On reset the 16 MHz internal RC oscillator is selected as the default CPU clock. The
16 MHz internal RC oscillator is factory-trimmed to offer 1% accuracy over the full
temperature range. The application can then select as system clock either the RC oscillator
or an external 4-26 MHz clock source. This clock can be monitored for failure. If a failure is
detected, the system automatically switches back to the internal RC oscillator and a
software interrupt is generated (if enabled). This clock source is input to a PLL thus allowing
to increase the frequency up to 168 MHz. Similarly, full interrupt management of the PLL
clock entry is available when necessary (for example if an indirectly used external oscillator
fails).
Several prescalers allow the configuration of the three AHB buses, the high-speed APB
(APB2) and the low-speed APB (APB1) domains. The maximum frequency of the three AHB
buses is 168 MHz while the maximum frequency of the high-speed APB domains is 84 MHz.
The maximum allowed frequency of the low-speed APB domain is 42 MHz.
The devices embed a dedicated PLL (PLLI2S) which allows to achieve audio class
performance. In this case, the I2S master clock can generate all standard sampling
frequencies from 8 kHz to 192 kHz.