Rotary encoder handler for arduino. v1.1

Copyright 2011 Ben Buxton. Licenced under the GNU GPL Version 3. Contact: bb@cactii.net

A typical mechanical rotary encoder emits a two bit gray code on 3 output pins. Every step in the output (often accompanied by a physical 'click') generates a specific sequence of output codes on the pins.

There are 3 pins used for the rotary encoding - one common and two 'bit' pins.

The following is the typical sequence of code on the output when moving from one step to the next:

Position Bit1 Bit2

Step1     0      0
 1/4      1      0
 1/2      1      1
 3/4      0      1
Step2     0      0

From this table, we can see that when moving from one 'click' to the next, there are 4 changes in the output code.

From an initial 0 - 0, Bit1 goes high, Bit0 stays low.
Then both bits are high, halfway through the step.
Then Bit1 goes low, but Bit2 stays high.
Finally at the end of the step, both bits return to 0.

Detecting the direction is easy - the table simply goes in the other direction (read up instead of down).

To decode this, we use a simple state machine. Every time the output code changes, it follows state, until finally a full steps worth of code is received (in the correct order). At the final 0-0, it returns a value indicating a step in one direction or the other.

It's also possible to use 'half-step' mode. This just emits an event at both the 0-0 and 1-1 positions. This might be useful for some encoders where you want to detect all positions.

If an invalid state happens (for example we go from '0-1' straight to '1-0'), the state machine resets to the start until 0-0 and the next valid codes occur.

The biggest advantage of using a state machine over other algorithms is that this has inherent debounce built in. Other algorithms emit spurious output with switch bounce, but this one will simply flip between sub-states until the bounce settles, then continue along the state machine.

A side effect of debounce is that fast rotations can cause steps to be skipped. By not requiring debounce, fast rotations can be accurately measured.

Another advantage is the ability to properly handle bad state, such as due to EMI, etc.

It is also a lot simpler than others - a static state table and less than 10 lines of logic.