Digital-electronics 简明教程

Digital Electronics - Priority Encoder

在数字电子领域， encoder 是一种组合逻辑电路，它接受十进制数字和字母字符作为输入，并生成编码后的输入表示作为输出。换句话说，将数字和符号转换成其对应编码格式的电子组合电路称为 encoder 。编码器执行的操作称为 encoding ，它将熟悉的数字和字符转换为其等效代码的过程。

In digital electronics, an encoder is a combinational logic circuit which accepts inputs as decimal digits and alphabetic characters, and produces the outputs as the coded representation of the inputs. In other words, an electronic combinational circuit that converts numbers and symbols into their corresponding coded format is called an encoder. The operation performed by the encoder is called encoding which is a process of converting familiar numbers and characters into their equivalent codes.

编码器有 2n 个输入线和 n 个输出线。一次仅激活 2n 个输入线中的一个。编码器的编码输出取决于已激活的输入线。有多种类型的编码器可用，例如“八进制到二进制编码器”、“十进制到 BCD 编码器”、“键盘编码器”等。

An encoder has 2n input lines and n-output lines. At a time, only one of the 2n input lines is activated. The coded output of the encoder depends upon the activated input line. There are several types of encoders available such as "octal to binary encoder", "decimal to BCD encoder", "keyboard encoders", etc.

What is a Priority Encoder?

对于普通编码器，在任何给定时间，只能激活一个十进制输入。但在某些实际数字系统中，两个或更多个十进制输入会无意中同时变得活跃，这可能会造成混乱。例如，在键盘上，用户先按下键 4，然后释放另一个键 2。在这种情况下，输出将对应于 (6)10，而不是 (4)10 或 (2)10。借助优先编码器可以解决这类问题。

In case of an ordinary encoder, one and only one decimal input can be activated at any given time. But in the case of some practical digital systems, two or more decimal inputs can unintentionally become active at the same time that might cause a confusion. For example, on a keyboard, a user presses key 4 before releasing another key 2. In such a situation, the output will be corresponding to (6)10, instead of being (4)10 or (2)10. This kind of problems can be solved with the help of priority encoder.

在数字电子学中，对在所有可能同时被激活的输入中仅响应一个输入而产生输出的组合逻辑电路称为优先级编码器。为此，它使用优先级系统，因此得名。

In digital electronics, a combinational logic circuit which produces outputs in response to only one input among all those that may be activated at the same time is called a priority encoder. For this, it uses a priority system, and hence it is named so.

使用的一种最流行的优先级系统是基于输入的相对大小。根据优先级系统，编码的是同时输入的所有输入中具有最大大小的十进制输入。因此，根据此优先级编码系统，如果 4 和 2 同时处于活动状态，优先级编码器将编码为 4。

One most popular priority system used is based on the relative magnitudes of the inputs. According to the priority system, the decimal input having largest magnitude among all the simultaneous inputs is encoded. Hence, as per this priority encoding system, the priority encoder would encode 4 if both 4 and 2 are active at the same time.

在某些实际系统中，优先级编码器有多个同时例行活动的输入。在这些情况下，编码器的主要功能是选择具有最高优先级的输入。优先级编码器的此功能称为仲裁。例如，在计算机系统中，连接了多个输入设备，其中几个可能试图同时向系统提供数据。在这种情况下，优先级编码器负责启用在所有输入设备中具有最高优先级的输入设备。

In some practical systems, priority encoders have several inputs which are routinely active at the same time. In such cases, the primary function of the encoder is to select the input with the highest priority. This function of the priority encoder is known as arbitration. For example, in a computer system, multiple input devices are connected, and several of them may try to supply data to the system at the same time. In this case, the priority encoder is responsible for enabling that input device which has the highest priority among all the input devices.

Types of Priority Encoders

有几种类型的优先级编码器。下面列出并解释了一些最重要的优先级编码器类型。

Several types of priority encoders are there. Some most important types of priority encoders are listed and explained below.

4 Input Priority Encoder
Decimal to BCD Priority Encoder
Octal to Binary Priority Encoder

让我们详细讨论每种类型的优先级编码器。

Let us discuss each type of priority encoder in detail.

4-Input Priority Encoder

4 输入优先级编码器的逻辑电路如图 1 所示。

The logic circuit of the 4-input priority encoder is shown in Figure-1.

它有三个由 A、B 和 V 指定的输出。其中，A 和 B 是普通输出，而 V 是充当有效位指示器的输出。当一个或多个输入等于 1 时，此第三个输出 V 设置为 1。在编码器的所有输入等于 0 的情况下，没有有效的输入，因此输出 V 设置为 0。当 V 等于 0 时，编码器的另外两个输出，即 A 和 B 未确定。因此，当 V 等于 0 时，输出 A 和 B 指定为“不关心条件”。

It has three outputs designated by A, B, and V. Where, A and B are the ordinary outputs and V is the output that acts as a valid bit indicator. This third output V is set to 1 when one or more inputs are equal to 1. In the case, when all the inputs to the encoder are equal to 0, there is no any valid input, and thus the output V is set to 0. The other two outputs, i.e. A and B of the encoder are not determined when V is equal to 0. Therefore, when, V is equal to 0, the outputs A and B are specified as "don’t care conditions".

