DocID024597 Rev 5 211/1830
RM0351 Reset and clock control (RCC)
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The enable bit of each PLL output clock (PLLPEN, PLLQEN, PLLREN, PLLSAI1PEN,
PLLSAI1QEN, PLLSAI1REN, PLLSAI2PEN and PLLSAI2REN) can be modified at any time
without stopping the corresponding PLL. PLLREN cannot be cleared if PLLCLK is used as
system clock.
6.2.6 LSE clock
The LSE crystal is a 32.768 kHz Low Speed External crystal or ceramic resonator. It has the
advantage of providing a low-power but highly accurate clock source to the real-time clock
peripheral (RTC) for clock/calendar or other timing functions.
The LSE crystal is switched on and off using the LSEON bit in Backup domain control
register (RCC_BDCR). The crystal oscillator driving strength can be changed at runtime
using the LSEDRV[1:0] bits in the Backup domain control register (RCC_BDCR) to obtain
the best compromise between robustness and short start-up time on one side and low-
power-consumption on the other side. The LSE drive can be decreased to the lower drive
capability (LSEDRV=00) when the LSE is ON. However, once LSEDRV is selected, the
drive capability can not be increased if LSEON=1.
The LSERDY flag in the Backup domain control register (RCC_BDCR) indicates whether
the LSE crystal is stable or not. At startup, the LSE crystal output clock signal is not released
until this bit is set by hardware. An interrupt can be generated if enabled in the Clock
interrupt enable register (RCC_CIER).
External source (LSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
1 MHz. You select this mode by setting the LSEBYP and LSEON bits in the AHB1 peripheral
clocks enable in Sleep and Stop modes register (RCC_AHB1SMENR). The external clock
signal (square, sinus or triangle) with ~50 % duty cycle has to drive the OSC32_IN pin while
the OSC32_OUT pin can be used as GPIO. See Figure 17.
6.2.7 LSI clock
The LSI RC acts as a low-power clock source that can be kept running in Stop and Standby
mode for the independent watchdog (IWDG), RTC and LCD. The clock frequency is 32 kHz.
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 Control/status register
(RCC_CSR).
The LSIRDY flag in the 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 Clock interrupt enable register (RCC_CIER).
6.2.8 System clock (SYSCLK) selection
Four different clock sources can be used to drive the system clock (SYSCLK):
• MSI oscillator
• HSI16 oscillator
• HSE oscillator
• PLL
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