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Combinational Components

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Chapter 4 − Standard Combinational Components Page 1 of 46 Contents Combinational | Chapter 4 - Standard Combinational Components Page 1 of 46 Contents Combinational Components.2 4.1 Signal Naming Conventions.3 4.2 Adder.3 4.2.1 Full Adder.3 4.2.2 Ripple-Carry Adder.5 4.2.3 Carry-Lookahead Adder.6 4.3 Two s Complement Binary Numbers.7 4.4 Subtractor.9 4.5 Adder-Subtractor Combination.10 4.6 Arithmetic Logic Unit.13 4.7 Decoder.18 4.8 Encoder.20 4.8.1 Priority Encoder.21 4.9 Multiplexer.21 4.9.1 Using Multiplexers to Implement a Function.24 4.10 Tri-state Buffer.24 4.11 Comparator.26 4.12 Shifter-Rotator.28 4.12.1 Barrel Shifter.30 4.13 Multiplier.31 4.14 Summary Checklist.33 4.15 Problems.34 Index . 46 Digital Logic and Microprocessor Design with VHDL Last updated 6 16 2004 6 04 PM Chapter 4 - Standard Combinational Components Page 2 of 46 Chapter 4 Standard Combinational Components Control Inputs Data Inputs Control unit register Nextstate Logic State Memory Output Logic K. . y P K. P Control Signals P K. I P register vÿ. ALU 1 0 I Datapath Status Signals 1 8 _ 8 Control Outputs Data Outputs Digital Logic and Microprocessor Design with VHDL Last updated 6 16 2004 6 04 PM Chapter 4 - Standard Combinational Components Page 3 of 46 As with many construction projects it is often easier to build in a hierarchical fashion. Initially we use the very basic building blocks to build slightly larger building blocks and then from these larger building blocks we build yet larger building blocks and so on. Similarly in constructing large digital circuits instead of starting with the basic logic gates as building blocks each time we often start with larger building blocks. Many of these larger building blocks are often used over and over again in different digital circuits and therefore are considered as standard components for large digital circuits. In order to reduce design time these standard components are often available in standard libraries so that they do not have to be redesigned each time that they are needed. For example many digital circuits .