RM0440 Rev 4 279/2126
RM0440 Reset and clock control (RCC)
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External crystal/ceramic resonator (HSE crystal)
The 4 to 48 MHz external oscillator has the advantage of producing a very accurate rate on
the main clock.
The associated hardware configuration is shown in Figure 18. Refer to the electrical
characteristics section of the datasheet for more details.
The HSERDY flag in the Clock control register (RCC_CR) indicates if the HSE 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 Clock interrupt enable register (RCC_CIER).
The HSE Crystal can be switched on and off using the HSEON bit in the Clock control
register (RCC_CR).
External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
48 MHz. You select this mode by setting the HSEBYP and HSEON
bits in the Clock control
register (RCC_CR). The external clock signal (square, sinus or triangle) with ~40-60 % duty
cycle depending on the frequency (refer to the datasheet) has to drive the OSC_IN pin while
the OSC_OUT pin can be used as GPIO. See Figure 18.
7.2.2 HSI16 clock
The HSI16 clock signal is generated from an internal 16 MHz RC Oscillator.
The HSI16 RC oscillator has the advantage of providing 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 selected as system clock after wakeup from Stop modes (Stop 0,
Stop 1). Refer to Section 7.3: Low-power modes. It can also be used as a backup clock
source (auxiliary clock) if the HSE crystal oscillator fails. Refer to Section 7.2.9: Clock
security system (CSS).
Calibration
RC oscillator frequencies 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 T
A
=25°C.
After reset, the factory calibration value is loaded in the HSICAL[7:0] bits in the Internal
clock sources calibration register (RCC_ICSCR).
If the application is subject to voltage or temperature variations this may affect the RC
oscillator speed. You can trim the HSI16 frequency in the application using the
HSITRIM[6:0] in the Internal clock sources calibration register (RCC_ICSCR).
For more details on how to measure the HSI16 frequency variation, refer to Section 7.2.16:
Internal/external clock measurement with TIM5/TIM15/TIM16/TIM17.
The HSIRDY flag in the Clock control register (RCC_CR) indicates if the HSI16 oscillator is
stable or not. At startup, the HSI16 output clock is not released until this bit is set by
hardware.
The HSI16 oscillator can be switched on and off using the HSION bit in the Clock control
register (RCC_CR).
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