Example 2: Automatic clock switching

AN2669

4/8

2

Example 2: Automatic clock switching

2.1

Overview

The purpose of this example is to demonstrate the automatic clock switching feature

available in the STM8A. It provides an easy to use, fast and secure way for the application to

switch from one master clock source to another using very few instructions.

This example is provided as CLK example 2 in the STM8 firmware library.

2.2

Hardware description

The STM8/128-EVAL evaluation board is used for this example. The main hardware

connections are shown in Figure 1.

2.3

Firmware description

The user can change the clock selection at any time, by pressing the joystick in the desired

direction according to Table 1.

The current clock selection is displayed on the LCD.

Each clock source is associated with a LED as shown in Table 2.

The LED blinking frequency depends on the clock speed. Clock switching can also be

monitored by connecting an oscilloscope to the CCO pin (PE0 ).

Table 1.

Joystick control

Joystick movement

Clock source to switch to

Up

HSE clock

Down

LSI clock

Right

HSI clock

Table 2.

Status of LEDs display

LED

Master clock source

LD1

HSE clock

LD2

LSI clock

LD3

HSI clock