Electronic – Scrolling wheel encoder with button push button action into wheel axis

"Do you know a dedicated IC that has only the count (in both direction) features"?

You may be surprised to learn that a few simple, cheap, generic logic chips are sufficient to decode a rotary encoder -- it doesn't require some special chip specifically customized for rotary decoders. The Codewheel Generator Page has a few relatively simple circuits that decode the 2 rotary quadrature outputs into a "direction" and "count" outputs. Those outputs can be wired into the "direction" and "clock" inputs of any up/down counter -- such as, for example, the CD4029, the CD4516, the CD40193, 74HC193, etc.

"is it possible to make à simple one with an attiny"?

Yes, you can make a simple quadrature encoder with an ATtiny. Some tips for connecting a quadrature encoder to a Atmel chip and decoding it in software are at:

• Alex Brown's Robotics Page: Using Encoders
• Arduino Playground wiki: Reading Rotary Encoders
• Keith's Electronics Blog: LED Calculator with Rotary Quadrature Encoder

If you have relatively few pulses-per-second, you might be able to run the 2 signals from each encoder directly into your main ATmega and use the above techniques to do it all in software, without any external conversion hardware.

This isn't the usual rotary encoder pattern, but it should still work. See this wikipedia section for the normal gray-code implementation.

Basically, you will need to:

1. Store the current state of the encoders to a register
2. Wait for some length of time. (This must be short enough that only one transition occurs)
3. Compare the new encoder state with the old encoder state to see if they are different.
4. Based on your previous state, and the new state, decode the direction. (This step/algorithm would be easier if you used a typical implementation.)

The disadvantage of your setup is that it takes two transmitters, probably has a more complicated pattern drawn, and there's no way to identify if the encoder has gone 2 steps instead of 1. Additionally, only one bit changes with the gray code count, so the comparison routine is easier and not a look-up table.