicating with the MCP23S08 then the DAC60508 would not be "listening" to the bus, so there is no risk of response to contents in bits 22:20.

    When would you sort of "accidentally" be communicating to the DAC with MCP data?
  • 0 in reply to Kevin Duke
    TI__Mastermind 46290 points
    Hello Andy,

    Any update on this or would you like to take the thread to email?
  • 0 in reply to Kevin Duke
    TI__Prodigy 40 points
    Hello Kevin,
    I was trying to share the same CS signal between the MCP and the DAC. Maybe one way to get that to work would be to send 32 bits, and those bits would have an incorrect CRC for the DAC. The unknown part though, was that the reserved bits were being written to.

    Thanks,
    Andy
  • 0 in reply to Andy Alleman
    TI__Mastermind 46290 points
    Andy,

    The DAC also offers daisy-chain operation. You could put the MCP device as the second device in the chain and just issue valid frames to each device - basically just providing 24-bits of NOOP for the DAC when you want to talk to the MCP device.