Nuts and Volts - August 2008

of course, no reason why the bit order could not be reversed and the output sequence changed correspondingly.)

To produce a number in the sequence shown, place Outputs A and B in a byte in working storage like this:

bit 7

0

B

bit 0

A

At startup, the program must read the encoder value, since it could output any of its four values. The value is saved in memory as the previous value. The encoder is then polled until the value changes. Comparing the previous value and current value to the sequence 0, 1, 3, 2, you can tell which way the shaft has been rotated and act accordingly. Then the current value is saved as the previous value and polling continues.

Note that the encoder outputs require pullup resistors. In my demonstration example, I use the internal pullups available at port B of the PIC16F84A for the encoder and the pushbutton. Otherwise, 10K resistors to the +5V supply are recommended.

The pushbutton is debounced and read like any pushbutton. According to the specifications, the longest settle time is 10 milliseconds — which is pretty fast — so your code can be responsive to button presses.

Finally, the encoder supply pin requires a 150 ohm resistor between it and the +5V supply. The encoder’s pinouts are shown in Table 1.

Pin

1

2

3

4

5

6

Description

Supply (+ 5 VDC through a 150 ohm resistor)

Ground

Output B

Output A

Pushbutton

■ TABLE 1

■ FIGURE 3. Pin numbers viewed from the top.

What Can I Do With It?

The obvious use of the encoder is for the clockwise rotation to signal an “up” type event, the counter-clockwise rotation “down,” and the pushbutton to signal some kind of request or mode selection. For example, let’s say you are designing a programmable thermostat with an LCD readout. You want it to control either a furnace or air conditioner, so it has three states: cool, heat, and off. You could use up and down buttons for the temperature, and a three position switch for the state. Or, you could use an encoder. Rotating the shaft raises or lowers the temperature setting. Pressing the button changes the mode, activating a menu which allows you to select heat, cool, or off (by rotating the shaft). Pressing the button