Efficiently Reading Quadrature With Interrupts

Once you start wanting to control the position of a motor through feedback control, you will likely end up using a rotary encoder as the feedback device. Encoders can be broadly categorized as:

• Absolute position
• Incremental position

Absolute position encoders will tell you what the angular position is all the time, even between power cycles. Most use "grey code" (a modified form of binary) with several "tracks" (bits) to read position.

Incremental position encoders will tell you what the angular position is, but only relative to where it was when you started paying attention (usually on power-up). Two common types of incremental outputs are:

• Incremental (clever name)
• Quadrature

Incremental is rather useless for position control because it doesn't give you any information about what direction you are turning, just that you are turning. Quadrature encoders give you direction as well as incremental position. This article deals with efficiently reading quadrature output on an Arduino by utilizing external interrupts and some AVR bare-bones code.

What is Quadrature?

There are two channels of output in quadrature lovingly referred to as channels A and B. They are each a square wave, but are offset from each other by 90 degrees. Whether channel A is leading or lagging channel B depends on the direction the shaft is turning, which is what allows you to determine direction. For example, both channels are low and then channel A goes high, you know that you are spinning CCW. If channel B had instead gone high before channel A, you would then know you are spinning CW. Of course, this can be deduced starting from any state as can be seen in the diagram. The output channels can be produced by a variety of means, usually either magnets in a disk attached to the shaft and a pair of hall effect sensors, or a disk with slots cut out and a pair of optical sensors looking through the slots. A google image search of "quadrature encoder" will come up with plenty of pictures to explain it. 

How many magnets or slots are in a revolution of the encoder is known as pulses per revolution or p/r. Only the pulses on one channel are counted, the other channel will of necessity have the same number of pulses. Encoders can range anywhere from <10 p/r to 1000's of p/r, the higher numbers giving a higher resolution. If you are looking for maximum resolution, multiply the p/r by 4 since for each pulse there are two detectable events (rising edge and falling edge) on each channel. For example, a 1024 p/r encoder will give you 1024 pulses on channel A and 1024 pulses on channel B. If you detect both rising and falling edges on each channel, you have 4096 events in one revolution, giving an angular resolution of $$\frac{360^{\circ}}{1024 \tfrac{pulses}{rev} \times 4 \tfrac{events}{pulse}} = 0.0879^{\circ}/event$$

Selecting an Encoder

Selecting the best encoder for your design is more than just getting the highest resolution you can afford since the higher the p/r the faster you have to read the events. If you need to keep track of your position while spinning at 1,000 rpm with a 1024 p/r encoder, that means that between each event there is only

$$\frac{1}{1024 \tfrac{pulses}{rev} \times 4 \tfrac{events}{pulse} \times \frac{1000 \tfrac{rev}{min}}{60 \tfrac{sec}{min}} } = 0.000014648 sec \approx 15\mu s$$
You can see how a higher resolution than needed could result in not being able to keep up with the incoming information. Of course, if you needed to you could always just look for one edge on one channel and increase the 15 microseconds to 60 microseconds, but you loose resolution and direction when doing so.

From this example we can see some important factors that must be considered when selecting an appropriate encoder:

• What is your required angular resolution
• What is your maximum speed you need to be able to track
• How fast can you read and process the encoder output

The remainder of this article will deal with the third bullet point, allowing you to read the encoder output as fast as possible. This will give you more freedom when considering the other two points.

Connect Quadrature Output to External Interrupts

Too often I see people trying to connect the quadrature output to some digital inputs and read them in the program loop. The problem with this is that if your loop is slower than the incoming quadrature information, then you loose track of where you are. Wouldn't it be nice if there was a way to have the encoder tell us when it's ready to send information, rather than asking for updates every time around the loop?

There is! It's called external interrupts, and an Arduino has two of them. Baically, you just connect the two quadrature output channels to the two interrups pins (digital pins 2 and 3 on a standard form factor arduino) and connect the interrupts in software:

volatile long enc_count = 0;

void setup() {
    // all your normal setup code
    attachInterrupt(0,encoder_isr,CHANGE);
    attachInterrupt(1,encoder_isr,CHANGE);
}

Some important things here. First, I've defined enc_count as a global variable for future use. Next, an ISR (Interrupt Service Routine) is a bit of code that gets called whenever the interrupt is triggered. ISR's take no inputs and give no outputs (hence the need for a global enc_count). You'll define this bit of code yourself, and I'll cover that a bit later. Both interrupts (interrupts 0 and 1 on pins 2 and 3, respectively) are calling the same ISR. In general the can call different ISR's, but for our purposes we want them both to behave the same. Next, we see the word "CHANGE". This is telling the program to interrupt every time there is a change on the pin, whether it goes from low to high, or high to low. Other options in place of "CHANGE" are "FALLING", "RISING", and "LOW". Check out http://arduino.cc/en/Reference/AttachInterrupt for additional info on using interrupts.

