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TCA6408A - What would cause nibble inversion for the 8 bits?

Luciano Mougenot
Prodigy 20 points

We use this part as an io expander for input of 8 discretes, active low.  4 out of 500 units are failing tests because the nibbles are being read back swapped.

Ex:

0000 0001 reads 0001 0000

0000 0010 reads 0010 0000

0000 0100 reads 0100 0000

.

.

1000 0000 reads 0000 1000

I see no method of configuring the device to do this via either HW or SW.  The part has "ZVU" then "43H" printed on its top, which matches one of the working units.  I just received a new order of 10 parts which have "ZVU" and "4AH" printed on the top, which we'll attempt to swap onto one of these problematic units to see what happens.

Any ideas on what could cause this or what to try next?  I2C works and allows me to read the swapped nibbles.

Thanks

  • 0
    TI__Genius 16510 points
    Hello Luciano,

    Thanks for the question.

    This is interesting. There is no register to perform such a task. I have a few questions in regards to this:

    1) Is the nibble reversing consistent per unit? easily reproducible?
    2) If so, can you capture the I2C waveform of the transaction when it comes back with the nibble flipped?
    3) Does this issue follow a device if you put it onto another board?
  • 0 in reply to Jonathan Valdez
    Prodigy 20 points

    Hi Jonathan,

    I had our tech re-look at the 1st unit, they found that the solder was "thin" and after adding more it now works.  Its 43H part was replaced with a 4AH part.  

    I grabbed the 2nd failed unit back to test for nibble reversal and it now works, after having opened it up to inspect it.  So now I'm asking them to simply inspect and clean all the units that are having these issues before we look any further.

    I don't know if we put the original 43H part onto another board if it would reproduce the nibble reversal again, but assume it wouldn't be worth the time unless more units behave similarly.

    Thanks, I'll update this post if anything changes but for now I'm pushing this issue back to the MFG group to clean/re-test.

  • 0 in reply to Luciano Mougenot
    TI__Genius 16510 points
    Glad to hear this. Please let me know if you require any additional help.