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1、Digital Electronics-Lecture 7ForsChapter 6-MSI Combinational Circuits II6-5Decoders译码器A Decoder is a combinational logic circuit that detects the presence of a specified combinationofbits (code) on its inputs and indicates thepresence of that code by a specified output level.The basic binary decoder

2、2YJ, Department ofElectrical & ElectronicTechnology, SAEE, USTBThe output is 1 only when:A0 = 1A1 = 0A2 = 0A3 = 1More Examples:A0 A1 A2A3A0 A1A2A3 XXdecoder for 0011Active LOW outputdecoder for 0011Active HIGH output3YJ, Department ofElectrical &Electronic Technology, SAEE, USTBA 0 is the active out

3、put stateA 1 is the active output stateExercise:Determine the logic required to decodethe binary number 1011 by producing a HIGH output level on the output.active-HIGH output4YJ, Department ofElectrical& Electronic Technology, SAEE,USTBThe 4-bit decoder4-bit decoder, also called 4-line-to-16-line de

4、coder, which can decode all possible combinations of 4 bits.Active-low outputs5YJ, Department ofElectrical & Electronic Technology, SAEE, USTBTtable of the 4-bit decoder6YJ, Department ofElectrical & Electronic Technology, SAEE,USTBIC: Pin diagram and logic symbol for the 74HC154The enable function:

5、CS1CS 27YJ, Department ofElectrical & Electronic Technology, SAEE, USTBExpansion:A 5-bit decoder using 74HC154s8YJ, Department ofElectrical & Electronic Technology, SAEE, USTBApplication: A simplified computer I/O port system with a port address decoder with only four address lines shown.9YJ, Depart

6、ment ofElectrical & Electronic Technology, SAEE, USTBSupplement：Use the binary decoder as a logicfunction generatorY0 = A2 A1 A0= m0= m1=m2Y1 = A2 A1 A0= A2 A1 A0Y2 Y3 Y4Y5= A2 A1 A0=m3=m4=m5=m6= A2 A1 A0= A2 A1 A0Y = m= A A AYii6210= A A A =m7Y721010YJ, Department ofElectrical &Electronic Technolog

7、y, SAEE, USTB二进制译码器的每一项输出对应一个最小项，所有输出包含所有的最小项。Ex 1: Use a decoder with a few gates necessary to implement the following expression.Y=AB+BC+CAA B CSolution：1) Choose IC3 inputs， Use 74LS1382) Change the expression to the standard SOP form.Y = ABC + ABC + ABC + ABC= Y3Y5Y6Y7= Y3 + Y5 +Y6 +Y711YJ, Depa

8、rtment ofElectrical & Electronic Technology, SAEE,USTBEx 2: Use a 2-line-to-4-line decoder 74LS139 and a few gates necessary to implement the following 2-outputs function.Z1 = AB + ABZ2 = AB + ABFunction table of 2-line-to-4-line decoderY0 = A1 A0 =m0Y1 = A1 A0 =m1Y = A A =m2210Y = A A =m331012YJ, D

9、epartment ofElectrical & ElectronicTechnology, SAEE, USTBInputsOutputsSA1A0Y3Y2Y1Y0100000101001111111110110110110111Solution：Y = A AY = A A010110= A1 A0Y3 = A1 A0Y2= AB + AB= AB + ABZ2Z1Z2Z1Let：A=A ；B=A10= Y 1 + Y 2= Y 1Y 2Then：Z1= Y 0+ Y 3= Y 0Y 3Z2AB13YJ, DepartmentofElectrical &Electronic Technol

10、ogy,SAEE, USTB&Y0Y1Y2Y374LS139SA1A0Exercise: Use a 3-line-to-8-line decoder to realize the following combinational function.Y = AB + BC14YJ, Department ofElectrical &Electronic Technology, SAEE, USTBExercise: Write the Boolean expression of Z1 inminimum SOP form.= miYiZ1 (A,B,C) = Y3Y4Y5Y6 = Y3 + Y4

11、 + Y5 + Y6= ABC + ABC + ABC + ABC= m(3，4，5，6)15YJ, Department ofElectrical & Electronic Technology, SAEE,USTBZ1 = ABC + AB + AC用最小项译码器实现组合逻辑函数：n/2n 线译码器，包含了n变量所有的最小项。加上或与非门，可以组成的输入变量不大于n的组合逻辑函数。实现n变量逻辑函数：将逻辑表达式写成n变量最小项形式16YJ, Department ofElectrical & Electronic Technology,SAEE, USTBThe BCD-to-decim

