Digital-electronics 简明教程

Digital Electronics - Sequential Circuits

数字电路分为两大类，即 combinational circuits 和 sequential circuits 。我们已经在本教程的前几章讨论了组合电路。本章将重点介绍顺序电路的详细信息。

Digital circuits are classified into two major categories namely, combinational circuits and sequential circuits. We have already discussed about combinational circuits in the earlier chapters of this tutorial. This chapter will highlight the details of sequential circuits.

顺序电路是一种数字逻辑电路，其输出取决于当前输入以及电路的过去运算。让我们从本教程的本节开始对顺序电路进行基本的介绍。

A sequential circuit is a type of digital logic circuit whose output depends on present inputs as well as past operation of the circuit. Let us start this section of the tutorial with a basic introduction to sequential circuits.

What is a Sequential Circuit?

sequential circuit 是一个逻辑电路，它包含一个存储电路过去运算历史的存储元件。因此，顺序电路的输出取决于当前输入以及电路的过去输出。

A sequential circuit is a logic circuit that consists of a memory element to store history of past operation of the circuit. Therefore, the output of a sequential circuit depends on present inputs as well as past outputs of the circuit.

block diagram of a typical sequential circuit 如以下图片所示−

The block diagram of a typical sequential circuit is shown in the following figure −

sequential circuit

这里可以看出，顺序电路基本上是组合电路和存储元件的组合。组合电路执行指定逻辑运算，而存储元件记录电路运算的历史记录。此记录随后用于将来执行各种逻辑运算。

Here, it can be seen that a sequential circuit is basically a combination of a combinational circuit and a memory element. The combinational circuit performs the logical operations specified, while the memory element records the history of operation of the circuit. This history is then used to perform various logical operations in future.

顺序电路之所以这样命名，是因为它们使用一系列最新和以前的输入来确定新的输出。

The sequential circuits are named so because they use a series of latest and previous inputs to determine the new output.

Main Components of Sequential Circuit

顺序电路由若干不同的数字元件组成，以处理和存储系统中的信息。以下是顺序电路的一些关键组件的说明−

A sequential circuit consists of several different digital components to process and hold information in the system. Here are some key components of a sequential circuit explained −

Logic Gates

与、或、非等逻辑门用于实现顺序电路的数据处理机制。这些逻辑门基本上以特定方式互连，以实现组合电路，对输入数据执行逻辑运算。

The logic gates like AND, OR, NOT, etc. are used to implement the data processing mechanism of the sequential circuits. These logic gates are basically interconnected in a specific manner to implement combinational circuits to perform logical operations on input data.

Memory Element

在顺序电路中，存储元件是另一个关键组件，它持有电路运算的历史记录。一般来说，触发器用作顺序电路中的存储元件。

In sequential circuits, the memory element is another crucial component that holds history of circuit operation. Generally, flip-flops are used as the memory element in sequential circuits.

在顺序电路中，在输出和输入之间提供一个反馈路径，该反馈路径将信息从输出端传输到存储元件，并从存储元件传输到输入端。

In sequential circuits, a feedback path is provided between the output and the input that transfers information from output end to the memory element and from memory element to the input end.

所有这些组件相互连接在一起，以设计能够执行复杂运算并将状态信息存储在存储元件中的顺序电路。

All these components are interconnected together to design a sequential circuit that can perform complex operations and store state information in the memory element.

Types of Sequential Circuits

根据结构、操作和应用，顺序电路分为以下两种类型−

Based on structure, operation, and applications, the sequential circuits are classified into the following two types −

  1. Asynchronous Sequential Circuit

  2. Synchronous Sequential Circuit

让我们详细讨论这两种顺序电路。

Let us discuss both of these sequential circuits in detail.

Asynchrnous Sequential Circuit

一种顺序电路，其操作不依赖于时钟信号，称为 asynchronous sequential circuit 。这种类型的顺序电路使用输入脉冲工作，这意味着它们的状态会随着输入脉冲的变化而改变。

A type of sequential circuit whose operation does not depend on the clock signals is known as an asynchronous sequential circuit. This type of sequential circuits operates using the input pulses that means their state changes with the change in the input pulses.

异步顺序电路的主要组件包括非时钟触发器和组合逻辑电路。 block diagram of a typical asynchronous sequential circuit 如以下图片所示。

The main components of the asynchronous sequential circuits include un-clocked flip flops and combinational logic circuits. The block diagram of a typical asynchronous sequential circuit is shown in the following figure.

asynchronous sequential circuit

从该图表中，很明显异步时序电路类似于带反馈机制的组合逻辑电路。

From this diagram, it is clear that an asynchronous sequential circuit is similar to a combinational logic circuit with a feedback mechanism.

异步时序电路主要用于在时钟信号不可用或不适合使用的情况中。例如，在任务执行速度很重要的条件下。

Asynchronous sequential circuits are mainly used in applications where the clock signals are not available or practical to use. For example, in conditions when speed of the task execution is important.

