ST STM32L4+ Series Reference Manual page 248

Reset and clock control (RCC)
The HSI16 signal can also be used as a backup source (Auxiliary clock) if the HSE crystal
oscillator fails. Refer to
6.2.3 MSI clock
The MSI clock signal is generated from an internal RC oscillator. Its frequency range can be
adjusted by software by using the MSIRANGE[3:0] bits in the
(RCC_CR). Twelve frequency ranges are available: 100 kHz, 200 kHz, 400 kHz, 800 kHz,
1 MHz, 2 MHz, 4 MHz (default value), 8 MHz, 16 MHz, 24 MHz, 32 MHz and 48 MHz.
The MSI clock is used as system clock after restart from Reset, wakeup from Standby and
Shutdown low-power modes. After restart from Reset, the MSI frequency is set to its default
value 4 MHz. Refer to
The MSI clock can be selected as system clock after a wakeup from Stop mode (Stop 0,
Stop 1 or Stop 2). Refer to
clock source (auxiliary clock) if the HSE crystal oscillator fails. Refer to
security system
The MSI RC oscillator has the advantage of providing a low-cost (no external components)
low-power clock source. In addition, when used in PLL-mode with the LSE, it provides a
very accurate clock source which can be used by the USB OTG FS device, and feed the
main PLL to run the system at the maximum speed.
The MSIRDY flag in the
stable or not. At startup, the MSI RC output clock is not released until this bit is set by
hardware. The MSI RC can be switched on and off by using the MSION bit in the
control register
Hardware auto calibration with LSE (PLL-mode)
When a 32.768 kHz external oscillator is present in the application, it is possible to configure
the MSI in a PLL-mode by setting the MSIPLLEN bit in the
When configured in PLL-mode, the MSI automatically calibrates itself thanks to the LSE.
This mode is available for all MSI frequency ranges. At 48 MHz, the MSI in PLL-mode can
be used for the USB OTG FS device, saving the need of an external high-speed crystal.
Software calibration
The MSI RC oscillator frequency can vary from one chip to another due to manufacturing
process variations, this is why each device is factory calibrated by ST for 1 % accuracy at an
ambient temperature, TA, of 25 °C. After reset, the factory calibration value is loaded in the
MSICAL[7:0] bits in the
application is subject to voltage or temperature variations, this may affect the RC oscillator
speed. You can trim the MSI frequency in the application by using the MSITRIM[7:0] bits in
the RCC_ICSCR register. For more details on how to measure the MSI frequency variation
please refer to
TIM15/TIM16/TIM17.
6.2.4 HSI48 clock
The HSI48 clock signal is generated from an internal 48 MHz RC oscillator and can be used
directly for USB and for random number generator (RNG) as well as SDMMC.
The internal 48 MHz RC oscillator is mainly dedicated to provide a high precision clock to
the USB peripheral by means of a special Clock Recovery System (CRS) circuitry. The CRS
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Section 6.2.10: Clock security system (CSS) on page
Section 6.3: Low-power
Section 6.3: Low-power
(CSS).
Clock control register (RCC_CR)
(RCC_CR).
Internal clock sources calibration register
Section 6.2.17: Internal/external clock measurement with
RM0432 Rev 6
Clock control register
modes.
modes. It can also be used as a backup
indicates wether the MSI RC is
Clock control register
(RCC_ICSCR). If the
RM0432
252.
Section 6.2.10: Clock
Clock
(RCC_CR).