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TCA8418: maximum capacitor

Sylvain Mariel
Prodigy 20 points
Other Parts Discussed in Thread: TCA8418

Hello,


I have a TCA8418 on my design with Vcc=1.8V. Any time I press a key, other ones are detected.

On each column and each row i have a CEM filters with 2x47pF. When a key is pressed, there are 4*47pF charged by the internal 100kohm pull-up.

I measured a 300µs periodic 0V pulse. Due to those capacitors, when the pulse goes from 0 to Vcc, the edge is "good" but not perfect: 10µs between 0V and Vihmin (=1.8*0.7).

Everything works when I completely remove those filters.

I concluded that the problem are the capacitors.

So my question is : what is the maximum capacitor that the TCA8418 can receive on its inputs ?

Or, say differently, what is the maximum acceptable delay between the rising edge of the pulse and Vihmin ?

NB: I can't add external pull-up.

  • 0
    TI__Genius 16430 points

    Hello Sylvain,

    As you have concluded, the addition of a capacitor on the line introduces a longer delay than the device can handle.

    TO answer your question, the TCA8418 scans through each row at ~50 ms. This means that your maximum delay from the start of the rising edge to VIH min must be less than 50 ms. Since the internal oscillator has some tolerance, you should give yourself some margin, +/- 30% would be safe.

  • 0 in reply to Jonathan Valdez
    Prodigy 20 points

    I'm not surer I was clear in my precedent message.

    Here is a screen capture of a Line with 2x470pF (and 2x470pF on the col).

    I measure 343µs, which is obviously too much.

    With 2x47pF the rise time is around 10µs, but i still detect ghost keys.

    With 2x10pF everything seems to work properly.

    So, I wonder what is the maximum input capacitance a line can manage.

    Or, on the following capture, what is the maximum acceptable rising time?

  • 0 in reply to Sylvain Mariel
    TI__Genius 16430 points

    Hello Sylvain,

    Thanks for the clarification, I understand what you're asking for now.

    We do not have a specification for this scanning speed. I would expect the maximum allowed delay to be near 50-100 us.