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Fundamentals of digital circuits (III) encoder and decoder
2022-07-06 14:42:00 【ブリンク】
Fundamentals of digital circuits ( 3、 ... and ) Encoder and decoder
This paper will introduce the logic function and application of encoder and decoder in detail .
One 、 Encoder
Compile information with specific meaning into several binary codes , This process is called encoding , The digital circuit that realizes this function is called encoder .
1. Binary encoder
The following figure shows the truth table realized by the three bit binary encoder :
Due to the uniqueness of coding , Only one input signal can be encoded at a certain time , Therefore, among the eight input levels , Only one is low , The rest are high level . A 0 A_0 A0, A 1 A_1 A1, A 2 A_2 A2 There are three outputs , Their logical expressions are as follows :
A 0 = I 1 ‾ ⋅ I 3 ‾ ⋅ I 5 ‾ ⋅ I 7 ‾ ‾ A_0 = \overline{\overline{I_1}\cdot \overline{I_3}\cdot \overline{I_5}\cdot \overline{I_7}} A0=I1⋅I3⋅I5⋅I7
A 1 = I 2 ‾ ⋅ I 3 ‾ ⋅ I 6 ‾ ⋅ I 7 ‾ ‾ A_1 = \overline{\overline{I_2}\cdot \overline{I_3}\cdot \overline{I_6}\cdot \overline{I_7}} A1=I2⋅I3⋅I6⋅I7
A 2 = I 4 ‾ ⋅ I 5 ‾ ⋅ I 6 ‾ ⋅ I 7 ‾ ‾ A_2 = \overline{\overline{I_4}\cdot \overline{I_5}\cdot \overline{I_6}\cdot \overline{I_7}} A2=I4⋅I5⋅I6⋅I7
The following figure is their logic diagram :
Just connect the same pins together to form an integrated three bit binary encoder .
2. Priority encoder
The problem with the above binary encoder is : Only one input is allowed to be a valid signal , Otherwise, the encoder will produce an error . To solve this problem , A priority encoder is designed .
This article will introduce 8-3 Line priority encoder 74148 The logical function of . First list its truth table :
It can be seen from the picture that , The input effective level of the encoder is low , The output code is in the form of inverse code .
I S ‾ \overline{I_S} IS It's the control end , Its function is to control whether the encoder works , Only when I S I_S IS Input is 0 when , Encoder can encode .
E ‾ \overline{E} E It is the extension end , If the input levels are all 1 There may be wrong level input , The output 1.
When a valid level is input , It will be encoded according to priority , stay I 0 I_0 I0~ I 7 I_7 I7 in , I 7 I_7 I7 The highest priority , I 0 I_0 I0 The lowest . No matter I 0 I_0 I0~ I 6 I_6 I6 Whether the input is a valid signal , as long as I 7 I_7 I7 The input is a valid signal , Will be encoded first I 7 I_7 I7 The signal at the input .
Two 、 Decoder
Decoding is the inverse process of coding . He can translate a code into actual meaning . The logic circuit used to realize it is called decoder .
1. Binary decoder
This paper mainly introduces 3-8 line decoder 74138 The logical function of . Its truth table and integrated chip pin diagram are given below :
F 0 ‾ \overline{F_0} F0 ~ F 7 ‾ \overline{F_7} F7 by 8 Outputs , S 1 ‾ \overline{S_1} S1 ~ S 3 ‾ \overline{S_3} S3 by 3 Two control terminals , Only when S 1 ‾ \overline{S_1} S1 by 1 And S 2 ‾ \overline{S_2} S2 and S 3 ‾ \overline{S_3} S3 by 0 The decoder is only in working state . When the input side enters a specific value in the table , The output terminal will also output the corresponding value . And each output value of the binary decoder is the value of the minimum term of the input code ( Low level active ), So we can use binary decoder 74138 Implement logical functions . It is also a kind of decoder often used in digital circuits .
2. Seven segment font decoder
The figure below shows a seven segment digital tube :
7 The light-emitting diodes are arranged in this pattern , Each segment is controlled by a pin .7 There are two kinds of segment nixie tubes , Common anode digital tube and common cathode digital tube , among , Common anode nixie tube 7 The anodes of two light-emitting diodes are connected together ,7 Each cathode is independent ; And common cathode digital tube 7 The cathodes of two light-emitting diodes are connected together ,7 The two anodes are independent .
When a cathode of the common anode nixie tube is connected to the low level , The corresponding diode emits light , Therefore, the common anode digital tube needs to output a low-level effective decoder drive , The common cathode digital tube needs to output high-level and effective decoder drive .
As shown in the figure below, it is a seven segment font decoder with effective output low level 74LS47 Integrated chip pin diagram and its truth table :
7447 The chip is decoding at the same time , Some other pins have the following functions :
(1) Test lamp input L T ‾ \overline{LT} LT
As the name suggests, the test lamp input is set to detect whether each segment of the nixie tube works normally , When the input is 0 when , No matter what the rest of the input is , The decoder outputs low level , It can ensure that each LED turns on .
(2) Light off input B I ‾ \overline{BI} BI
Light off input is just as the name suggests , It is set up to turn off the light of each section of nixie tube . When the input is 0 when , No matter what state other inputs are , All decoders input high level , Make all digital tubes go out
(3) Zero out input B R I ‾ \overline{BRI} BRI
The function of zero input is to make it unnecessary to display 0 Extinguished and set , for example : When we are showing 0088.8800 when , In fact, just show 88.88 that will do , Put other positions 0 Just go out .
(4) Zero out output R B O ‾ \overline{RBO} RBO
The zero off input and the light off input share one port , They are usually used together , It can realize the zero elimination control of multiple digits , There is no detailed introduction here .
2022/5/6 first draft
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