AN2669

Example 1: Clock security system (CSS) capability

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Example 1: Clock security system (CSS) capability

1.1

Overview

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.

1.2

Hardware description

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

Figure 1.

STM8A CLK interface

1.3

Firmware 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).

CCO

24 MHz

20 pF

20 pF

1 M

Ω

60

Ω

Oscilloscope

OSCIN

OSCOUT

STM8A

PH0

PH1

PH2

PH3

1 k

Ω

1 k

Ω

1 k

Ω

1 k

Ω

LD1

LD2

LD3

LD4