Clocks

The MSIS is used as system clock source after startup from reset, configured at 4 MHz.

Each clock source can be switched on or off independently when it is not used, to optimize power consumption.

Several pre-scalers can be used to configure the AHB and the APB frequencies domains with a maximum frequency of 160 MHz.

The resonator and the load capacitors must be placed as close as possible to the oscillator pins in order to minimize output distortion and startup stabilization time. The loading capacitance values must be adjusted according to the selected oscillator.

Table 3. HSE/LSE clock sources

Clock source	Hardware configuration
External clock
Crystal/ceramic resonators	CL1 and CL2 values depend on the quartz. Refer to document AN5373_general-info.html#id_14462__entry_phr_xrh_dvb for more details.

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 HSE/LSE clock sources. Refer to the electrical characteristics section of the datasheet for more details.

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 HSE/LSE clock sources).

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 AN5373_general-info.html#id_14462__entry_msl_1fh_dvb.

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.

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 AN5373_general-info.html#id_14462__entry_msl_1fh_dvb 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 AN5373_general-info.html#id_14462__entry_msl_1fh_dvb).

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.

The LSE crystal is a 32.768 kHz low-speed external crystal or ceramic resonator (see Table 3 HSE/LSE clock sources). It provides a low‑power but highly accurate clock source to the RTC (real-time clock) peripheral for clock/calendar or other timing functions.

The crystal oscillator driving strength can be changed at runtime using the LSEDRV[1:0] bits in RCC_BDCR, to obtain the best compromise between robustness and short startup time on one side, and low-power-consumption on the other side.

The BOOT0 value may come from the PH3-BOOT0 pin or from an option bit depending on the value of a user option bit to free the GPIO pad if needed.

When TrustZone® is enabled by setting the TZEN option bit (TZEN = 1), the boot space must be in a secure area. The SECBOOTADD0[24:0] option bytes are used to select the boot secure memory address. A unique boot entry option can be selected by setting the BOOT_LOCK option bit. All other boot options are ignored.