Clock management AN3029
10/42 DocID16139 Rev 5
4 Clock management
The STM8L101xx devices have no external clock, so no precautionary measures are
needed.
4.1 Clock management overview
STM8L05xxx, STM8L15xxx, STM8L162xx, STM8AL31xx and STM8AL3Lxx devices offer a
flexible way of selecting the core and peripheral clocks (ADC, memory, and digital
peripherals). The devices have internal and external clock source inputs, both of which have
a high speed and a low speed version. Any of those four clocks can be use for the CPU and
most of the peripherals through a programmable prescaler. An I/O can be programmed as
output clock (CCO) to reflect one of the four clocks (with or without prescaling).
The signal which leaves the I/O represents an output clock (CCO) divided by a division
factor.
4.2 Internal clocks
STM8L and STM8AL 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 38 kHz.
After reset, the CPU starts with the internal RC (HSI clock signal) divided by 8, i.e. 2 MHz.
4.3 External clock
STM8L05xx, STM8L15xxx, STM8L162xx, STM8AL31xx, and STM8AL3Lxx devices have
two kinds of external clock: A high speed external clock (HSE) running at up to 16 MHz and
a low speed external clock (LSE) running at 32.768 kHz.
4.3.1 HSE clock
STM8L05xx, STM8L15xxx, STM8L162xx, STM8AL31xx, and STM8AL3Lxx 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 4 describes the external clock connections.
External clock
• Frequency: 0 kHz … 16 MHz
• Input hysteresis: 100 mV
Caution: Without prescaler, a duty cycle of 45/55 % maximum must be respected at high speed
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