Circuits that use digital signals to perform arithmetic and logical operations on digital quantities are called digital circuits, or digital systems. Because it has logic operations and logic processing functions, it is also called a digital logic circuit. Modern digital circuits are constructed from a number of digital integrated devices made by semiconductor technology.
The development of digital circuits, like analog circuits, has gone through several eras from electron tubes, semiconductor discrete devices to integrated circuits. But its development is faster than that of analog circuits. Since the 1960s, digital integrated devices have been made into small-scale logic devices with a bipolar process. Then it developed to medium-scale logic devices; in the late 1970s, the appearance of microprocessors brought a qualitative leap in the performance of digital integrated circuits.
Digital systemThe composition of the digital system is shown in the figure.
The digital system usually consists of input interface, output interface, data processing and controller. The main task of input interface and output interface is to convert analog quantity into digital quantity or digital quantity into analog quantity. The main function of the processor is to control the work of various components in the system and make them operate according to certain procedures. Usually, the presence or absence of a controller is used as a sign to distinguish between functional components and digital systems. Any system that contains a controller and can be operated in sequence, regardless of scale, is called a digital system.
Digital circuit characteristics1. It has both arithmetic operation and logic operation functions. Digital circuits are based on binary logic algebra and use binary digital signals to perform arithmetic operations and logical operations conveniently. Therefore, they are extremely suitable for applications such as operations, comparison, storage, transmission, control, and decision-making.
2. The realization is simple and the system is reliable. Digital logic circuit based on binary system has strong reliability. The small fluctuation of the power supply voltage has no influence on it, and the influence of temperature and process deviation on the reliability of its work is much smaller than that of the analog circuit.
3. High integration and easy function realization. High integration, small size, and low power consumption are one of the outstanding advantages of digital circuits. Circuit design, maintenance, and maintenance are flexible and convenient. With the rapid development of integrated circuit technology, the integration of digital logic circuits is getting higher and higher. The functions of integrated circuit blocks follow the trend of small-scale integrated circuits, medium-scale integrated circuits, and large-scale integration. The development of circuits and very large-scale integrated circuits has also risen from the component level, device level, component level, and board level to the system level.
Advantages of digital circuits: good stability, high reliability, long-term storage, convenient for computer processing, and high integration.
Digital circuit classificationAccording to function, it is divided into combinational logic circuit and sequential logic circuit.
The output of the former at any moment depends only on the input state of the circuit at the moment, and has nothing to do with the state of the circuit in the past, and they do not have a memory function. Commonly used combinational logic devices include adders, decoders, and data selectors.
The output of the latter at any time depends not only on the input state of the circuit at the moment, but also on the state of the circuit in the past. They have a memory function.
According to the structure, it is divided into discrete component circuits and integrated circuits. The former is a circuit that connects independent transistors, resistors and other components with wires.
In the latter, the components and wires are fabricated on a semiconductor silicon chip, packaged in a shell, and the lead circuit is soldered out. The degree of integration of integrated circuits is different.
Application of digital circuitAt present, the application of digital circuits has been extremely extensive. In digital communication systems, in image and TV signal processing, several 0s and 1s can be used to compile various codes, which represent different information meanings.
In automatic control, you can use the logic function of digital circuits to design a variety of digital control devices. In measuring instruments, digital circuits can be used to process the measurement signals, and the test results can be displayed in decimal numbers; especially in digital electronic computers, digital circuits can be used to process digital information with various functions. Digital electronic computers have penetrated into all areas of the national economy and people's lives, and have brought about fundamental changes in many aspects.
The main research object of digital circuit is the logical relationship between the output and input of the circuit, so the analysis method of analog circuit cannot be used in the digital circuit, for example, the small signal model analysis method. Since the devices in the digital circuit mainly work in the switching state, the analysis tools used are mainly logic algebra, and the main functions of the circuit are expressed with function tables, truth tables, logic expressions, and waveform diagrams.
With the development of computing technology, in order to analyze, simulate and design digital circuits or digital systems, it is also possible to use hardware description language, use software such as ABEL language, and use computers to analyze, simulate and design digital systems.
Digital Circuit Recognition Skills1. Right or wrong depends on the logic. Read the circuit specification to understand the structure, function, and purpose of the logic circuit. You can also read the truth table to understand the logical relationship of "yes" or "not" between output and input, and master the logical functions of each unit module.
2. The function of the components depends on the pins. Digital circuits often use integrated circuits with various logic functions, which will make the entire circuit simpler and more reliable. But it also brings certain difficulties for recognizing pictures. Because you can’t see the internal components and circuit composition of the integrated circuit, you can only see many external pins. These pins have their own functions. These pins are connected with other external components or circuits to achieve certain functions. . In fact, many times you don't need to know the internal circuit composition of the integrated circuit, just understand the function of the external pins. The function of each pin of the integrated circuit is marked with text. If no text description or parameter is given in the circuit, you should consult the relevant manual to understand the logic function of the integrated block and the role of each pin. For some commonly used integrated circuits, such as the commonly used operational amplifier LM 324, four two-input NAND gates, seven four LS zero zero, five five five time base circuits, etc., readers should remember the function of each pin. Quick and accurate picture recognition is helpful.
3. Look at the module for functional decomposition. For digital circuits, the system can be divided into several functional modules according to the signal flow direction. Each module completes a relatively independent function, analyzes the interaction status of the modules, and lists the input and output logic truth tables of each module if necessary.
4. Take a comprehensive look at the whole. Connect the modules to analyze the complete working process of the circuit from input to output, and draw relevant working waveforms when necessary to help analyze and understand the logic functions of the circuit.
Research in digital circuitsThe main problem studied in digital circuits is the logical relationship between the state of the output signal ("0" or "1") and the input signal ("0" or "1"), that is, the logical function of the circuit.
The research methods of digital circuits are logic analysis and logic design, and the tools needed are logic algebra. (Under positive logic, "0" is low level, "1" is high level, and there is no clear boundary between high and low levels)
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