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TCA8418: Carbon Touch with TCA8418

OLIVIER BARBEDETTE
Prodigy 20 points
Part Number: TCA8418

Tool/software:

Hello,

We have some carbon touch detection problems on a product.

I think it's more caused by the Keyboard definition than the TCA8418.

I try to understand how TCA8418 works.

Datasheet shows that a pull-up of 105K is on each pin ROW / COL.

So i supposed that a low output level is imposed on each ROW (sequentially) in order to detect which key is activated (Through the reading of each COL pin)

Is it correct ?

Second question. I measure an impedance of 100 ohms between pin ROW and pin COL when Key is activated. Is it problematic for the TCA8418 to have such a high impedance.

Best regards  

  • +1
    Guru 267780 points

    1. Yes, the internal pull-up makes the default pin state high, and the row/column connection, if active, pulls the pin low.

    2. The 100 Ω and the 105 kΩ form a voltage divider. The resulting voltage is less then 0.1 % of VCC, which is low enough.