17CHAPTER ONE
Review of Electronics Fundamentals
curve, Vo at Io max. As a result, |
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the best we can do is to look |
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at the output characteristics |
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graphically, as shown in |
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Figure 1-17. |
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Logic Symbols |
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Figure 1-17: Output voltage Vo versus current Io. |
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Logic symbols are used to represent the logic functions in a more abstract way, allowing the designer to specify the logical function of a circuit without getting into the details of the underlying components (such as the transistors and resistors). The logic symbols used in this text represent those that are most commonly used in commercial documentation. There are other standards, such as the ANSI/IEEE standard gate level symbols, but they are not encountered as frequently in practice. Figure
1-18 shows the logic symbols for |
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different gates, and their functions |
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F = A |
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are described in the truth tables. |
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The logic symbols in Figure 1-18 |
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show the shapes and Boolean logic |
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functions for the most common |
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gate configurations. The buffer |
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device is a triangle—the symbol |
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for an amplifier—because it |
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F = A |
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F = AB |
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amplifies the input signal, allowing |
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an increase in the number of loads |
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that can be driven. Note that a |
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small circle, often referred to as a |
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“bubble,” on an input or output |
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Figure 1-18: Logic symbols, symbolic notation, |
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terminal designates a logical inver- |
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and truth tables. |
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sion. Thus the inverter is shown as |
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a triangle (amplifier) with a bubble on the output to signify the logic level |
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inversion on the output. The logic voltage levels for TTL logic are: |
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Positive Logic |
Corresponding TTL Logic Voltages |
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0 = false = lowest voltage level |
0 = input voltages 0 to 0.8 volts (low) |
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1 = true = highest voltage level |
1 = input voltages 2 to 5 volts (high) |
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18EMBEDDED CONTROLLER
Hardware Design
This means that a TTL compatible logic input is guaranteed to respond to an input signal between 0 and 0.8 volts as a logic zero, and input voltages from 2 to 5 volts as a logic one. Note that voltages between 0.8 and 2 volts are not valid logic levels.
Logic voltage levels are different for different types of logic, but the most common logic levels are those corresponding to the original TTL (transistor transistor logic), using a 5 volt power supply. CMOS levels, using 3 or 5 volt power, are also common. TTL and CMOS logic—like almost every other type of logic in common use —are called positive logic because the most positive voltage corresponds to the logic one value.
Tri-State Logic
Tri-state logic does not refer to orderly thinking in a three state geographic region! When we speak of binary (base two number) values, we mean that a given bit or logic signal can take on either one of two valid states (zero or one) at any instant in time. A logic gate that is not forcing its output to be either one or zero is said to be tri-stated. Tri-state logic does not refer to base three numbers, but rather to a third invalid logic state when the output of a logic device is neither sinking nor sourcing current. This so-called third state is really an undefined
condition, because the |
Tri-State Inverting Buffer |
Output ENabled |
Output DISabled |
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device output is not |
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forcing a logic level on |
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its output. It is said to be |
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Truth Table |
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in a floating, high impedance, passive, or Hi-Z state, since the output circuits are effectively disconnected. A tri-state driver connected to one signal wire of the bus is shown in Figure 1-19.
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Output |
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Switch |
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Symbol and Function |
Equivalent Circuit – Active and Passive |
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Figure 1-19: Active and passive states of a tri-state buffer.
On the left is an inverting buffer with an enabled tri-state output. On the right side is an example showing two of the same type of buffers, with the top device in the disabled or passive state, and the lower device is enabled