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📜  数字计数器

📅  最后修改于: 2021-01-12 04:56:50             🧑  作者: Mango

计数器是一个时序电路。用于计数脉冲的数字电路是已知的计数器。计数器是触发器的最广泛应用。它是一组施加了时钟信号的触发器。计数器有两种类型。

  • 异步或波纹计数器。
  • 同步计数器。

异步或波纹计数器

图中显示了一个2位波纹上升计数器的逻辑图。使用了触发器(T)触发器。但是我们也可以将JK触发器与J和K永久连接到逻辑1一起使用。外部时钟应用于触发器A的时钟输入，Q A输出应用于下一个触发器的时钟输入，即FF-B。

逻辑图

异步或波纹计数器的逻辑图

操作方式

S.N. Condition Operation
1 Initially let both the FFs be in the reset state QBQA = 00 initially
2 After 1st negative clock edge

As soon as the first negative clock edge is applied, FF-A will toggle and QA will be equal to 1.

QA is connected to clock input of FF-B. Since QA has changed from 0 to 1, it is treated as the positive clock edge by FF-B. There is no change in QB because FF-B is a negative edge triggered FF.

QBQA = 01 after the first clock pulse.
3 After 2nd negative clock edge

On the arrival of second negative clock edge, FF-A toggles again and QA = 0.

The change in QA acts as a negative clock edge for FF-B. So it will also toggle, and QB will be 1.

QBQA = 10 after the second clock pulse.
4 After 3rd negative clock edge

On the arrival of 3rd negative clock edge, FF-A toggles again and QA become 1 from 0.

Since this is a positive going change, FF-B does not respond to it and remains inactive. So QB does not change and continues to be equal to 1.

QBQA = 11 after the third clock pulse.
5 After 4th negative clock edge

On the arrival of 4th negative clock edge, FF-A toggles again and QA becomes 1 from 0.

This negative change in QA acts as clock pulse for FF-B. Hence it toggles to change QB from 1 to 0.

QBQA = 00 after the fourth clock pulse.

真相表

异步或波动计数器真值表

同步计数器

如果同时将“时钟”脉冲施加到计数器中的所有触发器，则该计数器称为同步计数器。

2位同步递增计数器

FF-A的J A和K A输入与逻辑1相关联。因此FF-A将用作触发触发器。 J B和K B输入连接到Q A。

逻辑图

同步计数器逻辑图

操作方式

S.N. Condition Operation
1 Initially let both the FFs be in the reset state QBQA = 00 initially.
2 After 1st negative clock edge

As soon as the first negative clock edge is applied, FF-A will toggle and QA will change from 0 to 1.

But at the instant of application of negative clock edge, QA , JB = KB = 0. Hence FF-B will not change its state. So QB will remain 0.

QBQA = 01 after the first clock pulse.
3 After 2nd negative clock edge

On the arrival of second negative clock edge, FF-A toggles again and QA changes from 1 to 0.

But at this instant QA was 1. So JB = KB= 1 and FF-B will toggle. Hence QB changes from 0 to 1.

QBQA = 10 after the second clock pulse.
4 After 3rd negative clock edge

On application of the third falling clock edge, FF-A will toggle from 0 to 1 but there is no change of state for FF-B.

QBQA = 11 after the third clock pulse.
5 After 4th negative clock edge

On application of the next clock pulse, QA will change from 1 to 0 as QB will also change from 1 to 0.

QBQA = 00 after the fourth clock pulse.

柜台分类

根据计数进行的方式，同步或异步计数器分为以下几种：

  • 柜台
  • 柜台
  • 递增/递减计数器

上/下计数器

递增计数器和递减计数器结合在一起以获得递增/递减计数器。还提供了模式控制(M)输入以选择上或下模式。为了实现上/下操作，需要在每对触发器之间设计和使用组合电路。

  • 加/减计数器的类型
  • 上下波动计数器
  • 上/下同步计数器

上下波纹计数器

在UP / DOWN纹波计数器中，所有FF都以触发模式运行。因此，将使用T型触发器或JK型触发器。 LSB触发器直接接收时钟。但是，从前一个FF的(Q = Q bar)输出获得每个其他FF的时钟。

  • 向上计数模式(M = 0) -如果要进行向上计数，则前一个FF的Q输出连接到下一级的时钟。对于此模式，模式选择输入M为逻辑0(M = 0)。

  • 向下计数模式(M = 1) -如果M = 1，则前一个FF的Q bar输出连接到下一个FF。这将使计数器在计数模式下运行。

例

3位二进制上下波动计数器。

  • 3位-因此需要三个FF。

  • 上/下-因此模式控制输入必不可少。

  • 对于脉动上升计数器，前一个FF的Q输出连接到下一个FF的时钟输入。

  • 对于脉动上升计数器，前一个FF的Q输出连接到下一个FF的时钟输入。

  • 对于纹波下降计数器，前一个FF的Q bar输出连接到下一个FF的时钟输入。

  • 令前一个FF的Q和Q bar输出的选择由模式控制输入M控制，如果M = 0，则进行UP计数。因此，将Q连接到CLK。如果M = 1，则向下计数。因此，将Q bar连接到CLK。

框图

递增/递减计数器的框图

真相表

递增/递减计数器真值表

操作方式

S.N. Condition Operation
1 Case 1 − With M = 0 (Up counting mode)

If M = 0 and M bar = 1, then the AND gates 1 and 3 in fig. will be enabled whereas the AND gates 2 and 4 will be disabled.

Hence QA gets connected to the clock input of FF-B and QB gets connected to the clock input of FF-C.

These connections are same as those for the normal up counter. Thus with M = 0 the circuit work as an up counter.
2 Case 2: With M = 1 (Down counting mode)

If M = 1, then AND gates 2 and 4 in fig. are enabled whereas the AND gates 1 and 3 are disabled.

Hence QA bar gets connected to the clock input of FF-B and QB bar gets connected to the clock input of FF-C.

These connections will produce a down counter. Thus with M = 1 the circuit works as a down counter.

模数计数器(MOD-N计数器)

2位波纹计数器称为MOD-4计数器，而3位波纹计数器称为MOD-8计数器。因此，通常将n位波纹计数器称为N模计数器。其中，MOD号= 2 n 。

模数类型

  • 2位向上或向下(MOD-4)
  • 3位向上或向下(MOD-8)
  • 4位上下(MOD-16)

柜台申请

  • 频率计数器
  • 数码时钟
  • 时间测量
  • 模数转换器
  • 分频器电路
  • 数字三角波发生器。