So what happens is your loop is running happily along with no thought of checking the quadrature. Once one of the pins changes, the loop is interrupted, the ISR is executed, and once that's done the loop picks up where it left off. You must be careful to make the ISR as fast as possible, because your regular loop stops dead in it's tracks, and if you have any time sensitive stuff in the loop it may not work right. Also, you have to make sure the ISR ends before another one is called.

The Look-up Table

I wish I could claim credit for what I'm about to share with you. But the truth is, I can't. I found the concept in some dusty corner of the internet. At the time I didn't understand what was going on, but through copy and pasting code and some trial and error I was able to get it working for the project I was working on at the time. Since then I have figured out what was going on, and have expanded and generalized the concept a fair amount. I went to find the website I originally got this from, but was unable to locate it. If anybody has some idea of where it came from, please post in the comments.

UPDATE: The source has been found! http://www.circuitsathome.com/mcu/reading-rotary-encoder-on-arduino

The idea here is simple enough: whenever there is a call to the ISR, the previous levels of the channels is stored in memory and the new levels are read. For convenience I will refer to the levels of the channels as xx where the first number is the level of channel A and the second number is the level of channel B. For example, progressing through states 1-4, the channel levels are written as: 

1. 00
2. 10
3. 11
4. 01 

Imagine we were in state 1 and both channels were reading low, and then the rising edge of channel A triggers an ISR and puts us into state 2. The old reading of "00" is remembered for future reference and the new levels "10" are read. Let's concatenate these two pieces of information into "0010" for convenience. Progressing from state 2 to state 3, we are then left with "1011", where "10" is the levels the channels were at in state 2, and "11" is the levels the channels are at in state 3. Continuing through the remaining state changes, we go through "1101" and "0100" to complete one period. There's nothing too tricky about any of this. We're just listing the 4 numbers in each row of the table in the image. If we were to continue to a state 5 and beyond, the strings of numbers would then repeat.

We can go through a similar exercise in the reverse direction and get the following:

With a quick examination of the two tables we see that there are 8 unique strings of numbers, 4 describing clockwise rotation, and 4 describing counter-clockwise rotation. Now, all those rows look an awful lot like binary numbers, so let's make it official and call them binary. With 4 bits there are 16 unique numbers represented in binary (2^4 = 16). So I'm just going to fill out a table with all 16 numbers, and where ever there's a clockwise rotation I'll add a 1 in a new column, where ever there's a counter-clockwise rotation I'll add a -1, and everywhere else I'll add a 0.

By now maybe you see where this is going. It's quite elegant, really. If you don't see it yet, don't worry. I'll continue.

Typically, what I see done by others when reading quadrature is to check a bunch of conditions and either add 1 or subtract 1 from the encoder variable. What if instead we had a predefined array 16 entries long, and the index of the array told us whether to add or subtract 1 from our encoder variable?

int8_t lookup_table[] = {0,-1,1,0,1,0,0,-1,-1,0,0,1,0,1,-1,0};

Seems simple enough, but how do we get which index we're looking for? If you're not too familiar with how a compiler takes the program you write and turns it into a program readable by a computer/microcontroller, it might surprise you to hear that it doesn't matter one bit if you enter a number in decimal or binary (or octal or hex), the end result will be the same:

lookup_table[5] == lookup_table[0b0101]

So now, if we find a way to store the previous and current channel levels as a binary number, we can use that information to index the look-up table.

Binary Operators and PINs

I'll skip straight to the punchline here. The ISR that gets called whenever there's an event on either of the channels looks like this:

void encoder_isr() {
    static int8_t lookup_table[] = {0,-1,1,0,1,0,0,-1,-1,0,0,1,0,1,-1,0};
    static uint8_t enc_val = 0;
    
    enc_val = enc_val << 2;
    enc_val = enc_val | ((PIND & 0b1100) >> 2)

    enc_count = enc_count + lookup_table[enc_val & 0b1111];
}

This might look like a cryptic mess to you, but don't fret. It's not too difficult. Let's go through line by line.

Line 8 is just the ISR function. Remember that ISR's take no inputs and give no outputs..

Line 9 is the look-up table we worked out previously. The "static" in the front means that this is only set the first time the code is run, and it's remembered (and not reset) each subsequent time through the function.

Line 10 is for holding the binary number derived from the quadrature channels. We'll use this to index into the look-up table.

Line 12 is where we keep track of what the channels were last time around. the "<< 2" you see there says to shift the binary bits 2 places to the left. For example:

00110011 << 2 = 11001100

This is taking the 2 channel levels from last time and moving them left to match what's in our table images above. Whenever you shift bits, 0's will always fill in the new slots.

Line 13 is a little more complicated. You see a ">> 2" in there, you can probably guess it means take whatever is in "(PIND & 0b1100)" and shift it 2 places to the right. "PIND" is a very handy, very fast way of reading digital pins on an Arduino. Whenever the code comes across the "PIND" word, it takes the input readings on pins 0-7 and gives them as an 8-bit binary number with each bit representing a single pin. If pins 2, 4 and 7 are high, and all the others low, PIND would return "10010100". There are other PINs on an Arduino, and similar ways of writing to the pins and setting the pull-up resistors. I'll refer you to http://www.arduino.cc/en/Reference/PortManipulation for further reading on the subject.

