Clock management AN2752
12/42 Doc ID 14651 Rev 3
4 Clock management
4.1 Clock management overview
Root part number 2 devices offer a flexible way of selecting the core and peripheral clocks
(ADC, memory, digital peripherals). The devices have internal and external clock source
inputs and one output clock (CCO).
Figure 5. System clock distribution internal clock
4.2 Internal clock
The RC oscillator has an internal capacitor (C) and an internal resistor ladder (R). STM8S
devices have two kinds of internal clock: a high speed internal clock (HSI) running at
16 MHz and a low speed internal clock (LSI) running at 128 kHz.
After reset, the CPU starts with the internal RC (HSI clock signal) divided by 8, i.e. 2 MHz.
4.3 External clock
STM8S devices can connect to an external crystal or an external oscillator.
Note: When no external clock is used, OSCIN and OSCOUT can be used as general purpose
I/Os.
Figure 6 describes the external clock connections.
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