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Hardware Design

Of course there are disadvantages too, including:

•Reduced flexibility since you can’t easily change the functions designed into the chip.

•Expansion of memory or I/O is limited or impossible.

•Limited data transfer rates due to practical size and speed limits for a single-chip.

•Lower performance I/O because of design compromises to fit everything on one chip.

Digital Hardware Concepts

In addition to the CPU, memory, and I/O building blocks, other logic circuits may also be required. Such logic circuits are frequently referred to as glue logic because they are used to connect the various building blocks together. The most difficult and important task the hardware designer faces is the proper selection and specification of this “glue logic.” Devices such as registers, buffers, drivers and decoders are frequently used to adapt the control signals provided by the CPU to those of the other devices. While TTL gate level logic is still in use for this purpose, the programmable logic device (PLD) has become an important device in connecting the building blocks. Contemporary microcontroller designers need to acquire the following skills:

•Interpretation of manufacturers specifications

•Detailed, worst case timing analysis and design

•Worst case signal loading analysis

•Design of appropriate signal and level conversion circuits

•Component evaluation and selection

•Programmable logic device selection and design

The glue logic used to join the processor, memories, and I/O is ultimately composed of logic gates, which are themselves composed almost entirely of transistors, diodes, resistors, and interconnecting wires. In order to understand the basic operation of the glue logic, we are going to begin at the component level with a review of basic electronics concepts. These concepts will be presented as fluid flow analogies.

7CHAPTER ONE

Review of Electronics Fundamentals

Voltage, Current, and Resistance

pressure “motivation,” if you will,

vides a limiting constraint on the

amount of current that will actually flow. The resistor will allow a current to flow through it that is proportional to the voltage across it, and inversely proportional to the resistance value. Higher resistance is like a smaller aperture for the fluid to flow through. The

resistance results in a voltage, or pressure drop, across the resistance as long as current is

flowing in the resistor. Figure 1-4 illustrates this.

The wiring connecting the components in a circuit is like the piping connecting plumbing components that let a fluid flow. The flow of current in the circuit

is controlled by the magnitude of the voltage (pressure) and the resistance (pressure drop) in the circuit. In Figure 1-5, the battery provides a voltage to force current through the resistor. The magnitude of the voltage (V) generated by the battery is developed across the resistor, and the magnitude of the resistance (R), determine the current (I). Note the “return” current path is often shown as “ground,” which is the reference voltage used as the “zero volts” point. In this case, current flows from the positive battery terminal, through the wire, then the resistor, then through the “ground” connection to the minus terminal of the battery. This is usually not the same as earth ground, which provides a connection to a stake or pipe literally stuck in the ground. The magnitude of the current in this case is I = V / R by re-arranging the

8EMBEDDED CONTROLLER

Hardware Design

equation V = I * R, as shown in Figure 1-5. This is known as Ohm’s law.

sitic values that limit

their performance in the real world and are subject to other limitations, such as operating temperature, power limits, etc. Current flows only through a complete circuit, and in most cases (for a positive power supply) current flows from the power source through the circuitry and returns to the power supply through the common “ground” connection. Current flowing through any resistance results in the dissipation of power as heat. The power dissipated is P = I2R = V*I = V2/R. Note that voltage is sometimes denoted by the variable V and sometimes by E, for electromotive force.

All practical components have some resistance. Real batteries have an internal resistance, for example, which provides an upper limit to the current the battery can supply to an external circuit. Real wires have resistance as well, so the actual performance of a circuit will deviate somewhat from the ideal. These effects are obvious in some cases, but not in others. In an automobile starting circuit, it’s not surprising that the battery, supplying 12 volts to a starter with internal resistance on the order of 0.01 to 0.1 ohms, will result in currents of hundreds of amperes in order to start the engine. On the other hand, while consulting with a prominent notebook computer manufacturer, I uncovered a design error resulting in an internal current of hundreds of amperes flowing in the circuit for a few nanoseconds. Obviously, this wreaked havoc on the operation of the computer, and generated a great deal of electromagnetic noise!

One of the things you will learn in this book is how to avoid those kinds of mistakes. It’s also important to remember that power is dissipated in any resistance present in the circuit. The power is proportional to the voltage times