Reset and clock control (RCC) RM0453
288/1461 RM0453 Rev 1
indicate when LSE system clock is ready (due clock synchronization) after having been
enabled by the LSESYSEN.
External source (LSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
1 MHz. This mode is selected by setting the LSEBYP and LSEON bits in the RCC Backup
domain control register (RCC_BDCR). The external clock signal (square, sinus or triangle)
with ~50 % duty cycle must drive the OSC32_IN pin while the OSC32_OUT pin can be used
as GPIO (see Figure 30: LSE clock sources).
7.2.6 LSI clock
The LSI RC acts as a low-power clock source that can be kept running in Stop and Standby
modes for the independent watchdog (IWDG) and RTC. The clock frequency is ~32 kHz or
can be divided by 128 (~250 Hz) using LSIPRE. For more details, refer to the electrical
characteristics section of the datasheets.
The LSI RC can be switched on and off using the LSION bit in the RCC control/status
register (RCC_CSR).
The LSIRDY flag in the RCC control/status register (RCC_CSR) indicates if the LSI
oscillator is stable or not. At startup, the clock is not released until this bit is set by hardware.
An interrupt can be generated if enabled in the RCC clock interrupt enable register
(RCC_CIER).
7.2.7 Clock source stabilization time
The different clock sources require a stabilization time, during which no clock is forwarded to
the system (see the table below).
7.2.8 System clock (SYSCLK) selection
The following clock sources can be used to drive the system clock (SYSCLK):
• MSI oscillator
• HSI16 oscillator
• HSE32 oscillator (either 32 MHz or divided by 2 for 16 MHz)
• PLLRCLK
Table 57. Clock source stabilization times
Clock source Stabilization time
MSI Refer to the device datasheet.
HSI Refer to the device datasheet.
HSE Refer to the device datasheet.
LSI
2 cycles (~85 μs LSIPRE = 0)
2 cycles (~2 ms LSIPRE = 1)
LSE 4096 cycles (125 ms)
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