Designing Embedded Hardware, 2nd Edition

Chapter 4. Electronics 101

...in reality, nothing but atoms and void

—Democritus

In writing this book, my hope is to bring to you an understanding of the design process involved in producing an embedded computer system. To this end, I have kept the electronics, the chips, and the systems I have used as simple as possible. I want you to understand the big picture without getting lost in detail. But, however simple I keep the computer designs, you won't get very far without at least a very rudimentary understanding of electronics. So what I want to do in this chapter is give some basic background theory to guide you on your way. Electronics is a truly vast and complicated multidisciplinary field, and it is not possible to cover even a thousandth of it here. I won't even attempt to. What I will do is give you an understanding of the basic principles necessary for embedded computer engineering in a simplified, and hopefully easy to understand, way. The rest of the vast mountain, I will leave unvisited. If you want to learn more, pick up a copy of Paul Horowitz and Winfield Hill's The Art of Electronics (Cambridge University Press). It's a great introductory text.

Voltage and Current

It's all about electrons. It is from their very name that we derive the term electronics. Electrons are subatomic particles with a negative charge. They are bound to positively charged atomic nuclei through Coulombic attraction. The classical physics view was to think of electrons "orbiting" the nucleus, analogous to planets orbiting in a solar system. While not at all correct,^[*] it makes it easier to visualize what goes on. The strength by which electrons are bound to the nucleus varies from atomic element to atomic element, and from molecule to molecule. Substances are either conductors , insulators , or semiconductors . In a conductor, such as a metal, the energy required to shift an electron from one nucleus to another is negligible, and the electrons may easily exchange with nearby atomic nuclei. In effect, the metal is a collection of nuclei surround by a "sea" of semi-free electrons. In an insulator, the opposite is true. The energy required to shift an electron from a nucleus is excessive, and so electrons tend to stay put. In a semiconductor, the substance may act either as a conductor or as an insulator, depending on external influences. By controlling the external influences, you change the conductivity of the substance and therefore change the way electrons move within that substance. In effect, a semiconductor is a switch that may be controlled by other semiconductors. It is this basic principle that is the basis of all modern electronics. It is the cornerstone upon which everything digital is founded.

The flow of electrons through a conductor or a semiconductor is known as current . Current is measured in Amperes, more commonly called just plain Amps (with the unit symbol "A," equation symbol "I"). For an electron to move through a conductor,^[*] there must be a "vacancy" at the next nucleus into which it can shift. (If the next nucleus has a full complement of electrons, the Coulombic repulsion of those electrons will prevent any others from slotting in.) Semiconductor physicists term these vacancies holes . An electron shifting into a neighboring hole leaves a new hole behind it. This new hole is then filled by another electron further down the line, which, in turn, creates another new hole. So current flow is, in effect, a movement of electrons in one direction and a "movement of holes" in another. The electrons are negatively charged, and the holes may be thought of as positive charges. (A missing electron at a nucleus means that the positive charge of the nucleus isn't fully canceled, and so a net positive charge exists at that location.) So while electrons move from negative to positive, the holes move from positive to negative, and it is the movement of holes (rather than electrons) that we refer to when we talk about current. Current flow, which we work with in electronics, is deemed to be from positive to negative. For continued current flow, there must be a continuous circular flow of electrons in one direction and holes in the other direction. It is from this circular flow that we derive the term circuit .

For current flow to occur between two points, there must exist an imbalance between electrons at one end and holes at the other. The size of this imbalance is known as the potential difference , or voltage difference , between two points. (It is also sometimes termed "the voltage drop across an electronic component.") The unit of voltage difference is the Volt (unit symbol "V"). The greater the voltage difference, the greater the opportunity for current flow. It is very important to note that voltage refers to the difference between two points. A voltage cannot exist in isolation. Although you will sometimes see a statement like "the voltage at this point is...," it is taken as given that it is relative to some common reference point, usually ground (the zero-volt reference point).

Warning

A common beginner's mistake in testing electronic circuits is to wire up only one lead of a piece of test equipment. Without both leads, there is no common reference point, and, therefore, any measurement taken is meaningless.

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^[*] The truth, as always, is far stranger. The quantum view is both beautiful and bizarre. For a simple and elegant introduction, read Richard Feynman's brilliant QED (Quantum Electro Dynamics) (Princeton University Press).

^[*] I'm treating a conducting semiconductor as though it were an ordinary conductor.