异步时序电路设计起来相对困难，有时会产生不确定的输出。

Asynchronous sequential circuits are relatively difficult to design and sometimes they produce uncertain output.

ripple counter 便是异步时序电路的常用示例。

The ripple counter is a common example of asynchronous sequential circuit.

Synchronous Sequential Circuit

同步时序电路是一种时序电路，其中所有存储单元都由一个公共时钟信号同步。因此，同步时序电路除了输入信号外，还要接受一个时钟信号。

A synchronous sequential circuit is a type of sequential circuit in which all the memory elements are synchronized by a common clock signal. Hence, synchronous sequential circuits take a clock signal along with input signals.

在同步时序电路中，输出脉冲的持续时间等同于应用的时钟脉冲的持续时间。示例请参见 block diagram of a typical synchronous sequential circuit −

In synchronous sequential circuits, the duration of the output pulse is equivalent to the duration of the clock pulse applied. Take a look at the block diagram of a typical synchronous sequential circuit −

synchronous sequential circuit

在该图中，可以看到时序电路的存储单元由一个时钟信号同步。

In this figure, it can be seen that the memory element of the sequential circuit is synchronized by a clock signal.

同步时序电路的主要缺点是，其运行非常慢。这是因为，电路每次进行操作时都必须等待时钟脉冲。然而，同步时序电路最显著的优势是其运行可靠且可预测。

The major disadvantage of the synchronous sequential circuits is that their operation is quite slow. This is because, every time the circuit has to wait for a clock pulse for the operation to take place. However, the most significant advantage of synchronous sequential circuits is that they have a reliable and predictable operation.

同步时序电路的一些常见示例包括计数器、寄存器、存储单元、控制单元等。

Some common examples of synchronous sequential circuits include counters, registers, memory units, control units, etc.

Sequential Circuits vs. Combinational Circuits

以下是时序电路相较于组合逻辑电路的一些主要优势：

Given below is a list of some of the main advantages of using sequential circuits over combinational logic circuits −

  1. Sequential circuits can retain the operation history which is important in various applications like data storage, feedback control systems, etc.

  2. Sequential circuits exhibit dynamic behavior and can execute complex operation in real time.

  3. Sequential circuits comprise a feedback mechanism which improves the stability and optimizes the system performance.

  4. Synchronous sequential circuits use a common clock signal for synchronization that ensures reliable operation of the circuit.

  5. Sequential circuits can perform more complex operations using simpler circuit designs than combinational circuits. Hence, their hardware complexity is lesser.

Disadvantages of Sequential Circuits

但是，时序电路也确实存在一些缺点。下面列出了时序电路的一些主要缺点：

Sequential circuits do have several disadvantages too. Some of the key disadvantages of using sequential circuits are listed below −

  1. Sequential circuits have higher propagation delay because the input signal passes through multiple stages of logic circuits and memory elements.

  2. Sequential circuits are relatively complicated and time taking process to design and analyze.

  3. Sequential circuits require a proper synchronization and clock distribution to work as intended.

  4. As compared to combinational circuits, sequential circuits consume relatively more power due to complex design and use of additional components like clock and memory element.

Applications of Sequential Circuits

顺序电路在数字电子学领域中的应用非常广泛。以下是一些顺序电路应用的常见示例 −

Sequential circuits are used in a wide range of applications in the field of digital electronics.The following are some common examples of applications of sequential circuits −

  1. Sequential circuits are used in digital counters employed in applications like frequency division, event counting, time keeping, and more.

  2. Sequential circuits are also used in digital memory devices like flip-flops, registers, etc. to store and retrieve data.

  3. Sequential circuits are used to design control circuits in digital systems.

  4. Sequential circuits play an important role in sequential logic and state-based data processing operations.

  5. Sequential circuits are also used in automation systems to control the operation of machines based on predefined logics.

  6. In communication systems, sequential circuits are used to implement communication protocols and data transmission standards.

Conclusion

顺序电路是数字电子系统中的重要组件。顺序电路只不过是组合逻辑电路和存储单元的组合，其中存储单元与组合电路通过反馈机制连接。

Sequential circuits are important components in digital electronic systems. A sequential circuit is nothing but a combination of combinational logic circuit and a memory element, where the memory element is connected in a feedback mechanism with the combinational circuit.

顺序电路最重要的是需要注意的是，它们的输出受当前输入和以前的输入和输出共同决定。

The most important thing to be noted about sequential circuits is that their output is determined by both present inputs and previous inputs and outputs.

顺序电路用于设计复杂的数字系统，这些系统可以执行高级操作，如实时数据处理、数据存储和传输、事件计数等。

Sequential circuits are used to design complex digital systems that can perform advance operations like real-time data processing, storage and transmission of data, counting events, and more.