Nuts and Volts - December 2010

decrement in value based upon rotational direction. As a final function, the interrupt service routine determines the knob switch state (ON, OFF) and provides debounce for this switch. The Timer 1 interrupt service code is shown in Figure 7.

Scrolling Text Display Application Using the Rotary Encoder

The first demo verifies operation of the rotary encoder. How about a demo that uses the encoder in a real application? In this second demo (Scrolling demo.MCP), we use the encoder to assist the user in scrolling through a large amount of text that is stored in the microcontroller Flash memory and viewed on the LCD. Our Experimenter LCD is a 16 x 3 display, limiting how much text we can view at a time. In this demo, we have hardcoded President Lincoln’s entire Gettysburg Address in microcontroller Flash. The speech is approximately 1,172 characters and 64 lines of text. The rotary encoder will be used to scroll backward and forward to facilitate viewing the entire speech one line at a time.

We use the same interrupt and timer set-ups as in the earlier demo, and to this add two new functions:

■ FIGURE 8. Scrolling demo.

• Scroll Down() — This function advances forward to the next line of text in the speech. For each rotary detent, it retrieves this line for display and halts when it reaches the last line of the speech.

• Scroll Back() — This function moves backwards to the previous line of text in the speech. For each rotary detent, it retrieves this line for display and halts when it reaches the first line of the speech.

Timers are a broad subject and there are many uses for them. A good reference book for this material and other applications is available in the Nuts & Volts Webstore entitled Programming 16 bit Microcontrollers in C by Lucio Di Jasio. For your convenience, there is mini-kit for this article containing all interface electronics, mechanical connectors, and the associated printed circuit board available from Nuts & Volts. The mini-kit not only contains a rotary knob interface but also a full PS/2 keyboard interface. We’ll cover more of this keyboard interface at another time. NV

Figure 8 shows the scrolling demo in action. Enjoy!

The “Mini Kit” used in this article can be purchased online from the Nuts & Volts Webstore at www.nutsvolts.com or call our order desk at 800-783-4624.

Where To Go From Here

■ FIGURE 9. Prototype hook-up.

We have covered a new interface capability for the Experimenter. The rotary encoder really increases our “bag of tricks’ and has wide applicability. We also discussed the PIC24F timer and its interrupt capability, applying timers to interface to these input devices.