参考手册上是这么写的：
Fast clock wakeup
As described for Halt mode, in order to get the shortest wakeup time, it is recommended to
select HSI as the fMASTER clock source. The FHWU bit is also available to save switching
time.
A fast wakeup time is very important in Active-halt mode. It supplements the effect of CPU
processing performance by helping to minimize the time the MCU stays in Run mode
between two periods in low power mode, and thus reduces the overall average power
consumption.
Since the clock is not automatically switched to the original master clock, the application
must restore the clock source before entering Halt/Active-halt mode as soon as it is ready.
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