Quick Start Guide for the System Clock Management service (SAM4S)

This is the quick start guide for the System Clock Management service, with step-by-step instructions on how to configure and use the service for specific use cases.

System Clock Management use cases

• Basic usage of the System Clock Management service
• Advanced use case - Peripheral Bus Clock Management (SAM4S)

Basic usage of the System Clock Management service

This section will present a basic use case for the System Clock Management service. This use case will configure the main system clock to 120MHz, using an internal PLL module to multiply the frequency of a crystal attached to the microcontroller.

Prerequisites

• None

Initialization code

Add to the application initialization code:

sysclk_init();

Workflow

1. Configure the system clocks according to the settings in conf_clock.h:

   sysclk_init(); 

Example code

Add or uncomment the following in your conf_clock.h header file, commenting out all other definitions of the same symbol(s):

#define CONFIG_SYSCLK_SOURCE        SYSCLK_SRC_PLLACK

// Fpll0 = (Fclk * PLL_mul) / PLL_div
#define CONFIG_PLL0_SOURCE          PLL_SRC_MAINCK_XTAL
#define CONFIG_PLL0_MUL             (120000000UL / BOARD_FREQ_MAINCK_XTAL)
#define CONFIG_PLL0_DIV             1

// Fbus = Fsys / BUS_div
#define CONFIG_SYSCLK_PRES          SYSCLK_PRES_1

Workflow

1. Configure the main system clock to use the output of the PLL module as its source:

   #define CONFIG_SYSCLK_SOURCE          SYSCLK_SRC_PLLACK 

2. Configure the PLL module to use the fast external fast crystal oscillator as its source:

   #define CONFIG_PLL0_SOURCE            PLL_SRC_MAINCK_XTAL 

3. Configure the PLL module to multiply the external fast crystal oscillator frequency up to 120MHz:

   #define CONFIG_PLL0_MUL             (120000000UL / BOARD_FREQ_MAINCK_XTAL)
   #define CONFIG_PLL0_DIV             1

   Note

   For user boards, BOARD_FREQ_MAINCK_XTAL should be defined in the board conf_board.h configuration file as the frequency of the fast crystal attached to the microcontroller.

4. Configure the main clock to run at the full 120MHz, disable scaling of the main system clock speed:

   #define CONFIG_SYSCLK_PRES         SYSCLK_PRES_1

   Note

   Some dividers are powers of two, while others are integer division factors. Refer to the formulas in the conf_clock.h template commented above each division define.