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TCA6424A: Using PWM with IO expander

Robert Price
Prodigy 20 points
Part Number: TCA6424A
Other Parts Discussed in Thread: TCA6507, TCA9544A

Hello.

I am considering using this IO expander to route a PWM signal through this device to an attached LED to control brightness.  Looking at the manual, I should be able to put the PWM signal on VCC and the device should route it to the selected output channel.

I would like to know if this will work?  

Thank you

  • 0
    TI__Genius 15345 points

    Hi Robert,

    I don't believe that would work. Are you trying to toggle the port outputs to work like a PWM signal? If that is the case you would have to essentially configure the registers of the device so that one of the ports is turning on and off. In order to do this it requires at least 3 bytes of data to be sent to the device through the SDA pin. This would limit how fast your PWM could be.

    We actually make an I2C device made for controling LEDs. It has PWM functionality and is call the TCA6507. Here is a write up on on how to control the device. Let me know if you have any further questions.

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    Prodigy 20 points

    No, we are not trying to create a PWM signal by cycling the outputs on this device.  We have a PWM signal generated elsewhere on the board.  We would like to selectively route the PWM signal via the TCA to assorted output circuits.  Therefore the question is can we supply a PWM signal to this board for distribution through the circuits we select?

    Thank you

  • 0 in reply to Robert Price
    TI__Guru 74252 points

    Robert,

    Thanks for your patience, Chris will be getting back to you by tomorrow.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Genius 15345 points

    Hi Robert,

    What is the maximum frequency of this PWM signal? Also is there a reason why you would like to use an I2C device for this application?

    I believe that the best device for your problem would be a MUX and in fact using a generic MUX would be even better if I2C is not required. We do make I2C MUX devices like the TCA9544A however they are mainly used to send I2C signals between different devices. This means that there are rise and fall time requirements on the input signals and therefore a maximum frequency that you can send. For the TCA9544A the maximum frequency is 400 kHz.

    Even then, every time you wanted to send your PWM signal to a different circuit you would have to send I2C commands to enable and disable specific channels on the device and then you could send your PWM signal on the SDA line if it met the timing requirements of the device. This would add delay to your circuit and might cause further issues.

    If you would like to go with just a regular MUX you wouldn't have to worry about sending I2C commands and you would just use a control signal to change which output you want to send your PWM signal to. We have a dedicated Switches and MUX team and if you would like I could forward the thread to them.

    Let me know how you would like to proceed.

    Best,

    Chris