ENGR/ECE 212 21 February, 2003
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1 ENGR/ECE 212 21 February, 2003 All slides except those with blue backgrounds are taken from John Wakerly’s Stanford EE 121, Digital Design Laboratory
description
ENGR/ECE 212 21 February, 2003. All slides except those with blue backgrounds are taken from John Wakerly’s Stanford EE 121, Digital Design Laboratory. Decoders. General decoder structure Typically n inputs, 2 n outputs 2-to-4, 3-to-8, 4-to-16, etc. with a few enable inputs. - PowerPoint PPT Presentation
Transcript of ENGR/ECE 212 21 February, 2003
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ENGR/ECE 21221 February, 2003
All slides except those with blue backgrounds
are taken from John Wakerly’s
Stanford EE 121, Digital Design Laboratory
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Decoders• General decoder structure
• Typically n inputs, 2n outputs– 2-to-4, 3-to-8, 4-to-16, etc.– with a few enable inputs
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Binary 2-to-4 decoder
Note “x” (don’t care) notation.
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MSI 2-to-4 decoder
• Input buffering (less load)• NAND gates (faster)
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Decoder Symbol
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3-to-8 decoder
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74x138 3-to-8-decoder symbol
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Decoder cascading
4-to-16 decoder
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More cascading
5-to-32 decoder
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ABEL / PAL Version of 74x138module Z74X138
title '74x138 Decoder PLD J. Wakerly, Stanford University‘
"Z74X138 device 'P16L8';
" Input pins
A, B, C, !G2A, !G2B, G1 pin 1, 2, 3, 4, 5, 6;
" Output pins
!Y0, !Y1, !Y2, !Y3 pin 19, 18, 17, 16 istype 'com';
!Y4, !Y5, !Y6, !Y7 pin 15, 14, 13, 12 istype 'com';
" Constant expression
ENB = G1 & G2A & G2B;
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ABEL decoder program (continuedequationsY0 = ENB & !C & !B & !A; Y1 = ENB & !C & !B & A; Y2 = ENB & !C & B & !A; Y3 = ENB & !C & B & A; Y4 = ENB & C & !B & !A; Y5 = ENB & C & !B & A; Y6 = ENB & C & B & !A; Y7 = ENB & C & B & A;
end Z74X138
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Decoder applications
• Microprocessor memory systems– selecting different banks of memory
• Microprocessor input/output systems– selecting different devices
• Microprocessor instruction decoding– enabling different functional units
• Memory chips– enabling different rows of memory depending on
address
• Lots of other applications
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Some decoder datasheets
• 74x138– 3-line to 8-line decoders/demultiplexers
• 74x139– Dual 2-line to 4 line decoders/demultiplexers
• 74x154– 4-to-16 line decoders/demultiplexers
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Encoders vs. Decoders
Decoder Encoder
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Binary encoders
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Need priority in most applications
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8-input priority encoder
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74x148 8-input priority encoder
– Active-low I/O– Enable Input– “Got Something”– Enable Output
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74x148circuit
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Cascading priority encoders
• 32-inputpriority encoder
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Some encoder datasheets
• 74x148– 8-line to 3-line priority encoders
• 74x147– 10-line to 4-line priority encoders
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Multiplexers
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74x1518-input
multiplexer
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Some multiplexer datasheets
• 74x151– 8-Input Multiplexer
• 74x153– Dual 4-Input Multiplexer
• 74x157, 74x158– Quad 2-Input Multiplexer
