STM32L时钟

Four different clock sources can be used to drive the system clock (SYSCLK):

1、HSI ((high-speed internal) oscillator clock

2、HSE (high-speed external) oscillator clock

3、PLL clock
4、MSI (multispeed internal) oscillator clock
The MSI is used as system clock source after startup from Reset, wake-up from Stop or
Standby low power modes.

The devices have the following two secondary clock sources:

1、37 kHz low speed internal RC (LSI RC) which drives the independent watchdog and
optionally the RTC used for Auto-wakeup from Stop/Standby mode.
2、 32.768 kHz low speed external crystal (LSE crystal) which optionally drives the realtime
clock (RTCCLK)