
RM0453 Rev 2
287/1454
RM0453
Reset and clock control (RCC)
363
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
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
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
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
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
The system clock maximum frequency in range 1 is 48 MHz. After a system reset, the MSI
oscillator, at 4 MHz, is selected as system clock. When a clock source is used directly or
through the PLL as a system clock, it is not possible to stop it.
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
μ
s LSIPRE = 0)
2 cycles (~2 ms LSIPRE = 1)
LSE
4096 cycles (125 ms)