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Keyscanning question

Ok,

I know this has come up recently...

I am doing a keyscanning app, around 96 keys or so.

  1. Can I put the blocking diodes in such a way that I don't incurr a voltage drop?

I am working with a low-voltage system and schottkys still erode noise margin. I haven't come up with a clever way on my own yet.

  1. Keyboard PCB is two layers. Should I flood fill the bottom layer with GND to reduce EMI?

I am worried that with additional GND copper, I will increase capacitance on the long keyboard traces (2 ft).

Thank you. HW.

Reply to
Hw
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if voltage drop is a concern you have a bad input circuit in use. Rubber keys have a reasonable resistance >0 Ohm and they should work. Diodes are only required for 3 key combinations otherwhile you can drop them completely

dont use matrix or change the input threshold to proper value. Eg. use schmitt trigger buffers for input.

You can make a GND plane but there are more important things for reliability: For proper ESD behavior decouple XY matrix with resistor networks of several kOhms against IO buffers. Make a clear gap trace in pcb and bridge this gap for signals with high impedance resistors only. VCC and GND crosses that gap with differential coil only what is high impedance for AC and low impedance for DC. Note: Keyboards and mainly thin membrane keyboards are one of the practically most vulnerable entrance for static discharge becouse charged users most time touch the keyboard first

Reply to
Jürgen Veith

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