Clock Management; Clock Management Overview; Internal Clock; External Clock - ST STM8S Application Note

Clock management

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.
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
12/42
System clock distribution internal clock
1 to 24 MHz
crystal
and external
clock
External clock
16 MHz/128 kHz
internal RC
lnternal clock
For more details please refer to the section on clock management in the datasheet
describes the external clock connections.
OSC
IN
OSC
OUT
Prescaler
Doc ID 14651 Rev 3
CCO pin
Clock unit
WDG/AWU
Timer
Clock distribution
AN2752