Am 21.02.2015 um 08:08 schrieb Matthias Urlichs:
>
> Or the capacitive sensors I want to use (can the BAE0911 even do
> that? an atmega supports them without any additional hardware).
>
You mean a capacitive keyboard? That's only software, on any µC.
The only difference between AVR and any other µC is Atmel bought Quantum
to get their AVR codebase on the topic. That was years ago and they now
make a lot of noise how great their product is. (I used the QT60160 in
two projects, it's okay. I does what I had expected.)
But you can built it on your own. Just connect the sensor plate to any
µC pin by a high-ohmic resistor, then continously charge/discharge that
small "capacitator" by toggling the pin. Start a timer with each toggle,
inbetween toggling, read the pin state. When it changes, read the timer
value. When a finger is near the sensor plate, the charging/discharging
cycle changes and so you can detect it.
"How" the cycle changes is not exactly defined. The QT60160 has IIRC
four values per key which affect the detection so I have to assume
Quantum/Atmel have only a small clue themselves. At least the datasheet
gives good advice what to try, how the layout of the keys and the board
thickness has to correspond to the overall key and trace sizes. A must-read!
---
Why I'd used the QT60160, even having an AVR as the main processor?
Because those chips are *small* and *cheap* (~2€) and I really haven't
wanted occupying interrupts and CPU time for scanning 8 keys. If you
have one single key or two, you may decide different, but *I* would
certainly avoid the hassle of doing that inside the µC and deal with all
the timing fallout from it. Implementing a Onewire slave already is a
big timing problem of its own.
And please be careful with the back side of those capacitive keys. It
has to be free of moveable parts, because the key works both ways and
may be influenced by moving relays etc. Been there, seen that.
Kind regards
Jan
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