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时序逻辑：计数器
计数器
示例：74VHC393，74VHC161
计数器在每个时钟（CK）脉冲上按顺序进行递增或递减计数。一个4位计数器的模数可达16；一个8位计数器的模数可达256；一个14位计数器的模数可达16384。某些计数器具有CLR输入，用于将内部状态初始化为已知值。
计数器集成在数字定时器、电子计算器、秒表和许多其它设备中。
计数器大致分为异步（纹波进位）和同步（并行进位）计数器。设单个触发器的传输延迟时间为tpd。然后，n级异步计数器将产生相当于n×tpd的大延迟。还应注意，当计数器输出馈送至逻辑门时，异步计数器可能产生风险。
74VHC393四位计数器
异步：波纹进位计数器
74VHC161四位计数器的逻辑示意图
同步：并行进位计数器
时序图：下面显示了一个典型的同步（并行进位）计数器的时序图，它在时钟（CK）的每一个边沿上按顺序进行递增计数。
