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ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Examples Barrel Shifter Speical purpose Encoders/Comparators

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Ex Ex HW solutions

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Design examples using combinational building blocks (decoders, encoders, multiplexers, comparators, three-state devices, adders). Barrel shifter Dual-priority encoder Cascading comparators, mode-dependent comparator

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Barrel shifter design example n data inputs, n data outputs Control inputs specify number of positions to rotate or shift data inputs Example: n = 16 DIN[15:0], DOUT[15:0], S[3:0] (shift amount) Many possible solutions, all based on multiplexers

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Multiplexers (mux) Select one of n sources of data to transmit on a bus. Eg. Put between Processor’s registers and ALU A 16-bit processor where 3-bit field specifies on of 8 registers. The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux.

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MSI: 74x151 8-input 1-bit multiplexer

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16-to-1 barrel shifter 16-to-1 mux = 2 x 74x151 8-to-1 mux + NAND gate

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4 16-bit 2-to-1 muxes 16-bit 2-to-1 mux = 4 x 74x157 4-bit 2-to-1 mux

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Properties of different approaches

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Dual-Priority Encoder A priority encoder identifies not only the highest but also the second-highest- priority asserted signal among a set of 8 request inputs.

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Single-priority encoder

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Parallel Cascading Comparators 74x85: serial cascading scheme, delay of propagating the cascading signals through a cascade of comparators. 74x682: Parallel approach, used to build very wide comparators. Build 24-bit comparator, using 3 74x682 PEQQ=EQ2.EQ1.EQ0 PGTQ=GT2+EQ2.GT1+EQ2.EQ1.GT0

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8 bit Comparator B0 A1 B1 A2 B2 A3 A0 B3 74x85 A**B Most Significant bitsLeast Significant bits
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Mode-dependent Comparator Design a c.c. whose inputs are two 8-bit unsigned binary integers(X,Y), and a control signal(MIN/MAX). The output is an 8-bit unsigned binary integer Z, such that Z=min(X,Y) if MIN/MAX=1; Z=max(X,Y) otherwise

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Next… Sequential Logic Introduction

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