Rotary encoders – Experiments

These articles will contain information about the tests that I‘ve done on using of multiple rotary encoders in Arduino and beyond.

Finally we will develop a module based on Altera CPLD that will allow simultaneous reading up to 32 encoders.

Encoders that we used in these experiments are the type EC12 and EC11.

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There are many methods, projects and libraries that aim to manage rotary encoders in Arduino enviroment.

These can be categorized into two major classes:

• Polling methods
• Interrupts methods

Polling methods

The microcontroller continuously monitors the input from the rotary encoder(s). The main drawback of this method is that microcontroller have to check very fast for pulses so don’t miss it. And the microcontroller does not have time to perform other tasks.

Interrupts methods

One or two pins of the rotary encoder(s) are connected at interrupt pins of microcontroller. The main drawback of this method is that external interrupts in microcontrollers are limited resources.

For example ATmega8 has two external interrupts: INT0 and INT1. Atmega32 has three: INT0, INT1, INT2. Atmega328 has INT0, INT1, PCI0, PCI1 and PCI2.

The correct method to read rotary encoders:

Before speaking of the “correct method” and why it is “correct”, we should see how it works effectively these encoders. These encoders (with which we will work further) are actually “incremental rotary encoders”. This type of encoders provides sequential output at pins A and B when the encoder is rotated.

There are 3 pins on on EC12, two for the Channel A and Channel B outputs and Common (which usually is connected to GND). EC11 has 2 extra pins that we will ignore for now (they correspond to a simple push button).

EC12 has 24 fixed position “detents” on 360º rotation. EC11 has 20 detents.

As we can see from the image above, in fixed positions(detents) both switches are open.We make a convention:

• MSB (most significant bit) = Channel A
• LSB (less significant bit) = Channel B
• OPEN = 0
• CLOSE = 1.

Transition from one detent to next detent will follow sequence:

• CW (clockwise): 00→10→11→01→00
• CCW(counterclockwise):00→01→11→10→00

If we try to implement this convention(which is very intuitive) we have the following (big)problem:

We need to connect common pin to VCC (high level signal). And we need pull-down resistors because we can’t leave input pins of microcontroller floating.

But AVR microcontrollers have already internal pull-up resistors. We want to use this and not add external pull-down resistors. We will connect common pin to ground and we will change our convention in according to this. These are values ​​that we will work further:

• MSB = Channel A
• LSB  = Channel B
• OPEN = 1 (pull-up resistor will keep level high when switch is open)
  
• CLOSE = 0 (when switch is closed, input pin of microcontroller will be connected to GND)
  

Our transitions will change as follows:

• CW (clockwise): 11→01→00→10→11
• CCW(counterclockwise) 11→10→00→01→11

 As we can see for each state of the encoder there are only two possible values ​​that can change its current state.

Because we want to detect CCW or CW movements we consider not 4 but 7 states.

CCWFinal← CCWSt3 ← CCWSt2 ← CCWSt1 ← START → CWSt1 → CWSt2 → CWSt3 → CWFinal

CCWFinal = START = CWFinal

The following diagram will clarify this more (or not?).

Although the diagram looks a bit non intuitive because we expect to have four states, if we analyze each of the states we see that it takes into consideration every possible movement, including incomplete movements. CW_Flag=0 and CCW_Flag=0 are default for all transitions less those marked with RED.

Next we move on to the actual implementation of the algorithm that results from the diagram / table .

Implementation

We have 7 states: START, CW_Step1, CW_Step2, CW_Step3, CCW_Step1, CCW_Step2, CCW_Step3. We put these states intro arbitrary order, but we will take care to maintain this order in the table which we will build.

The columns will be placed in ascending order so that we can directly use as a index in the table. Since all possible values ​​of our inputs are increasing sequence starting with 0 (00,01,10,11) we do not need to map these inputs to another index.

Our table is incomplete at this time because we not taken into account the most important thing CW_Flag and CCW_Flag. Usually we would have needed another table (or another method to select corresponding outputs). 7 states occupy only 3bits. In each cell, we have 5 bits available to store additional data. For our flags (CW_Flag and CCW_Flag) we need 2 bits.We define these flags using the most significant bits (arbitrary choice).

Our table will take this final form:

Updating current state is done as follows:

Example of using this in interrupt routine:

Here is a complete example in Arduino using Atmega32, which will put it all together.

This article will not stop here. Other sections will follow:

• Reading multiple rotary encoders with Arduino.(ATMEGA1284 DIP40).
• Verilog design for multiple rotary encoders with Altera FPGA development board.
• Downgrade to CPLD and PCB design for final project.

This sections are not ready yet for reasons of logistics.

I have already ordered 3 pieces ATMEGA1284 … about 1 week to delivery …arrived

Ordered ALTERA FPGA Development Board … about 3 weeks to delivery…arrived

Ordered TQFP100 carrier board for ALTERA CPLD.(Already have MAX II Cpld EPM570T100C5N) about 3 weeks to delivery. …arrived

If you’re interested in the evolution of the project,  check for updates once a week

Later edit (25.feb.2015):

Part 2 it’s now ready Rotary encoders experiments – part 2.