Lecture8

# Lecture8 - Chapter 3 Part 2 Functions and functional blocks...

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1 Chapter 3 Part 2 Functions and functional blocks Rudimentary logic functions Selecting Decoding Encoding Implementing Combinational Functions Using: Decoders and OR gates Multiplexers (and inverter)

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2 Functions and Functional Blocks The functions considered are those found to be very useful in design Corresponding to each of the functions is a combinational circuit implementation called a functional block . In the past, many functional blocks were implemented as SSI, MSI, and LSI circuits. Today, they are often simply parts within a VLSI circuits.
3 Rudimentary Logic Functions Functions of a single variable X Can be used on the inputs to functional blocks

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4 Multiple-bit Rudimentary Functions Multi-bit Examples: A wide line is used to represent a bus which is a vector signal In (b) of the example, F = (F 3 , F 2 , F 1 , F 0 ) is a bus. The bus can be split into individual bits as shown in (b) Sets of bits can be split from the bus as shown in (c) for bits 2 and 1 of F. The sets of bits need not be continuous as shown in (d) for bits 3, 1, and 0 of F. F (d) 0 F 3 1 F 2 F 1 A F 0 (a) 0 1 A 1 2 3 4 F 0 (b) 4 2:1 F(2:1) 2 F (c) 4 3,1:0 F(3), F(1:0) 3 A A
5 Selecting of data or information is a critical function in digital systems and computers Circuits that perform selecting have: A set of information inputs from which the selection is made A single output A set of control lines for making the selection Logic circuits that perform selecting are called multiplexers Selecting can also be done by three-state logic or transmission gates Selecting

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6 Multiplexers A multiplexer selects information from an input line and directs the information to an output line A typical multiplexer has n control inputs (S n 1 , … S 0 ) called selection inputs , 2 n information inputs (I 2 n 1 , … I 0 ), and one output Y A multiplexer can be designed to have m information inputs with m < 2 n as well as n selection inputs
7 S I 0 I 1 Decoder Enabling Circuits Y 2-to-1-Line Multiplexer Since 2 = 2 1 , n = 1 The single selection variable S has two values: S = 0 selects input I 0 S = 1 selects input I 1 The equation: Y = I 0 + SI 1 The circuit: S

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8 2-to-1-Line Multiplexer
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## This note was uploaded on 11/03/2009 for the course CS 20910 taught by Professor Taoxie during the Spring '09 term at San Diego State.

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Lecture8 - Chapter 3 Part 2 Functions and functional blocks...

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