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**Speical purpose Encoders/Comparators**

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

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HW solutions Ex. 5.46 Ex. 5.19

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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) 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. Select one of n sources of data to transmit on a bus.

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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 74x 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 +5V 74x85 74x85 A<B A<BIN A<BOUT A<BIN**

Least Significant bits Most 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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