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基于74LS194的m序列发生器.docx

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基于74LS194的m序列发生器 Introduction: Them-sequencegeneratorbasedon74LS194isacircuitthatgeneratesasequenceofbinarynumberswhicharepseudo-random.Thiscircuitiscommonlyusedinvariousapplicationssuchascryptography,spreadspectrumcommunications,andtestingandverificationofdigitalsystems.Them-sequencegeneratorisafeedbackshiftregisterwithafeedbackpolynomialthatdeterminesthesequencegenerated.The74LS194isahigh-speed4-bitripplecarryshiftregisterthatcanbecascadedtoformanm-sequencegenerator.Thispaperwilldiscussthedesignandoperationofthem-sequencegeneratorbasedon74LS194. Design: Thedesignofthem-sequencegeneratorinvolvesselectingafeedbackpolynomialfortheshiftregister.Thefeedbackpolynomialdeterminesthesequencegeneratedbytheshiftregister.Thefeedbackpolynomialmustbeaprimitivepolynomialinordertogenerateamaximum-lengthsequence.Aprimitivepolynomialisapolynomialthatgeneratesasequenceofmaximumlengthforagivennumberofbits.Thepolynomialfora4-bitshiftregisterisx^4+x+1.Thispolynomialgeneratesamaximum-lengthsequenceof15bits.Them-sequencegeneratorbasedon74LS194consistsoffour74LS194scascadedtogethertoforma16-bitshiftregister. Operation: Them-sequencegeneratorbasedon74LS194operatesbyshiftingthecontentsoftheshiftregistertotheright.Theoutputoftherightmostbitisfedbacktotheinputoftheleftmostbitthroughanexclusive-ORgate.Thefeedbackpolynomialdeterminesthelogicfunctionperformedbytheexclusive-ORgate.Theoutputoftheshiftregisteristhem-sequencegeneratedbythecircuit.Thelengthofthesequenceisdeterminedbythenumberofbitsintheshiftregisterandthefeedbackpolynomialused.Inthecaseofthe74LS194-basedm-sequencegenerator,thesequencelengthis2^4-1=15. Applications: Them-sequencegeneratorbasedon74LS194hasmanyapplicationsindigitalsystems.Oneofitsmainapplicationsisincryptography.Thegeneratedm-sequencecanbeusedasakeystreamforastreamcipher.Thesequenceispseudo-randomandhasalongperiodwhichmakesitdifficulttobreakthecipher.Them-sequencegeneratorisalsousedinspreadspectrumcommunications.Them-sequencegeneratedbythecircuitisusedtospreadtheorigina