Nuts and Volts - August 2008

Learning how using a pushbutton rotary

encoder can help you design and build better,

more user-friendly microcontroller projects.

I was bitten by the microcontroller bug a number of years ago and enjoyed building a variety of gadgets that use them. One of the tedious aspects of gadget building is the buttons and switches that make up the input controls. When I discovered the pushbutton rotary encoder, I realized that one control could replace many, simplifying the design, construction, and programming.

This article will introduce you to the Greyhill 62P22-L4 pushbutton rotary encoder¹. I will describe what it is, how it works, and how to integrate it into your project. A demonstration will be presented using a PIC16F84A², along with the project code written in C.

The Greyhill 62P22-L4

I chose this device more or less at random, because it was in stock from my favorite supplier. The shaft rotates with soft detents, and can be pressed in as a pushbutton. Each detent is

■ FIGURE 1. The encoder.

at 22° of rotation. There are no stops and no reason why it cannot be rotated endlessly either clockwise or counter-clockwise. This model comes in two different rotational torque and two different pushbutton forces; the L4 model is the lower torque and lower force model. (Other types with a variety of features such as integrated joystick are also available.)

This rotary encoder requires a + 5 VDC supply at 30 mA. It uses an internal LED and optical detector to produce its output. The pushbutton is a mechanical contact type and requires a few milliseconds to debounce (four at make and 10 at break³).

The rotary encoder outputs a two bit binary code on its Output A and Output B pins. This gives four unique values, after which the code repeats itself. Once you know the previous and current output values and the code sequence, you can determine which way the shaft has been rotated.

If you assume that Output A is the low order bit and Output B is the high order bit, when rotated clockwise the encoder outputs 0, 1, 3, 2. When rotated counter-clockwise, the output is 0, 2, 3, 1. (There is,