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TCA8418: Can I use diodes inline with the switches to prevent ghosting?

Richard Burnett
Prodigy 220 points
Part Number: TCA8418

I know that a diode configuration is recommended in key matrix designs to prevent ghosting.  In the data sheet, I do not see any mention of this.  If it is possible, can you recommend a diode (or characteristics) you recommend me using with the TCA8418.

Thanks

Rick

  • 0
    TI__Guru 73181 points
    Hey Richard,

    I don't have an immediate answer for you on this (first time this has come up). I've asked a more senior member of my team about this but he stated hes heard of people using diodes with the keypad but never had any personal experience with it.

    This is something I can look into and get back to you on (Likely Wednesday or Thursday).

    Sorry for the inconvenience/wait on this.
    -Bobby
  • 0 in reply to BOBBY
    Prodigy 220 points
    Thanks for looking into this.

    A follow on question if diodes are usable, can the TCA8418 detect more than 3 keys at that point as well within the key matrix?

    If 3 keys are the limit, does this also include any keys being utilized with standard GPIO?

    In my case I am planning on using a combination of both. (Since I do not need 80 keys)
  • 0 in reply to Richard Burnett
    TI__Guru 73181 points

    Hey Rick,

    Very sorry for the delay.

    "If it is possible, can you recommend a diode (or characteristics) you recommend me using with the TCA8418."

    During my research I was able to find this article:

    At the end of the article, they make a suggestion on the 1N4148 due to it being a fast switching diode. Other than the speed, looking for diodes you would want something with a lower enough forward voltage and low leakage current. I suspect the one recommended in the article to work.

    "can the TCA8418 detect more than 3 keys at that point as well within the key matrix?"

    I believe because we can store up to 10 events, you can press up to 5 keys at the same time and it will record when the key was pressed (one event) and when a key is released (another event). I haven't had a chance to play with this in the lab but this is my reasoning to say 5 keys should be the max you can input.

    "If 3 keys are the limit, does this also include any keys being utilized with standard GPIO?"

    Registers 0x14h, 0x15h, and 0x16h (GPIO data) are separate from the event registers so it would not include the "x keys" limit.

    Again, sorry for the delay in response.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 220 points

    Thanks Bobby!

    This is excellent, and in fact is the diode I already instantiated in my design in anticipation.  This was exactly what I wanted to hear!

    Rick