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IO-EXPANDER-EVM: Setup IO-Expander-EVM without MSP430

Tom Page
Prodigy 50 points
Part Number: IO-EXPANDER-EVM
Other Parts Discussed in Thread: TCA9539, TCA6424

I’m trying to communicate to the IO Expander EVM as a general I2C target to light up LEDs and I’m trying to do this without a MSP430 launchpad. I’m attempting to use the TCA9539 that’s on the board because it has more LEDs. However, I’m having some difficultly addressing it correctly.

I initially thought it might be the I2C host, but I can communicate to a different I2C target reliably so I think I have ruled that out.

My next thought is that I might have an incorrect setup for using the IO Expander EVM without an MSP430. The user guide is a little hard to follow. What jumpers do I need to connect to make the TCA9539 operate? I see that I may need to connect headers J9 and J10 but I’m not sure if this is for all 8-bit IO expanders or only a certain subset.

Alternatively, can you suggest another EVM that has a larger number (12+) of LED outputs with I2C communication?

  • 0
    TI__Guru* 75011 points

    Hi Tom,

    Is it possible for you to probe the SDA/SCL pins from the host and try to communicate to the TCA9539? I'd like to see what the I2C traffic looks like and if the communication looks like what we would expect. 

    Also, have you modified the IO expander EVM in anyway? 

    -Bobby

  • 0 in reply to BOBBY
    TI__Prodigy 50 points

    I attached a screenshot of some of the traffic I'm seeing on a scope.The SDA signal is on top in red and SCL is on the bottom in yellow. Logic level is 3.3V. I apologize for the blurriness, the scope doesn't have a good export function.

    Also, my host is running Debian linux and I have access to the i2cdetect command, which is telling me there is a single device connected to the I2C bus with an address of 0x22. From my understanding, this is the configured address for the TCA6424 on the EVM. If I had the TCA9539 enabled, I would expect to see a second address.

    I modified the EVM only to add a resistor and LED to one of the open areas on the EVM under the TCA9539. I also connected a jumper across header J10 in the hopes of enabling the TCA9539 as I thought that was indicated in the datasheet.

  • 0 in reply to Tom Page
    TI__Guru* 75011 points

    Hi Tom,

    The scopeshot looks okay in terms of signal integrity. 

    I was hoping to see a scopeshot of an attempted transaction with the TCA9539. I'm assuming you have TCA9539 populated on the EVM at U2. 

    I2Cdetect commands usually use an I2C read to try to get an ACK from whatever is on the bus. You may need to try to do an I2C write directly to the TCA9539 since the internal pointer after power up isn't set until after a write conditions, so it would NACK any attempts to read it until there is a write to it. I'd like to see what that looks like on the SDA/SCL lines to verify the device NACKs or ACKs it's address during a write transaction. 

    -Bobby