ST AN2669 Application note

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.

September 2008

Rev 2

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www.st.com

Contents	AN2669

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

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AN2669	Example 1: Clock security system (CSS) capability

1 Example 1: Clock security system (CSS) capability

1.1Overview

This example shows how to use the CSS capability. Enabling the CSS, is a way of making the application more reliable, by ensuring the microcontroller keeps running even if the main clock, (HSE) fails.

The example is provided as CLK example 1 in the STM8 firmware library. It uses most of the features of the CLK driver provided in the library.

1.2Hardware description

Figure 1 shows the main hardware connections needed to run the CSS.

Figure 1. STM8A CLK interface

24 MHz

OSCIN

20 pF

1 MΩ

1

Ω

OSCOUT

k

PH0

Ω

20 pF

60 Ω

1 k

PH1

1 kΩ

PH2

1 kΩ

PH3

CCO

Oscilloscope	LD1 LD2	LD3	LD4

STM8A

1.3Firmware description

The STM8 firmware library includes the CLK driver which supports all CLK features through a set of functions.

The current example shows how to configure the CLK to detect a failure on the HSE.

When the 24 MHz HSE clock is used as the system clock, the four LEDs (LD1, LD2, LD3, and LD4) connected to pins PH3, PH2, PH1 and PH0 respectively are toggled with a fast frequency derived from the system clock.

The CSS is enabled, so that when the HSE clock disappears (when it is broken or the external crystal is disconnected), the following clock recovery sequence is performed automatically:

●The HSE clock is disabled and the system clock switches to the HSI backup clock.

●The CPU continues processing operations but the frequency is changed to HSI/8 (CSS default clock).

●LED1, 2, 3, 4 toggle at a slower frequency.

●The clock switchover can be monitored by connecting an oscilloscope to the CCO pin (PE0).

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