Nuts and Volts - August 2008

again returns the unit to run mode.

This is a very simple thermostat, but it is now easy to extend the design with more features, taking advantage of the flexibility of the encoder and LCD display. You can add a clock and time of day programs; you can set the clock; you can add a fan on/off function; and so on, without having to add any more hardware to your design.

Another application is a power controller, possibly to control the temperature of a soldering iron. The pulse width modulation feature of many microcontrollers is ideal for power control of a resistive load, with the help of a power MOSFET. The device could have a one or two digit LED display showing the power level, and a rotary encoder to turn the heat up or down. The encoder’s pushbutton could be used as an on/off switch. (In fact, the power would always be on to the microcontroller, but turning the unit off instructs the processor to set the output power level to zero and disable the LED display.)

If you add hardware debouncing to the pushbutton and connect it to an external interrupt pin, a press of the button can be used to put the microcontroller into sleep mode and the interrupt caused by another press used to wake it up. Battery operated applications would benefit from the decreased power consumption when the unit was off.

The Demonstration Project

resistors are not needed. The PIC uses a 3. 58 MHz crystal oscillator, but any crystal up to the maximum frequency supported by the PIC is okay, as long as the timer0 interval parameters are adjusted accordingly. J1 is used for in-circuit Flash programming (I use a Pickit2⁴) and is not required for operation. The buzzer and the LED display are connected to the remaining I/O pins configured as outputs. (If you don’t want to bother with a buzzer, just replace it with an LED and 470 ohm limiting resistor.) U2 provides regulated five volts so that a battery or wall wart can be used to power the project.

The Software

This simple project uses a PIC16F84A to manage a rotary encoder as input and a single digit LED display and buzzer as output. On power up, ‘0’ is displayed. Rotate the encoder clockwise, and the number is incremented to 9, then back to zero again. Rotate counter-clockwise and the display becomes ‘ 9’ and decrements down to 0. Pressing the encoder pushbutton makes the buzzer sound.

The rotary encoder S1 is interfaced to U1 with three I/O pins configured as inputs. The port B internal pullups are used for each PORTB input port, so external pullup

■ Project top.

The hardware design of microcontroller projects is deceptively simple, because so much of the work is done in the software. Polling for input events is a major task, which must be carefully coordinated with the other work being done by the processor. Improper or poor design may result in input events being missed or responded to sluggishly. The way I manage this problem is to use the PIC’s timer to do all the input polling and switch debouncing. It then posts event flags to the foreground code, which is free to respond to events as they occur.

This architecture works well when an event signals an action that can be processed quickly. I do this for my encoder inputs, where I am incrementing or decrementing the display. Other kinds of events that require time to perform can be more difficult to handle. For my button press, I want to sound the buzzer for a period of time, but I do not want to tie up the main code with a long delay. In this case, the solution is to sound the buzzer in response to the button press, and then schedule an event to turn the buzzer off later (conveniently done in our timer interrupt routine). The benefit is no time-wasting delay loops at all in the program.

To see how the rotary encoder is managed in the software, have a look at encoder_demo.c and encoder_demo.h in the downloads available on the

Nuts & Volts website at www.nutsvolts.com.

1) In function InitCPU, the option register is loaded to assign the prescaler to timer0 and set the prescaler to 16:1. The timer0 counter TMR0 is initialized to the value in define TIMER0_INIT. Note also that PORTB pull-ups are enabled. Then timer0 and global interrupts are enabled.