集成计数器的级联问题 (Series Connecting of Integrated Counters)

由于集成计数器芯片使用除了级联，通常还需要修改计数周期，因此其连接方式存在先改模再级联和先级联再改模两种。
对于级联而言，分为同步级联和异步级联，其区别主要在于进位信号是通过边沿异步地触发下一级计数，还是通过使能端同步地触发下一级计数。
对于改模而言，根据芯片类型不同，大体分为同步反馈和异步反馈，区别则是在于改模信号反馈到同步输入端（可以是清零、置位或者置数），还是异步输入端。

先级联后改模只有一种选择反馈信号的方案，也就是从级联好的芯片整体状态中选择反馈信号。而另一种方法则存在两种选择进位信号的方案：前级们的状态或者前一级的改模信号。

综合地考虑，会发现如果选择状态产生改模或者进位信号，必须注意选择目前为止所有已经产生的状态而全局考虑；如果采用改模信号作为级间进位信号（对应上面先反馈再级联），则需要保证改模和级联的同步异步性匹配，异步改模信号通常是脉冲形式的，不能用于同步级联的进位信号，而同步改模信号有效时间要早于实际生效时间，也不能用于异步进位信号。

从模块化考虑，级联的时候引入改模信号较好，因为改模即意味着本级循环周期结束，因此就可以进位了；反馈的时候要引入状态信号，因为反馈的目的是修改状态循环。

Series connecting problem is always together with counting period modifying problem.

No matter in what order we implement the two steps, two tips are always useful.

• If using states to generate carry signal for series or reset signal for period changing, designers should always use all states before current section inclusively to ensure a valid signal, or the generated signal may be active just at the carrying or resetting moment of CURRENT section.
• If using reset signal to trigger next timer (let’ s call that carry signal), chip users should match the synchronous reset to synchronous series connecting, and vice versa. Because asynchronous signals can not trigger the synchronous enabling pin of next timer, and synchronous signals will trigger the next section in an earlier clock period.

To serially connect two counters, we can change their modus before connecting them, or do that after their connecting. The former has two ways to choose the trigger of the next counters: from states or from reset signals.
Considering a modules-style to keep its robustness, designers should simply choose reset signals to trigger the next timers and states to reset. In this way, we can just determine the scheme step by step.