Fundamentals of digital circuits (III) encoder and decoder

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 .$A_0$,$A_1$,$A_2$ There are three outputs , Their logical expressions are as follows ：
$A_0=\overline{\overline{I_1}\cdot \overline{I_3}\cdot \overline{I_5}\cdot \overline{I_7}}$
$A_1=\overline{\overline{I_2}\cdot \overline{I_3}\cdot \overline{I_6}\cdot \overline{I_7}}$
$A_2=\overline{\overline{I_4}\cdot \overline{I_5}\cdot \overline{I_6}\cdot \overline{I_7}}$
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 .

$\overline{I_S}$ It's the control end , Its function is to control whether the encoder works , Only when $I_S$ Input is 0 when , Encoder can encode .
$\overline{E}$ 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 $I_0$~$I_7$ in ,$I_7$ The highest priority ,$I_0$ The lowest . No matter $I_0$~$I_6$ Whether the input is a valid signal , as long as $I_7$ The input is a valid signal , Will be encoded first $I_7$ 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 ：

$\overline{F_0}$ ~ $\overline{F_7}$ by 8 Outputs ,$\overline{S_1}$ ~ $\overline{S_3}$ by 3 Two control terminals , Only when $\overline{S_1}$ by 1 And $\overline{S_2}$ and $\overline{S_3}$ 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 $\overline{LT}$

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 $\overline{BI}$

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 $\overline{BRI}$

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 $\overline{RBO}$

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