Using a optical coder

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Hello everyone

Currently I have one quadrature optical coder and my code works fine (increase and reduction according to the sense of rotation)

I have two questions

1) I want to increase the resolution (x4) of the optical codeur while decoding the fronts high and low of two channels A and B (see the picture in blue)

2) When I start the code if I decrease immediately the value displayed is 65535 65534.9 65534.8 etc....
but I will want -0.1 -0.2 -0.3 etc

Do you have an idea to begin?