Nuts and Volts - December 2010

www.nutsvolts.com/index.php?/magazine/article/december2010_Kibalo

samples any of these output waveforms and determines the amount of rotation. If time is also known, speed can be determined, as well. The final missing ingredient is the direction of shaft rotation CW or CCW; this is where both A and B waveforms are used together.

Waveform A is the same no matter what the direction of shaft rotation is. Waveform B reverses its order as the shaft turns from CW to CWW. By sampling both waveforms simultaneously and comparing them against one another, the microcontroller determines the direction of rotation.

With a rotary encoder, the microcontroller can determine direction, speed, and amount of rotation. All three of these physical phenomena can be used for operator input. For one of our demos, we will use the rotary encoder rotations and direction to cause up/down scrolling of text on the LCD display.

The specific encoder used in these demos is available from SparkFun (part number COM-09117). This encoder also has a built-in select switch (by pushing in on the encoder knob). We will use all these features in our first experiment.

■ FIGURE 2. Rotary encoder CW and

CCW commutation.

Let’s Review the PIC24F Peripheral Timer

We need the PIC24F timer peripheral and its interrupt capability to work with the rotary encoder. We have plenty of timers with the Experimenter — five total (Timer 1, Timer 2, Timer 3, Timer 4, and Timer 5). A timer high level block diagram is shown in Figure 3.

Each element within the timer has its own set of controls that the microcontroller needs to initialize. Starting on the left side of the block diagram is the input clock source selection for the timer controlled by TCS (timer clock select bit). The TCS setting allows for either the external clock or the internal PIC24F CPU clock. The PIC24F internal clock is 16 MHz which is the clock we will use for our demo. The next control is Timer Enable that turns on the timer. The next stage is the prescaler that allows the clock to be scaled down by factors of 1, 8, 64, and 256. The prescaler is set using an internal control register designated Timer Control (T1CON for Timer 1). The final element within the block diagram is the preset register and the timer register, both of which are16 bits wide. The microcontroller writes to the preset register to set the upper count range for the timer. During timer operation, the preset value is constantly compared against the running count value of the timer for a match condition. If a match occurs, a Timer Interrupt flag is latched to alert the microcontroller. Another action that occurs as a result of this latch is the reset of the timer register — this automatically starts the next timing cycle.