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Chapter 4 Gates and Circuits

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**Computers and Electricity**

Gate: A device that performs a basic operation on electrical signals. Circuits: Gates combined to perform more complicated tasks.

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**Computers and Electricity**

There are three different, but equally powerful, notational methods for describing the behavior of gates and circuits: Boolean expressions logic diagrams truth tables

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**Computers and Electricity**

Boolean expressions: Expressions in Boolean algebra, a mathematical notation for expressing two-valued logic. This algebraic notation is an elegant and powerful way to demonstrate the activity of electrical circuits.

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**Computers and Electricity**

Logic diagram: a graphical representation of a circuit Each type of gate is represented by a specific graphical symbol. Truth table: A table showing all possible input values and the associated output values.

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Gates Let’s examine the processing of the following six types of gates: NOT AND OR XOR NAND NOR Typically, logic diagrams are black and white, and the gates are distinguished only by their shape.

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NOT Gate A NOT gate accepts one input value and produces one output value. Figure 4.1 Various representations of a NOT gate

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NOT Gate By definition, if the input value for a NOT gate is 0, the output value is 1, and if the input value is 1, the output is 0. A NOT gate is sometimes referred to as an inverter because it inverts the input value.

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**AND Gate An AND gate accepts two input signals.**

If the input values for an AND gate are both 1, the output is 1; otherwise, the output is 0. Figure 4.2 Various representations of an AND gate

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OR Gate If the two input values are both 0, the output value is 0; otherwise, the output is 1. Figure 4.3 Various representations of a OR gate

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**XOR Gate XOR gate (eXclusive OR)**

An XOR gate produces 0 if its two inputs are the same, and a 1 otherwise. Note the difference between the XOR gate and the OR gate; they differ only in one input situation: When both input signals are 1, the OR gate produces a 1 but the XOR produces a 0.

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XOR Gate Figure 4.4 Various representations of an XOR gate

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NAND and NOR Gates The NAND and NOR gates are essentially the opposite of the AND and OR gates, respectively. Figure 4.5 Various representations of a NAND gate Figure 4.6 Various representations of a NOR gate 4–15

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**Review of Gate Processing**

A NOT gate inverts its single input value. An AND gate produces 1 if both input values are 1. An OR gate produces 1 if one or the other or both input values are 1.

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**Review of Gate Processing**

An XOR gate produces 1 if one or the other (but not both) input values are 1. A NAND gate produces the opposite results of an AND gate. A NOR gate produces the opposite results of an OR gate.

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Constructing Gates Transistor: A device that acts, depending on the voltage level of an input signal, either as a wire that conducts electricity or as a resistor that blocks the flow of electricity. A transistor has no moving parts, yet acts like a switch. It is made of a semiconductor material, which is neither a particularly good conductor of electricity, such as copper, nor a particularly good insulator, such as rubber.

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**Constructing Gates A transistor has three terminals:**

A source A base An emitter, typically connected to a ground wire If the electrical signal is grounded, it is allowed to flow through an alternative route to the ground (literally) where it can do no harm. Figure 4.8 The connections of a transistor

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Constructing Gates It turns out from the way a transistor works, the easiest gates to create are the NOT, NAND, and NOR gates. Figure 4.9 Constructing gates using transistors

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**Circuits Two general categories:**

In a combinational circuit, the input values explicitly determine the output. In a sequential circuit, the output is a function of the input values as well as the existing state of the circuit. As with gates, we can describe the operations of entire circuits using three notations: Boolean expressions logic diagrams truth tables

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**Combinational Circuits**

Gates are combined into circuits by using the output of one gate as the input for another. Page 99

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**Combinational Circuits**

Page 100 Because there are three inputs to this circuit, eight rows are required to describe all possible input combinations.

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**Combinational Circuits**

This same circuit using Boolean algebra: AB + AC

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**Combinational Circuits**

Now let’s go the other way; let’s take a Boolean expression and draw its circuit and its Truth Table. Consider the following Boolean expression: A(B + C) What does it mean?

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**Combinational Circuits**

A (B + C) Can you create its Truth Table?

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A (B + C) A B C B + C A(B + C) 1

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A (B + C) A B C B + C A(B + C) 1

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A (B + C) A B C B + C A(B + C) 1

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**Combinational Circuits**

Now compare the final result column in this truth table to the truth table for the previous example: A(B+C) (AB + AC) Their result columns are identical.

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**Combinational Circuits**

We have therefore just demonstrated circuit equivalence. That is, both circuits produce the exact same output for each input value combination. Boolean algebra allows us to apply provable mathematical principles to help us design logical circuits.

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**Properties of Boolean Algebra**

Page 101

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**Adders At the digital logic level, addition is performed in binary.**

Addition operations are carried out by special circuits called, appropriately, adders.

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Adders The result of adding two binary digits could produce a carry value. Recall that = 10 in base two. A circuit that computes the sum of two bits and produces the correct carry bit is called a half adder.

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**Adder Truth Table Examine the adder’s truth table carefully.**

The Sum column has the same results as the XOR gate. The Carry column has the same results as the AND gate.

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**Adders sum = A B carry = AB**

This circuit diagram represents a half adder. As do these two Boolean expressions: sum = A B carry = AB Page 103

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Adders A circuit called a full adder takes the carry-in value into account. Figure A full adder

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Multiplexers A multiplexer is a general circuit that produces a single output signal. The output is equal to one of several input signals to the circuit. The multiplexer selects which input signal is used as an output signal based on the value represented by a few more input signals, called select signals or select control lines.

