4.1.1 External crystal/ceramic resonator (HSE crystal)
The 4 to 50 MHz external oscillator has the advantage of producing a very accurate rate on the main clock. The
associated hardware configuration is shown in Table 3. Refer to the electrical characteristics section of the
datasheet for more details.
4.1.2 External source (HSE bypass)
In this mode, an external clock source must be provided, with a frequency up to 50 MHz. The external clock signal
(square, sinus or triangle) with ~40 to 60 % duty cycle depending on the frequency (refer to the datasheet), must
drive the OSC_IN pin while the OSC_OUT pin can be used as a GPIO (see Table 3).
Note: For details on pin availability, refer to the pinout section of the datasheet. To minimize the consumption, the
square signal is recommended.
4.2 HSI16 clock
The HSI16 clock signal is generated from an internal 16 MHz RC oscillator. The HSI16 RC oscillator provides a
clock source at low cost (no external components). It also has a faster startup time than the HSE crystal oscillator.
However, even with calibration, the frequency is less accurate than an external crystal oscillator or ceramic
resonator.
The HSI16 clock can be used as a backup clock source (auxiliary clock) if the HSE crystal oscillator fails.
For more details, refer to section Clock security system (CSS) in document [1].
4.3 MSI (MSIS and MSIK) clocks
The MSI is made of four internal RC oscillators: MSIRC0 (48 MHz), MSIRC1 (4 MHz), MSIRC2 (3.072 MHz), and
MSIRC3 (400 kHz). Each oscillator feeds a prescaler providing a division by 1, 2, 3 or 4.
Two output clocks are generated from these divided oscillators:
• MSIS that can be selected as system clock
• MSIK that can be selected by some peripherals as kernel clock
MSIS and MSIK frequency range can be adjusted by software, by using respectively the MSISRANGE [3:0] and
MSIKRANGE [3:0] fields in RCC_ICSCR1, with MSIRGSEL = 1. Sixteen frequency ranges are available,
generated from the four internal RCs (refer to document [1] for more details).
The MSI clock can also be used as a backup clock source (auxiliary clock) if the HSE crystal oscillator fails (refer
to section Clock security system (CSS) in document [1]).
The MSI oscillator provides a low-cost (no external components) low-power clock source. In addition, when the
MSI is used in PLL‑mode with the LSE, it provides a very accurate clock source that can be used by the USB
OTG_FS peripheral, and feeds the PLL to run the system at the maximum speed 160 MHz.
Hardware auto calibration with LSE (PLL-mode)
When a 32.768 kHz external oscillator is present in the application, either MSIS or MSIK can be configured in a
PLL‑mode. This mode is enabled as follows:
• for MSIS: by setting the MSIPLLEN bit to 1 in RCC_CR
• for MSIK: by setting the MSIPLLEN bit to 0 in RCC_CR
In case MSIS and MSIK ranges are generated from the same MSIRC source, the PLL‑mode is applied on both
MSIS and MSIK. When configured in PLL‑mode, the MSIS or MSIK automatically calibrates itself thanks to the
LSE. This mode is available for all MSI frequency ranges. At 48 MHz, the MSIK in PLL‑mode can be used for the
USB OTG_FS peripheral, avoiding the need of an external high‑speed crystal.
For more details on how to measure the MSI frequency variation, refer to section Internal/external clock
measurement with TIM15/TIM16/TIM17 in document [1].
Note: On STM32U5Ax/59x/5Fx/5Gx, the OTG_HS peripheral cannot be clocked by the MSI in PLL
‑
mode. The HSE
crystal must be used.
4.4
LSE clock
The LSE crystal is a 32.768 kHz low-speed external crystal or ceramic resonator (see Table 3). It provides a
low‑power but highly accurate clock source to the RTC (real-time clock) peripheral for clock/calendar or other
timing functions.
AN5373
HSI16 clock
AN5373 - Rev 6
page 26/47
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