July 2016 DocID029140 Rev 1 1/16
1
AN4850
Application note
STM32 MCUs spread-spectrum clock generation
principles, properties and implementation
Introduction
The STM32F2 Series, STM32F4 Series and STM32F7 Series embed a SSCG (spread-
spectrum clock generation) in their main PLL (phase-locked loop) system. This SSCG offers
some advantages regarding electromagnetic compatibility.
The main objectives of this application note are:
• to describe the SSCG principle in a simple way
• to present and explain its properties and features
• to compare the SSCG systems against non-SSCG systems.
This document covers a SSCG technique’s overview, an explanation of the SSCG principle
in a simple way as well as an overview of the SSCG advantages and disadvantages. This
application note also presents a parameters summary of the SSCG inside STM32F2 Series,
STM32F4 Series and STM32F7 Series and includes details on how to configure the SSCG
parameters in those products.
Table 1. Applicable products
Type Product series
Microcontrollers
STM32F2 Series, STMF4 Series, STM32F7 Series
www.st.com
Contents AN4850
2/16 DocID029140 Rev 1
Contents
1 Spread-spectrum clock generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Electromagnetic interference (EMI) problems and the clock generation . . 5
1.2 Using spread-spectrum technique for EMI reduction . . . . . . . . . . . . . . . . . 5
2 SSCG operation overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 SSCG mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Implementation of the SSCG controller . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 SSCG parameter calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.1 Example of configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 SSCG spectrum measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 SSCG basic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
