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TM4C123GH6PZ: Why is a data byte transmitted when slave address is NACKed?

Doug A
Prodigy 70 points
Part Number: TM4C123GH6PZ

While debugging my I2C master code, I observed that when I initiate a read or write to a slave that does not exist, the address byte is correctly NACKed (seen on oscilloscope and ADRACK is set in I2CMCS), but the data byte is still transmitted (also seen on oscilloscope). Is it correct behavior for a data byte to be transmitted if the slave address was NACKed?

  • 0
    TI__Guru**** 184346 points

    HI Doug,

      The state machine might have started the data byte before seeing the NACK. The important thing is that the I2C master must generate a STOP condition after receiving the NACK to abort the current transfer. Do you see the STOP generated?

  • 0 in reply to Charles Tsai
    Prodigy 70 points

    Charles,

    Yes, I see a STOP after the data byte is NACKed, although I expected the STOP to be generated after the address was NACKed. I do not think the I2C slave I am addressing will care that the data byte is transmitted if it NACKs its own address. My main concern is that I have configured the microcontroller's I2C registers correctly, which I think I have. Thanks for your help!

  • 0 in reply to Doug A
    TI__Guru**** 184346 points

    Hi Doug,

      Yes, you have configured the registers correctly.