3.2 STM32 Clock System There 5 clock sources in Stm32F103 HSI High Speed Internal) HSE(High Speed External LSI(Low Speed Internal) LSE(LOW Speed EXternal) PLL(Phase Locked Loop) Where, HIS, HSE, and pll can be used as system clock LSi and lse are second level clock The clock tree of stM32F103 is shown in Fig 11 on page 115 of the STM32 Reference Manual(RM)5 3.2 STM32 Clock System There 5 clock sources in STM32F103： HSI (High Speed Internal) HSE (High Speed External) LSI (Low Speed Internal) LSE (Low Speed External) PLL(Phase Locked Loop) Where, HIS, HSE, and PLL can be used as System Clock. LSI and LSE are second level clock. The clock tree of STM32F103 is shown in Fig.11 on page 115 of the STM32 Reference Manual (RM)