September 2008 Rev 2 1/8
AN2669
Application note
How to configure the STM8A microcontroller clocks
Introduction
The STM8A microcontroller has a very flexible clock controller (CLK) that allows the clocks
to be reconfigured at any time to suit different requirements or functional modes in the
application.
There are four clock sources that can be used.
■ 1-24 MHz high speed external crystal (HSE)
■ Up to 24 MHz high speed user-external clock (HSE user-ext)
■ 16 MHz high speed internal RC oscillator (HSI)
■ 128 kHz low speed internal RC (LSI)
This application note provides practical examples of how to program the STM8A CLK
registers. The source code of the examples is included in the STM8 firmware library.
For further information and a hardware description of the STM8A CLK, please refer to the
section on ‘Clock control (CLK)’ in the STM8A microcontroller family reference manual
(RM0009). Please contact STMicroelectronics for information on the STM8 firmware library.
www.st.com
Contents AN2669
2/8
Contents
1 Example 1: Clock security system (CSS) capability . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Example 2: Automatic clock switching . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Example 3: Manual clock switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4 Example 4: Configuring clocks for maximum speed . . . . . . . . . . . . . . . 6
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