12、al decoder4-line-to-10-line decoder1-of-10 decoder17YJ, Department ofElectrical & Electronic Technology, SAEE, USTBIC:The 74HC42 BCD-to-decimal decoder18YJ, Department ofElectrical & Electronic Technology, SAEE, USTBThe BCD-to-7-segment decoder（显示译码器）Block diagram of 7-segment logic and display19YJ,

13、 Department ofElectrical & Electronic Technology, SAEE, USTBP142 :Digital System Application7-segment displaysLED (light-emitting diode) displays20YJ, Department ofElectrical & Electronic Technology, SAEE, USTBTtable of the BCD-to-7-segment decoderabfced21YJ,DepartmentofElectrical & Electronic Techn

14、ology, SAEE, USTBgIC: Pin diagram and logic symbol for the 74LS47 BCD-to-7-segment decoder/driver.22YJ, Department ofElectrical & Electronic Technology,SAEE, USTB74LS47 Function Tableylamp tblankinzero suppress23YJ, Department ofElectrical & Electronic Technology, SAEE, USTBinputoutputdisplaLTRBIBIA

15、3A2A1A0a b c d e f gest 010 0 0 0 0 0 08g 01 1 1 1 1 1 1全灭i1on010 0 0 01 1 1 1 1 1 1全灭111110 0 0 08421码0 0 0 0 0 0 1译码019Zero suppression using the 74LS4700 0 0 000 0 0 000 0 1 110 0 0 0RBI LT8 4 2 174LS47g f e d c b a8 4 2 174LS47d c b a8 4 2 174LS47e d c b a8 4 2 1RBI LTRBI LTRBI LT74LS47d c b aBI

16、/RBOg f eBI/RBOgfBI/RBOgfeBI/RBOBlankedBlanked0101011100000000100Blanked& Electronic Technology,BlankedSAEE, USTB24YJ, Department ofElectricalRBI LT8 4 2 174LS47g f e d c b a BI/RBORBI LT8 4 2 174LS47g f e d c b a BI/RBORBI LT8 4 2 174LS47g f e d c b a BI/RBORBI LT8 4 2 174LS47g f e d c b a BI/RBOCh

17、ap06:P251-22Exercise: Determine the decimal number shown on the 7-segment displayer.25YJ, Department ofElectrical & ElectronicTechnology, SAEE, USTB6-6Encoders 编码器An encoder is a combinational logic circuit that essentially performs a “reverse” decoder function.An encoder accepts an active level on

18、one of its inputs which representing a digit, such as a decimal or octal digit, and converts it to a coded output, such as BCD or binary.26YJ, Department ofElectrical & Electronic Technology, SAEE, USTBDecimal-to-BCD encoderAt one moment only one input iive.27YJ, Department ofElectrical & Electronic

19、 Technology, SAEE, USTBIC: 74HC147 decimal-to-BCD priority encoderactive LOW inputsactive LOW outputsA 0-digit input is not needed because the BCD outputs are all HIGH when there are no active inputs.28YJ, Department ofElectrical & Electronic Technology, SAEE, USTBHPRI means highest value input has

20、priority74LS147 Function Table29YJ, Department ofElectrical & Electronic Technology, SAEE,USTBInputOutputI9I8I7I6I5I4I3I2I1Y3Y2Y1Y0111111111010 110 1110111101111101111110111111101111111101111011001111000100110101011110011011110Q: If LOW levels appear on pins 1,4 and 13, indicate the state of the fou

21、r outputs.(all other inputs aIGH)If all inputs are LOW ?If all inputs aIGH ?74HC14730YJ, Department ofElectrical & Electronic Technology, SAEE,USTBApplication:A simplified keyboard encoder31YJ, Department ofElectrical & Electronic Technology, SAEE, USTB8-line-to-3-line encoder32YJ, Department ofElectrical & ElectronicTechnology, SAEE, USTBIC: 74LS148 8-line-to-3-line encoder.EI: enable input EO: enable outputGS: expansion purposeEO is LOW when EI is LOW and none of theinputs iiveGS is LOW when EI isLOW and any of the inputsiive33YJ, Depa