How to interpret the direction of rotation from a digital rotary switch with a PIC

Step 2 Hardware Interface

A "simple" solution would be to have a "single pole-16 throw" switch with 16 connections to the microcontroller. Each switch output would then be tied to a pin on the microcontroller so that every dial position can be checked by the microcontroller. This is an excessive use of I/O pins. Things get even worse if we want more than 16 positions (detents) available to us on the switch. Each extra position on the switch would require an extra input to the microcontroller. This quickly becomes a very inefficient use of inputs on a microcontroller.

Enter the beauty of the rotary switch.

The rotary switch has only two outputs to the microcontroller listed as A and B on the data sheet. There are only four possible logic levels that these lines can take: AB = 00, 01, 10 and 11. This greatly reduces the number of input lines you must use in connecting the switch to the microcontroller. So, we've cut the number of input lines down to just two. Now what? It seems like we really need 16 different states but this new switch has only four. Have we shot ourselves in the foot? Nope. Read on. We'll cover a little bit of the theory behind the rotary switch operation to explain.