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TCA6408A: IO Expander not acknowledging

sameer
Prodigy 240 points
Part Number: TCA6408A

I am using two TCA6408APWR IO expanders on an I2C bus of a microcontroller

first expander address bit is high

second expander address bit is low

When i am writing data to first expander ,its acknowledging it ,i can configure all the ports and can read input and write output on respectively configured ports

When i am writing to second expander its not acknowledging the address

difference between two is 

First expander has vcci and vccp common and is on same board as microcontroller

whereas second expander has vcci = 3.3V and vccp = 5V and is on another pcb connected with a 3inch long wire with microcontroller pcb

checked on oscilloscope on second expanders scl and sda pins ,getting correct address but still the chip is not responding

What might be wrong ? 

  • 0
    TI__Guru 62910 points

    Hi Sameer,

    Since the main difference is that the non-responsive expander is on a different PCB, I'd recommend we check a couple of things first:

     - Are the MCU and working expander using VCCI = 3.3 V as well?  I want to make sure compatible voltage levels are being used across all the devices.

     - Are the power and ground connections shared between the two PCBs?  I'd like to make sure there isn't a shift in the ground reference potentials between the MCU and the expander that could cause a corresponding shift in the voltage levels of the I2C signals or address inputs.

     - Can we please take a look at the SDA/SCL waveforms at the expander input measured with respect to its ground pin?  I am wondering if there might be some signal integrity issues introduced by the board-to-board connections that might be causing communication to be unsuccessful.

    Lastly, if you could provide a schematic of the I2C implementation it would be helpful in making sure we understand the application.

    Regards,
    Max

  • 0 in reply to Max Robertson
    TI__Guru 62910 points

    Hi Sameer,

    Just checking back in on this - have you been able to resolve this issue?  If not, have you had a chance to look into any of my suggestions above?

    Regards,
    Max

  • 0 in reply to Max Robertson
    Prodigy 240 points

    Thanks for reply ,the ground was different now its working