When we write programs for AVR microcontrollers in C language, we rarely start from scratch. Most of the functionality like I/O control, timing, interrupts, and serial communication is already available to us through ready-made library files. These libraries make programming faster, easier, and more readable. They contain predefined functions, macros, constants, and register definitions that help us communicate with the AVR hardware efficiently. Instead of writing lengthy low-level assembly code, we can simply include a library and use its functions. For instance, turning an LED ON, configuring a timer, or sending serial data — all become one-line tasks with the help of proper header files. Understanding the role and structure of these library files is a fundamental part of embedded C programming because they serve as the bridge between our code and the AVR hardware registers. In this detailed note, we’ll discuss the types of library files used in AVR, their importance, how they are linked in the program, and the commonly used header files in AVR-GCC.
1. What Are Library Files?
A library file in AVR C programming is a collection of predefined code that provides specific functionality.
It can include register definitions, function declarations, macros, or constants which the compiler can reuse.
In simple words, a library file saves time and ensures standardisation of code across multiple projects.
Libraries are written once and used in many programs by using the #include directive.
#include <avr/io.h> // for input-output port definitions
#include <util/delay.h> // for delay functions
2. Types of Library Files in AVR
In AVR development, there are mainly two categories of library files used:
| Type | Description |
|---|---|
| Header Files (.h) | Contain declarations, macros, and function prototypes. Included using #include. |
| Object or Archive Files (.o / .a) | Contain the compiled binary code of library functions which the linker uses to build the final program. |
3. Commonly Used AVR Library Files
The AVR-GCC toolchain provides a wide range of standard libraries that make embedded programming smooth. Some of the most commonly used ones are listed below:
| Library File | Purpose / Functionality |
|---|---|
| <avr/io.h> | Defines all Input/Output registers, bit names, and port addresses for the specific AVR device. |
| <util/delay.h> | Provides the _delay_ms() and _delay_us() functions for accurate software delays. |
| <avr/interrupt.h> | Contains macros for enabling/disabling interrupts using sei() and cli(). |
| <avr/eeprom.h> | Allows reading and writing of data to EEPROM memory. |
| <avr/sleep.h> | Used to control sleep modes for power-saving applications. |
| <stdio.h> | Standard input/output library used for formatted printing or serial communication. |
4. How to Include Library Files
In C, library files are included using the #include preprocessor directive.
If the library is part of the standard toolchain, angle brackets < > are used.
If it is a user-defined library (custom code written for your project), double quotes " " are used.
// Standard library
#include <avr/io.h>
// User-defined library
#include "lcd.h"
5. Structure of AVR C Program Using Libraries
A typical AVR program starts by including necessary header files. These files automatically configure the microcontroller-specific registers and provide functions for I/O handling.
#include <avr/io.h>
#include <util/delay.h>
int main(void) {
DDRB = 0xFF; // Configure PORTB as output
while(1) {
PORTB = 0xFF; // Turn ON all LEDs
_delay_ms(500);
PORTB = 0x00; // Turn OFF all LEDs
_delay_ms(500);
}
}
Here, <avr/io.h> defines the register names like PORTB and DDRB,
and <util/delay.h> gives access to the _delay_ms() function.
Without these libraries, we would have to define addresses manually — which is difficult and error-prone.
6. Behind the Scenes — Linking of Libraries
When we compile the AVR program, the compiler converts the source code into object code. The linker then searches for all function definitions in the included library files and links them with the main program. This process ensures that all required functions are merged into the final hex file which is later uploaded into the microcontroller.
AVR Program Compilation Flow
To understand how library files and source code come together in AVR development, it’s useful to see the entire flow — from writing the C program to generating the final HEX file that’s uploaded into the microcontroller. The below diagram explains each step of this compilation and linking process.
7. Creating and Using Custom Library Files
Apart from built-in AVR libraries, developers can create their own library files for LCD, keypad, ADC, UART, or motor drivers. This approach improves code reusability and keeps large projects organised. A user-defined library typically consists of two files:
- Header file (.h): Contains function declarations and macros.
- Source file (.c): Contains function definitions.
// lcd.h
void lcd_init(void);
void lcd_cmd(unsigned char cmd);
void lcd_data(unsigned char data);
// lcd.c
#include "lcd.h"
void lcd_init(void){ /* code */ }
8. Advantages of Using Library Files
- Reduces code writing time.
- Improves readability and structure of the program.
- Encourages code reusability and modular programming.
- Ensures compatibility with different AVR devices.
- Provides access to optimized and tested low-level routines.
Conclusion
Library files are the backbone of C programming in AVR microcontrollers.
They hide complex hardware details and offer user-friendly functions for performing specific tasks.
By understanding and efficiently using these files, students can write cleaner and more reliable embedded programs.
In the next step of learning, try exploring each library individually and check their functions by reading header file contents
found inside the avr-libc directory. This practice will strengthen your knowledge of AVR internal working and make
you confident in developing professional embedded projects.
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