Functional overview
STM32F042x4 STM32F042x6
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DocID025832 Rev 5
3.6
Clocks and startup
System clock selection is performed on startup, however the internal RC 8 MHz oscillator is
selected as default CPU clock on reset. An external 4-32 MHz clock can be selected, in
which case it is monitored for failure. If failure is detected, the system automatically switches
back to the internal RC oscillator. A software interrupt is generated if enabled. Similarly, full
interrupt management of the PLL clock entry is available when necessary (for example on
failure of an indirectly used external crystal, resonator or oscillator).
Several prescalers allow the application to configure the frequency of the AHB and the APB
domains. The maximum frequency of the AHB and the APB domains is 48 MHz.
Additionally, also the internal RC 48 MHz oscillator can be selected for system clock or PLL
input source. This oscillator can be automatically fine-trimmed by the means of the CRS
peripheral using the external synchronization.
