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Characteristics of digital logic gate circuit

Last Update Time: 2018-08-23 11:25:16
Warm hints: The word in this article is about 3000 words and reading time is about 12 minutes

Characteristics of digital logic gate circuit


Logical algebra, also known as Boolean algebra or Commutative algebra, is a mathematical tool to express and deal with various logical relations between things. There are only two values in logic algebra, called binary logic. In binary logic, for any proposition p, P is true, or P is false, there is no other case.


The advantages of two element logic are simple and clear, and the disadvantage is that it can not directly describe many complex realities. Because binary logic is simple and straightforward, it is easy to implement by circuit, that is, binary logical albums can be implemented as long as the "through" and "off" (closed and off) states. The early switch circuit is composed of relay and switch, so it is called switch circuit. With the development of science and technology, digital electronic technology has gradually developed.


The core idea of digital electronic technology is binary logic. In logic algebra, 0 and 1 do not represent the size, but represent the two opposite states of the existence of things. 0 if it represents "yes", then 1 represents "non" and vice versa. In order to distinguish the size of ordinary mathematics, 0 and 1 are also called 0 states (0 states) and 1 states (1 states). Because there are only two kinds of logical algebra, the logical operations are very simple, and the logical operations are only three basic logical operations.or (OR), so other complex operations can be attributed to these three operations.


  1. The results of three basic logical operations


(1) operation of all conditions that determine the outcome of an event is called operation. If the condition used to determine Y is only A and B, the logical values corresponding to the relationships between A, B and Y are described as truth tables. The logical truth table is shown in Table 5-1. The logical expression is y=.B, read "a and B". Logical variables are usually represented by a capital letter, which can be abbreviated as Y=AB without misunderstandings.


(2) When all the conditions of the event result are determined, at least one condition is satisfied, the event occurs, and the call or operation is performed.


2. Introduction of digital logic gate circuit


(1) the characteristic of digital logic gate circuit is called digital circuit. It has the following characteristics:

1) strict logic.

2) strict timing.

3) there are only two logic levels (or pulses).

4) the magnitude of the level corresponding to the logical value (0 or 1) varies according to the actual circuit.

5) firmware features are obvious. Because digital circuits process logic level signals, from the point of view of signal processing, digital circuits have higher signal anti-interference ability than analog circuits.


(2) classification of digital circuits

1) digital circuits can be divided into small scale, medium scale, large scale and super large scale according to the integration degree. Small digital circuits (SS1) are usually logical unit circuits, such as logic gates, flip flops, etc. MSI is usually a logical function circuit, such as decoder, data selector, counter, register, etc. Large-scale digital circuits (LSI) and very large-scale digital circuits (VLSI) are usually small digital logic systems and have been made into large-scale modular digital circuits.

2) digital circuits can be divided into bipolar and unipolar circuits according to polarity. The representative of bipolar circuit is TTL circuit. In addition to the standard form, there are four other structural forms: high-speed TTL (74H series), low-power TTL (74L series), ultra-high-speed TTL (74S series) and low-power Schottky TTL (74LS series). CMOS circuit is the representative of unipolar circuit. The domestic CMOS circuit is mainly composed of C000 series. Its function and configuration of external leads are similar to that of international CD4000 series. In high-speed CMOS circuits, the 74HC and 74HCT series correspond to the TTL 74 series, the main difference is that the supply voltage is different, the 74HCT series and 74LS series are