VHDL语言描述，时钟分频,给定CPLD试验板系统时钟设置50M，但由于本作品的需要，我们将系统时钟经过20分频得到DS18B20所需的工作时钟，大约为1.25M。-VHDL language descr iption, the clock frequency, a given CPLD experiment board system clock set 50M, but as a result of this work, we will be the system clock frequenc