Nuts and Volts - December 2010

■ FIGURE 4. Timer initialization.

■ FIGURE 5. Timer interrupt service template.

■ FIGURE 6. Rotary encoder display demo.

other parts of the program, then this communication must occur through the use of shared variables.

Interrupt functions accept no parameters and return no parameters (note the use of void), and each interrupt source has its own specific function call. The code that executes during an interrupt function is called the “interrupt service routine.” General recommendations for writing an interrupt service are: First, perform the minimal essential processing needed for the service (that is, get in and out quickly); and secondly, do not call any other functions during the service. Finally, an interrupt service must reset the original interrupt flag that initiated the interrupt before exiting. If the interrupt service communicates with

Interfacing to a Rotary Encoder Using Timer 1

For this demo, we use Timer1 with interrupt service to process rotary encoder outputs. As the encoder knob is turned, the Experimenter LCD displays the direction and amount of rotation of the encoder. In addition, the display also shows the condition of the SELECT switch (that is part of the encoder) as ON or OFF, depending on whether you push in the knob. The demo uses the LCD library that was discussed in previous Experimenter articles. Figure 6 shows the LCD display. The hook-up diagram is shown in Figure 9. The demo in Figure 8 is an excellent way to test your rotary encoder and verify its proper operation.

Let’s take a closer look at the interrupt service. With a continuous five millisecond interrupt, the interrupt service code samples Output A and determines if it is changing in value from previous samples or remaining the same. If A goes high to low, the code will then look at Output B and make a determination on direction. Only during CCW operation — when A goes high to low — will Output B be zero. If this CCW condition is not detected, the rotation direction defaults to CW. In all cases where Output A changes from a low to a high, the condition will count as a detent movement.

The interrupt service reports the current encoder position and rotation using two variables: (D for direction (1 = CW, -1 = CCW) and RCOUNT (rotational count). RCOUNT will increment or