Bailey O.H.Embedded systems.Desktop integration.2005

190Chapter 7 / Hardware Development

Desoldering tool — I use a metal-barreled desoldering tool to fix my mistakes. There are many types of tools available from inexpensive bulbs (about $4) to desoldering irons (about $15). I also have a desoldering iron that I use if numerous parts are being desoldered.

Assorted stranded wire — A good assortment of 22 and 24 gauge stranded wire is a must. I always have four or five spools of assorted colors in my toolbox. Stranded wire won’t break as easily as solid wire and I put just a touch of solder on the end to prevent fraying (also known as “tinning” the end).

Assorted cutters and pliers — I use small and medium ones for cutting wires and holding small parts. I also use a product called Miracle Point for grasping small parts. These little tools are springloaded and have attached magnifying lenses. I also find small cutters from Xcelite to be very useful. These tools are very sharp and maintain their sharp edges well.

Magnifying glasses — When working with small parts a magnifying glass is a must. I use a product I found on the Internet called Helping Hands. It has a small metal base with two alligator clips and a magnifying glass and can hold small work pieces.

Small vise — I use a PanaVise Jr., which is designed to hold small circuit boards.

Assorted screwdrivers — Small and medium flat blade and Philips screwdrivers

Assorted tweezers — A good assortment of stainless steel tweezers

Multimeter — I have numerous multimeters that range in price from about $4 to over $60. Each has a purpose, with the most expensive meter having an RS-232 interface so I can do continuous recording. The least expensive meters I use to test components and low voltages.

nal timing, and debugging much easier. I own several oscilloscopes — one analog, one digital, and one USB digital scope. If you are just getting started and have never used an oscilloscope, I would recommend the Parallax USB oscilloscope. It comes with a book full of experiments to familiarize you with oscilloscope functions.

Logic analyzer (optional) — When developing any type of data communications product, a logic analyzer is almost a must. These devices capture electronic signals and reassemble them into readable data. While this can also be accomplished with a storage oscilloscope, a logic analyzer displays the bit patterns rather than just the signal transitions.

Soldering Techniques

I’m not going into tremendous details on how to solder because there are plenty of good sources of information on the subject already, so I’ll just cover the basics. Make sure your work area is well lit and ventilated. This is a must if you’re soldering for long periods of time or making printed circuit boards. If you’re using a regular soldering iron, make sure you let the iron warm up to the proper temperature before using it. The key to a good solder joint is providing the right amount of heat to the right spot for the right amount of time and the right amount of solder. The higher the wattage of the soldering iron element, the hotter the tip will get. A 20or 25-watt element provides enough heat to let the solder flow quickly. Do not heat the joint for more than a couple of seconds; applying too much heat for too long a period will damage components. If you’re soldering surface mount devices you may want to use a soldering station with low resistance to reduce the risk of static damage. This prevents ground resistance from

finding a path to the component. I don’t recommend you use surface mount devices (also known as SMDs) because they require special consideration when soldering and are hard to work with due to their small size. When soldering a joint, make certain that solder covers the entire wire end, not leaving “holes” around the connection. This will avoid a bad connection and uneven stress on the wire.

Building the Prototype Circuit Boards

There are many functions of this prototype that are common to all the microcontrollers we will use. We will start with the least difficult and progress to the most difficult. Most of the parts we will use can be obtained at the local Radio Shack store. Whenever possible the Radio Shack part number will appear in parentheses. Those parts that are not available at Radio Shack will be noted along with a source and part number. A current list of resources will also be available at www.wordware.com/files/

embsys and at www.time-lines.com. Since we have chosen to use an intelligent LCD display our task has been made somewhat easier.

To illustrate how we can build plug-compatible RS-232, USB, and Ethernet interfaces, we will put the RS-232 circuit on a separate board so we can create a true plug-and-play hardware interface for our microcontrollers. The first and easiest board to build is the RS-232 interface. The second board we will build contains the 1-Wire, 3-Wire, clock chip, temperature chip, and fan relay. We are using a very small relay that has a low current rating. As a result, don’t try attaching the fan blower on your furnace to this as this design is for concept proof only. The third and final board we will build has keypad functionality, power and alarm functions, an I2C interface, and a power supply. The reason we are providing power is because of the amount of current the Ethernet interface can draw. Let’s recap the purpose of each board.

Clock/temperature sensor/1-Wire/3-Wire interface board — This board consolidates all Dallas components and interfaces. This board will illustrate how to build a simple yet reliable 1-Wire interface that can support devices up to 2 feet away. It can also be used later as a model for remote temperature sensors.

The LCD, keypad, LED, alarm, I2C communications interface, and power supply board — This board interfaces to our intelligent LCD and contains the keys, LEDs, keypad interfaces, alarm buzzer and LED, and power supply.

The RS-232 Interface Board

The RS-232 interface is well established, as the last technical specification change made to it was in 1991. The interface is defined by the Electronics Industry Association (EIA) and has been widely used for over 40 years. The RS-232 interface connects two devices directly and works by attaching a 9-pin cable between the two devices. In order to facilitate an orderly conversation two types of RS-232 equipment are defined. The first is Data Terminal Equipment, which is also known as DTE. A personal computer is defined as DTE (even though a computer is an intelligent device). In simple terms, DTE is always an origination point of data (and usually in control of the data conversation). The second type of equipment defined in the RS-232 standard is Data Communications Equipment, also known as DCE. DCE is always a data carrier or transport. Figure 7-3 shows a typical RS-232 application.