External Crystal/Ceramic Resonator (Hse Crystal); External Source (Hse Bypass); Hsi16 Clock; Msi (Msis And Msik) Clocks - ST STM32U575 Series Application Note

Clock source
External clock
Crystal/ceramic resonators
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
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
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 the reference manual.
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 register, with MSIRGSEL = 1. Sixteen frequency ranges are available,
generated from the four internal RCs (see the reference manual for more details).
AN5373 - Rev 1
Table 3. HSE/LSE clock sources
Hardware configuration
OSC_IN
External souce
C
L1
CL1 and CL2 values depend on the quartz. Refer to the application note Oscillator
design guide for STM8AF/AL/S and STM32 microcontrollers (AN2867) for more
Table 3. Refer to the electrical characteristics section of the
OSC_OUT
GPIO
OSC_IN OSC_OUT
C
L2
Load capacitors
details.
Table 3).
AN5373
HSI16 clock
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