两个74ls290怎么构成六十进制计数器,要脚管具体链接
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发布时间：2023-08-15 19:44
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时间：2023-08-29 14:58
本设计方案提出一个用极少器件级联两个或更多Johnson计数器的实用方法。CD4017 Johnson十进制计数器应用于从声效到LED显示的简单电路中。计数器输出通常为低，只在各自解码时间内变高。每个解码输出在一个全时钟周期内保持高电平。直流供电电压范围从大约3V到18V。每个输出管脚（Q0到Q9）的直流电流消耗为10mA。电路通过了12V直流0到150°F温区内的无异常测试。
图1电路只使用四个IC，组成19阶连续计数器。由于供电后每个CD4017计数器的输出Q0都工作，所以不能在没有增加硬件的情况下得到20输出。因此，电路不能够使用IC3的Q0输出，而只能使用20个输出的19个。
初看上去，也许会考虑使用动作管脚（管脚12）从一个计数器接到第二个计数器的时钟输入管脚（管脚14），简单的级联两个计数器。但是这个结构一旦第一个计数器达到10，就开始重新计数，不能提供1到20的连续计数。这样的结构为0到99计数器，因为第一个IC计数器中每10个计数引发第二个计数器的一个计数。
通过连接两个计数器
，可以获得1到19的连续计数。电路使用IC4的CD4069反相器作为复位延时使能，在每个计数器开始计数前引入几毫秒的延时。管脚15的高电平复位使计数器到0位置。
没有延时时间，每个计数器上电后，计数输出是随机的，这样若干LED也许是亮的。电路用IC1的555定时器产生1.5Hz方波作为时钟。可以通过改变由R1、R2和C1组成的RC时间常数来改变频率。务必牢记，为获得占空比50%的输出，使R2远大于R1。IC2的管脚14为正边沿时钟触发。IC3的管脚13为负边沿时钟触发。因此，当时钟变为高电平时，IC2产生一个计数输出。当时钟变为低电平时，IC3产生一个计数输出。通过交叉输出，获得1到19的连续计数。因为每个时钟周期都是高低电平状态，第一个时钟脉冲后，两个LED通常是亮的——也就是说，LED1，LED1和2，LED2和3，LED3和4，等等。下面是完整电路动作的视频剪辑。（视频剪辑见原英文出处）
英文原文：
Cascade two decade counters to obtain 19-step sequential counter
Interleaving the outputs of a pair of Johnson counters yields a 19-step, sequential count.
Jeff Tregre, Dallas, TX; Edited by Charles H Small and Fran Granville -- EDN, 12/14/2007
This Design Idea offers a practical approach to cascading two or more Johnson counters together with a bare minimum of parts. The CD4017 Johnson decade counter finds use in simple circuits ranging from sound effects to LED displays. The counter’s outputs are normally low and go high only at their respective decoded time slot. Each decoded output remains high for one full clock cycle. The dc-supply voltage can range from approximately 3 to 18V dc. The dc-current drain per each output pin (Q0 to Q9) is 10 mA. The circuit has passed tests at 12V dc at 0 to 150°F without anomalies.
The circuit in Figure 1 uses only four ICs and yields a 19-step sequential count. You cannot get 20 outputs without adding more hardware because of the fact that, upon powering up, each CD4017 counter displays output Q0 as being on. Therefore, the circuit does not use output Q0 of IC3 and can use only 19 of the 20 outputs. ---------
At first blush, you might think that you could simply cascade two counters together using the carry-out pin, Pin 12, from one counter to feed the clock-input pin, Pin 14, of a second counter. But the problem with this configuration is that it does not provide sequential count from 1 to 20 because the first counter begins to count over again once it has reached 10. Such a configuration is a zero-to-99 counter
because every 10 counts on the first IC counter causes one count on the second IC counter.
By hooking together two counters, you can obtain a sequential count from 1 to 19. The circuit uses IC4, a CD4069 inverter, as a reset-delay enable to cause a few milliseconds of delay before each counter can begin to count. A high signal on the Pin 15 Reset clears the counter to its zero count.
Without the delay time, each counter powers up with a random output count such that several LEDs may be on. The circuit uses IC1, a 555 timer, as the clock to generate a 1.5-Hz square wave. You can change the frequency by changing the RC time constant comprising R1, R2, and C1. Keep in mind that, to obtain a 50% output ty cycle, make R2 much larger than R1. Pin 14 of IC2 has a positive-edge clock trigger. Pin 13 of IC3 has a negative-edge clock trigger. Therefore, when the clock goes high, IC2 proces an output count. When the clock goes low, IC3 proces an output count. By interleaving the outputs, you obtain a sequential count from 1 to 19. Because each clock cycle has both a high and a low state, after the first clock pulse, two LEDs will always be on—that is, LED 1, LED 1 and 2, LED 2 and 3, LED 3 and 4, and so on. See below for two short video clips of the finished circuit in action.
