system.c File Reference

Source file with implementation of system clock initialization. More...

#include "system.h"

Include dependency graph for system.c:

Functions
void	system_init (void)
	Initializes the system clock configuration of the microcontroller.

Detailed Description

Source file with implementation of system clock initialization.

This file contains the implementation of the system initialization routine, which configures the main system clock, optional prescaler, and oscillator standby behavior for AVR microcontrollers.

Functionality includes:

• Switching to a selected system clock source
• Waiting until the clock source is stable
• Optional configuration of the peripheral clock prescaler
• Enabling standby operation for the internal 20 MHz oscillator

Author

g.raf

Date

2025-09-18

Version

1.0 Release

Copyright

Copyright (c) 2025 g.raf Released under the GPLv3 License. (see LICENSE in repository)

Note

This file is part of a larger embedded systems project and subject to the license specified in the repository. For updates and the complete revision history, see the GitHub repository.

See also

https://github.com/0x007e/avr0 "AVR ATmega GitHub Repository"

Function Documentation

system_init()

void system_init

(

void

)

Initializes the system clock configuration of the microcontroller.

This function configures and enables the main system clock source and (optionally) the peripheral clock prescaler. The initialization sequence includes:

1. Writing the selected system clock source (SYSTEM_CLOCK) to CLKCTRL.MCLKCTRLA.
   
2. Waiting until the new clock source is active and stable, by checking CLKCTRL.MCLKSTATUS against SYSTEM_CLOCK_BIT.
   
3. If SYSTEM_PER_CLOCK_PRESCALER is defined, configuring the peripheral clock division ratio via CLKCTRL.MCLKCTRLB.
   
4. If the selected system clock is the internal 20 MHz oscillator (CLKCTRL_CLKSEL_OSC20M_gc), enabling the oscillator in standby mode by setting CLKCTRL.OSC20MCTRLA.
   

Note

• The function makes use of the Configuration Change Protection (CCP) mechanism (CCP = CCP_IOREG_gc) before writing to protected CLKCTRL registers.
  
• Changing the system clock affects all timing-sensitive peripherals (UART, timers, ADC clock, etc.).
  
• If no prescaler (SYSTEM_PER_CLOCK_PRESCALER) is defined, the peripheral clock will run at F_CPU / 1.
  

Warning

This function should be called once during system startup before any timing-dependent peripherals are initialized.