以下关于嵌入式系统时钟管理的叙述中，错误的是（)。
- A.系统的主时钟可以由外部时钟源提供，也可由外部晶体振荡器提供
- B.时钟控制逻辑可以在不需要锁相环的情况下产生慢速时钟
- C.利用锁相环可以对输入时钟进行倍频输出，但无法改变输出时钟的相位
- D.可以通过软件来控制时钟与每个外围模块的连接还是断开
正确答案及解析
正确答案
解析
系统的主时钟可以由外部时钟源提供，也可由外部晶体振荡器提供。时钟控制逻辑可以在不需要锁相环的情况下产生慢速时钟，并且可以通过软件来控制时钟与每个外围模块的连接还是断开，从而降低功效。系统时钟源直接采用外部晶振时，内部PLL电路可以调整系统时钟，使系统运行速度更快。由于片内的PPL电路可以兼具频率放大和信号提纯的功能，因此，系统可以以较低的外部时钟信号获得较高的工作频率，从而降低因高速开关时钟所造成的高频噪声。锁相环在工作的过程中，当输出信号的频率与输入信号的频率相等时，输出电压与输入电压保持固定的相位差值，即输出电压与输入电压的相位被锁住。
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