Nuts & Volts - November 2006

GETTING STARTED WITHPICs

THE LATEST IN PROGRAMMING MICROCONTROLLERS

■ BY CHUCK HELLEBUYCK

PIC HARDWARE INTERFACE

PROGRAMMING MICROCHIP’S PICS IS A LOT OF FUN, especially if you have a nice development board with all the connections pre-wired for you. My BasicBoard, built around the 40 pin Atom chip (16F877A with Atom bootloader installed), was designed just for that purpose. A beginner could get a lot of programming in before they ever have to build custom circuitry around their PIC micro-controller. What I’ve found from reader feedback is a lot of people are looking for a little guidance with those custom circuits. I’m more than willing to help.

Let’s start with the basics. A PIC needs four main connections to run: Vdd (typically 5V), ground (Vss), MCLR pull-up resistor, and an oscillator. Some PICs offer an internal oscillator option and also an internal MCLR pull-up. That reduces the connections down to Vdd and ground (Vss). I’ll let you explore the data sheets for that information and stick with the four connections as shown in Figure 1. I also throw in a standard 7805 regulator to produce the five-volt power. There are loads of other five-volt regulators you could choose, but the 7805 is so robust and cheap, I still like to use it.

This is a great way to show an error mode. Have one LED the color red and the other LED green. When the pin 0 in high and pin 1 is low, the green LED is lit possibly to indicate everything is running properly. When something goes wrong — such as a sensor signal lost or some other known problem — then the I/O is reversed making pin 0 low and pin 1 high. This will light the red LED and turn off the green LED.

Now take this concept a little further and drive four LEDs from three I/O pins as shown in Figure 3B. This saves I/O pins and allows you to control each LED individually. Table 1 shows how to light the various LEDs.

SWITCHES

High Side LED LED On: High 0 LED Off: Low 0 Low Side LED LED On: Low 0 LED Off: High 0

LEDS

These days, just about every project will have at least one LED connected to an I/O pin even if it’s just to indicate that the PIC is running. As simple as that sounds, there are several ways to do it. Most PIC I/O pins can supply 25 ma or sink 25 ma, so driving an LED directly is pretty easy, as shown in Figure 2. I show both a high side drive and a low side drive. The high side supplies the power to the anode through a current limiting resistor and the low side supplies the cathode ground path. Both work equally well, they just take a little different code setup as shown.

Another

■ FIGURE 2 LED option is to drive two LEDs from the same two I/O pins, as shown in Figure 3A.

■ FIGURE 1

12 November 2006

Switches are another area that initially seems straightforward, but get more complicated when you look at the hardware options. Switches can be high side or low side activation that either pulls an I/O pin high or low, but there is a little glitch to switches called switch bounce. Inside a momentary push button switch, or most switches for that matter, is a spring steel conductor that completes the connection between two pins. When you press the switch, the spring metal contact