The "& 0b1100" is what is sometimes referred to as a bit mask. When you use a single "&" like this (as opposed to the double "&&" in logical statements) it is a bit-wise AND, meaning each bit in the first number is ANDed with the corresponding bit in the second number to give an output. When reading the quadrature channels, we only care about the value on pins 2 and 3, but PIND is giving us 8 pins. So we apply a mask to ignore all the extraneous information:

  10010100  PIND
& 00001100  MASK
----------
  00000100  OUTPUT

The output only gets a "1" when both PIND and the mask have a "1" in that location, just like you would expect from an AND operation.

The last thing in line 6 is the "|". You're probably familiar with the OR logical operator "||". Just like the single "&", the single "|" works on a bit-wise level. So we are taking whatever is in enc_val and ORing it with everything to the left of it. Let's say enc_val contains the number 11001100, then:

  11001100 | ((PIND & 1100) >> 2)
= 11001100 | ((10010100 & 00001100) >> 2)
= 11001100 | (00000100 >> 2)
= 11001100 | 00000001

So:

  11001100
| 00000001
----------
  11001101

After all that, what's left in enc_val is 11001101, where the "01" on the far right is the current levels of channels A and B, and the "11" just left of those is what the channel levels were the previous time through the ISR. the "1100" on the left is leftover information that isn't important to us any more.

Line 15 should be pretty straight forward after all that. You can see that there's another bit mask going on:

  enc_val & 1111
= 11001101 & 00001111

So:

  11001101
& 00001111
----------
  00001101

The binary number 00001101 is equal to the decimal number 13, and is the index into the look-up table we've been after all along. To finish off, enc_count get's incremented by whatever value is in lookup_table[13], which is 1. Pretty nifty, eh?

Using Only 1 Interrupt

If you're using an arduino Mega, or the new Due, you have more external interrupts to work with, so if you want to hook up multiple encoders or you have interrupts dedicated to other hardware you're probably covered. But if you're working on an Uno or one of it's predecessors, you are limited to only 2 interrupts.

It is possible to use this same look-up technique with only 1 interrupt per encoder. The trade off is that you will loose half of your resolution. In this case, you would hook up the other channel to a regular digital pin, and then rework your look-up table keeping in mind that you can only detect when one of the channels is changing.

You might think I'm missing some information on the B channel, but remember that the microcontroller only sees when A changes, and reads B at that time. In the CW direction, when state 2 started A was high and B was low. When it gets to state 3, A is low and B is high. We have no information about when B changed from low to high, only that it is now high. That's why we loose half the resolution when using only one interrupt.

Working out the entire look-up table:

If you instead connected channel B to the interrupt, this table would be different. One way to tell if you've done the table correctly is that the direction column should always be symmetric about the middle. That is, entry 1 should equal entry 16, entry 2 = entry 15, entry 3 = entry 14, etc. Meeting this condition doesn't guarantee that you've done it right, but if you don't meet this condition I guarantee you've done it wrong.

You will need to make some changes to the ISR as well. Line 6 in the ISR above assumed that the quadrature channels were hooked up the pins 2 and 3. For convenience in the code, I suggest you keep the two channels adjacent to each other whenever possible, i.e. pins 3 and 4 or pins 1 and 2. But remember the pins 0 and 1 are used for TX and RX.

If you connect channel A to the interrupt on pin 3, and channel B to pin 4, the ISR becomes:

void encoder_isr() {
    static int8_t lookup_table[] = {0,0,0,-1,0,0,1,0,0,1,0,0,-1,0,0,0};
    static uint8_t enc_val = 0;
    
    enc_val = enc_val << 2;
    enc_val = enc_val | ((PIND & 0b11000) >> 3)

    enc_count = enc_count + lookup_table[enc_val & 0b1111];
}

A Word on Debouncing

Usually with optical or hall effect encoders, there is no bouncing of the channels that you need to worry about. But sometimes you want to use an encoder as an input knob, and these encoders usually have mechanical switches for the channels and are subject to bouncing. The proper way to handle bounce (called debouncing) is to build a low-pass filter out of a capacitor and resistor, and pass that through a schmitt trigger. Some people like to deal with debouncing in software, but that adds a lot of overhead to the code.

Using this look-up table method, you simultaneously eliminate most of the ill effects of bouncing. In the event that either one of the channels bounces, either you will switch back and forth between a +1 and -1 and finally settle on the correct value, or you will come up with a nonsensical combination of channel levels, in which case the look-up table says to add 0 to the encoder counter. Magical!

I've done this a few times, and it works quite well most of the time. There's a few times when the program seems to think the encoder has jumped back a tick, but you can usually ignore it without too much effort. I have also built circuits with the proper debouncing hardware, and the results are flawless. Keep that in mind when your designing your next project with an encoder knob input.

Here's an example schematic of proper debouncing hardware. The encoder is in the middle with a schmitt trigger on each of channels A, B and the encoder button. There's 6 schmitt triggers in the 74HC14N IC, so you only need that one for up to two encoders with buttons.