4 输入优先级编码器的真值表如下所示。

The truth table of the 4-input priority encoder is shown below.

Inputs (X = Don’t care)	Outputs
I0	I1
I2	I3
A	B
V	0
0	0
0	X
X	0
1	0
0	0
0	0
1	X
1	0
0	0
1	1
X	X
1	0
1	0
1	X
X	X
1	1
1	1

从这个真值表中可以看出，输入的下标数字越大，输入的优先级就越高。因此，输入 I3 具有最高优先级。因此，不管其他输入的值如何，当输入 I3 等于 1 时，AB 的输出为 11，即 3。输入 I2 具有紧随其后的较低优先级，然后是 I1，最后是 I0 具有最低优先级。

From this truth table, it can be observed that the higher the subscript number of the input, the higher the priority of the input. Thus, the input I3 has the highest priority. Therefore, regardless of the values of other inputs, when the input I3 is equal to 1, the output for AB is 11, i.e. 3. The input I2 has the next lower priority, and then I1, and finally I0 has the lowest priority.

我们可以从上表写出输出 A、B 和 V 的布尔表达式，如下所示：

We can write the Boolean expression for outputs A, B, and V from the above table as follows,

\mathrm{A \: = \: I_{3} \: + \: \bar{I_{3}} \: I_{2}\: = \: I_{3} \: + \: I_{2}}

\mathrm{B \: = \: I_{3} \: + \: \bar{I_{3}} \: \bar{I_{2}} \: I_{1} \: = \: I_{3} \: + \: \bar{I_{2}} \: I_{1}}

并且，

And,

\mathrm{V \: = \: I_{3} \: + \: I_{2} \: + \: I_{1} \: + \: I_{0}}

因此，输出 V 的条件是对所有输入变量进行的或运算。

Hence, the condition for the output V is an OR operation of all the input variables.

Decimal to BCD Priority Encoder

此类型的优先级编码器执行将十进制数字编码为 4 位 BCD（二进制编码十进制）输出的功能。由于它是一个十进制到 BCD 优先级编码器，因此它产生了对应于所有输入中优先级最高十进制数字的 BCD，并忽略所有其他数字。

This type of priority encoder performs the function of encoding the decimal digits into 4-bit BCD (Binary Coded Decimal) outputs. As it is a decimal to BCD priority encoder, therefore, it produces a BCD corresponding to the decimal digit of highest priority among all the inputs and ignores all others.

以下是十进制到二进制编码优先编码器的真值表。

The truth table of the decimal to BCD priority encoder is given below.

Decimal Inputs (X = Don’t care)	BCD Outputs
I1	I2
I3	I4
I5	I6
I7	I8
I9	A3
A2	A1
A0	1
1	1
1	1
1	1
1	1
1	1
1	1
X	X
X	X
X	X
X	X
0	0
1	1
0	X
X	X
X	X
X	X
0	1
0	1
1	1
X	X
X	X
X	X
0	1
1	1
0	0
0	X
X	X
X	X
0	1
1	1
1	0
0	1
X	X
X	X
0	1
1	1
1	1
0	1
0	X
X	X
0	1
1	1
1	1
1	0
1	1
X	X
0	1
1	1
1	1
1	1
1	0
0	X
0	1
1	1
1	1
1	1
1	1
0	1
0	1
1	1
1	1
1	1
1	1
1	1

十进制到二进制编码优先编码器的真值表清楚地表明十进制输入的大小决定了它们的优先级。如果任何一个十进制输入为高电平，它将在所有其他更高值输入为低电平时被编码，而不管所有较低值输入的状态如何。

The truth table of the decimal to BCD priority encoder clearly shows that the magnitudes of the decimal inputs determine their priorities. If any decimal input is HIGH, it will be encoded if all other higher value inputs are LOW regardless of the state of all lower value inputs.

Octal to Binary Priority Encoder

这种类型的优先级编码器用于执行八进制代码到二进制代码的编码。因此，这种类型的优先级编码器有八个输入和三个输出，产生相应的二进制代码，如下面的真值表所示。

This type of priority encoder is used to perform encoding of octal code into binary code. Hence, this type priority encoder has eight inputs and three outputs that produce corresponding binary code as given in the truth table below.

Inputs	Outputs
I0	I1
I2	I3
I4	I5
I6	I7
A2	A1
A0	V
0	0
0	0
0	0
0	0
X	X
X	0
1	0
0	0
0	0
0	0
0	0
0	1
X	1
0	0
0	0
0	0
0	0
1	1
X	X
1	0
0	0
0	0
0	1
0	1
X	X
X	1
0	0
0	0
0	1
1	1
X	X
X	X
1	0
0	0
1	0
0	1
X	X
X	X
X	1
0	0
1	0
1	1
X	X
X	X
X	X
1	0
1	1
0	1
X	X
X	X
X	X
X	1
1	1
1	1

这就是有关优先级编码器及其在数字电子学中的主要类型的全部内容。

This is all about the priority encoder and its major types in digital electronics.