Nuts and Volts - December 2010

●●●●

BY THOMAS KIBALO

ENHANCED USER INTERFACE FOR THE 16-BIT MICRO

NTER

This is the seventh in a series of articles describing the capabilities for the 16-Bit Micro Experimenter (Experimenter for short) and its associated hardware features (see previous articles in even months, starting with Dec ‘09). In this article, we will look at enhancing the Experimenter user interface with a rotary encoder using the PIC24F timer peripheral set and its interrupt capability. The timer peripheral is a significant component in the PIC24F peripheral arsenal. With the PIC24F timer, we can count external pulses or optionally count the internal 16 MHz PIC24F CPU clock as a timing source. All timer operations occur in hardware with a minimal need for software intervention — a really powerful capability to incorporate into Experimenter applications.

There is a lot of ground to cover here, but no worries, because we’ll supply code examples and schematics, as well as a mini-kit to help with these examples. As stated in earlier articles, all software will be kept to general level functional calls. However, some basic familiarity with C language syntax is required, as we will be incorporating some of the ‘C’ code libraries introduced in earlier Nuts & Volts articles that discussed the 16-bit Experimenter.

output signal is the result of the opening and closing of switches internal to the rotary encoder as its turns. To insure that digital signals are generated, the switch outputs are tied through pull-up resistors to + 3. 3 VDC, and the common is connected to ground. If a switch is open, a high voltage is generated; if closed, zero or low voltage is generated.

Understanding the Rotary Encoder

Let’s look at the rotary encoder and understand its operation. Most modern home and car stereos use mechanical rotary encoders for volume. They are also used in a variety of home appliances — like ovens — for temperature and timing settings. The rotary encoder is different from a potentiometer in that the encoder can undergo full rotation without limits. It is also different from a potentiometer in that we have two outputs: A and B. Each

32 December 2010

Now let’s imagine that the encoder internal shaft has two cams: one for A and one for B. Each cam has 12 “detent” or protrusions along its circumference that provide 360 degrees/12 or 30 degree resolution with each detent. As a cam turns, each detent opens or closes a switch to generate a waveform (see Figure 2). The A and B cams are placed 90 degrees from each other. Because of this placement, as the encoder shaft turns, the A and B output waveforms are 90 degrees out of phase (again look at Figure 2).