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Multiplexers The control lines S0, S1, and S2 determine which of eight other input lines (D0 through D7) are routed to the output (F). Page 105

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Sequential Circuits Digital circuits can also be used to store information. This application employs sequential circuits, because the output of the circuit is also used as input to the circuit.

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**Circuits as Memory An S-R latch stores a single binary digit (1 or 0).**

There are several ways an S-R latch circuit could be designed using various kinds of gates. Figure An S-R latch

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Circuits as Memory The design of this circuit guarantees that the two outputs X and Y are always complements of each other. The value of X at any point in time is considered to be the current state of the circuit. Therefore, if X is 1, the circuit is storing a 1; if X is 0, the circuit is storing a 0.

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**Circuits as Memory (S-R latch)**

S=1, R=1, X=1, Y=0 S NAND Y = 1 R NAND X = 0 The latch is storing 1. S=1 X=1 Y=0 R=1

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**Circuits as Memory (S-R latch)**

Momentarily set R=0 R NAND X => 0 NAND 1 = 1 (at Y) Note: At this point in its explanation, the textbook states “Y becomes 0”. This is a typo! S=1 X=1 Y=0 R=0

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**Circuits as Memory (S-R latch)**

Momentarily set R=0 R NAND X => 0 NAND 1 = 1 (at Y) Y NAND S => 1 NAND 1 = 0 (at X) S=1 X=1 Y=1 R=0

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**Circuits as Memory (S-R latch)**

Momentarily set R=0 R NAND X => 0 NAND 1 = 1 (at Y) Y NAND S => 1 NAND 1 = 0 (at X) S=1 X=0 Y=1 R=0

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**Circuits as Memory (S-R latch)**

When R is set back to 1 R NAND X = 1 S NAND Y = 0 The latch stores 0. S=1 X=0 Y=1 R=1

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**Circuits as Memory (S-R latch)**

S=1, R=1, X=0, Y=1 R NAND X = 1 S NAND Y = 0 Momentarily set S=0… S=1 X=0 Y=1 R=1

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**Circuits as Memory (S-R latch)**

S NAND Y => 0 NAND 1 = 1 (at X) S=0 X=0 Y=1 R=1

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**Circuits as Memory (S-R latch)**

S NAND Y => 0 NAND 1 = 1 (at X) X NAND R => 1 NAND 1 = 0 (at Y) S=0 X=1 Y=1 R=1

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**Circuits as Memory (S-R latch)**

S NAND Y => 0 NAND 1 = 1 (at X) X NAND R => 1 NAND 1 = 0 (at Y) S=0 X=1 Y=0 R=1

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**Circuits as Memory (S-R latch)**

When S returns to 1 S NAND Y = 1 R NAND X = 0 The latch stores 1 again. S=1 X=1 Y=0 R=1

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Circuits as Memory There are many ways to construct memory circuits. The SR latch is cheap to build (only 4 transistors) but it requires that its inputs by 1 normally. The flip-flop is a more expensive device that requires inputs of 0.

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Circuits as Memory The flip-flop.

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**Circuits as Memory (flip-flop)**

What happens if A becomes 1?

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**Circuits as Memory (flip-flop)**

1 OR anything = …

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**Circuits as Memory (flip-flop)**

1 OR anything = 1 So now 1 AND 1 = …

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**Circuits as Memory (flip-flop)**

1 AND 1 = 1 Now, notice what happens when the input at A returns to 0.

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**Circuits as Memory (flip-flop)**

The flip-flop continues to hold the value 1 as its output.

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**Circuits as Memory (flip-flop)**

Let’s go back to the original state with the output unknown. What happens if input B is set to 1?

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**Circuits as Memory (flip-flop)**

NOT 1 = …?

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**Circuits as Memory (flip-flop)**

NOT 1 = 0 0 AND anything = …?

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**Circuits as Memory (flip-flop)**

0 AND anything = 0 0 OR 0 = …?

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**Circuits as Memory (flip-flop)**

Now, notice what happens when the input at B returns to 0.

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**Circuits as Memory (flip-flop)**

The flip-flop continues to hold the value 0 as its output.

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**Circuits as Memory (flip-flop)**

Since we didn’t know the initial value of X we can summarize the behaviour of the flip-flop as follows: Input at A causes it to store a 1. Input at B causes it to store a 0.

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**Circuits as Memory (flip-flop)**

There are other ways to construct memory devices. See if you can figure out how this one works. Can you redesign it to be more efficiently manufactured?

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Integrated Circuits Integrated circuit (also called a chip) - A piece of silicon on which multiple gates have been embedded. These silicon pieces are mounted on a plastic or ceramic package with pins along the edges that can be soldered onto circuit boards or inserted into appropriate sockets.

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Integrated Circuits Integrated circuits (IC) are classified by the number of gates contained in them. Page 107

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Integrated Circuits Figure An SSI chip contains independent NAND gates

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CPU Chips The most important integrated circuit in any computer is the Central Processing Unit, or CPU. Each CPU chip has a large number of pins through which essentially all communication in a computer system occurs.

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CPU Chips A CPU adaptor. Each hole receives a pin from the CPU.

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