The Ultimate Guide to IAR AVR 7.30.5 Build Errors 2025

Error[Pe020]: identifier "DDRB" is undefined ( E:\Projects\AVR_Blinky\main.c 42 )

Common Causes & Solutions:

1. Missing Header File: Most often, you've forgotten to include the necessary header. For AVR-specific registers like DDRB, PORTB, or TCNT0, you absolutely need to include the main I/O definition file.

   // FIX: Add this line at the top of your main.c file
   #include <avr/io.h>

2. Typo: A simple spelling mistake. Did you type PORTB or PROTB? Double-check your spelling against the microcontroller's datasheet.
3. Incorrect Scope: The variable or function was declared in a different scope (e.g., inside another function) and is not visible where you're trying to use it.
4. Wrong Device Selected: In IAR, if you have selected an ATmega328P in your project options but are trying to access a register only available on an ATxmega, you'll get this error. Ensure your project target (Project > Options > General Options > Target) matches your hardware.

Warning[Pa082]: undefined behavior: the order of volatile accesses is undefined

This warning is crucial. While only a warning, ignoring it can lead to bizarre and hard-to-debug runtime bugs. It occurs when you access a volatile variable (like a hardware register) multiple times in a single statement.

Why is this bad? The C standard doesn't guarantee the order of operations within a single expression. The compiler might reorder your reads and writes in a way that breaks your logic.

Incorrect Code:

// WARNING: This can cause undefined behavior!
TIFR0 |= (1 << TOV0) | (1 << OCF0A); // Writing two bits to a volatile register

Correct, Safe Solution:

Break the operation into multiple, distinct statements. This creates "sequence points" that force the compiler to complete one operation before starting the next.

// CORRECT: Each access is a separate statement
TIFR0 |= (1 << TOV0);
TIFR0 |= (1 << OCF0A);

Wrestling with Lethal Linker Errors (The 'Lp' Series)

Linker errors appear after the compiler has successfully created all the object files. The linker's job is to stitch these files together and place them into the microcontroller's memory map. Linker errors are almost always about memory: either you've run out of it, or you're trying to put something where it doesn't belong.

Error[Lp005] & Error[Lp011]: Memory Placement and Overflow Errors

These two errors are symptoms of the same family of problems: memory management.

• Error[Lp005]: an absolute segment [...] cannot be placed in a cluster [...]: You're trying to place code or data at a specific memory address (using the @ operator or a pragma) that is already occupied.
• Error[Lp011]: could not find a memory range for the segment '...': The linker ran out of a specific type of memory (usually FLASH or SRAM) while trying to place a segment (like .text for code or .bss for zero-initialized variables).

Solutions: Your best friend for solving these is the Map File. Enable it under Project > Options > Linker > List > Generate linker map file. This file gives you a detailed report of what is placed where and how much space everything consumes.

Problem Symptom	Likely Cause	Primary Solution
Linker error indicates no space for .text or .rodata	Your compiled code and constants exceed the available FLASH memory.	Go to Project > Options > C/C++ Compiler > Optimizations and increase the optimization level (e.g., to High/Size). Analyze the .map file to find unexpectedly large functions or constant tables.
Linker error indicates no space for .bss or .data	Your global and static variables exceed the available SRAM.	Review your variables. Can any large arrays be made smaller or stored in FLASH using the PROGMEM utility? Can you reduce the scope of variables from global to local?
Error[Lp005] with absolute segments	You have two pieces of code/data competing for the same explicit address.	Search your code for the @ operator or #pragma location. Check your linker script (.xcl file) for custom segment definitions. Ensure there are no overlaps.

Solving Configuration & Environment Pitfalls

Sometimes the code is perfect, but the project setup is wrong. These errors are often the quickest to fix once you know where to look.

Error: cannot open source file "..."

This is a classic include path problem. The compiler knows it needs a file (e.g., my_custom_library.h), but it doesn't know where to find it.

Solution: You need to tell IAR where your header files are located.

1. Go to Project > Options.
2. Select C/C++ Compiler from the left-hand pane.
3. Go to the Preprocessor tab.
4. In the Additional include directories box, add the path to the folder containing your header file. Use relative paths (e.g., $PROJ_DIR$\..\libs\) for portability.

Proactive Strategies to Prevent Errors

The best way to fix errors is to prevent them from happening in the first place.

• Use Version Control: Use Git. Always. When a mysterious error appears, being able to see exactly what changed since the last successful build is invaluable.
• Compile Often: Don't write hundreds of lines of code before compiling. Make small changes and build frequently. It's much easier to find an error in 5 new lines of code than in 500.
• Understand `PROGMEM`: For AVRs with limited RAM, learning to properly use the `PROGMEM` utilities to store constant data (like strings, lookup tables, and bitmaps) in FLASH instead of RAM is a non-negotiable skill.
• Leverage `static_assert`: If your logic depends on certain assumptions (e.g., the size of a data type or a struct), you can validate them at compile time. This is far better than discovering the issue at runtime.

  #include <static_assert.h>
  
  // This will cause a build error if a long is not 4 bytes,
  // preventing incorrect pointer arithmetic.
  static_assert(sizeof(long) == 4, "This code assumes long is 32-bit.");

Conclusion: Building with Confidence

IAR AVR build errors, while initially daunting, are not malicious roadblocks. They are signposts from the toolchain, guiding you toward a more correct, efficient, and robust firmware. By understanding their anatomy, learning to recognize common patterns—from simple missing includes to complex linker memory overflows—and adopting proactive coding habits, you can transform your debugging sessions from frustrating slogs into efficient problem-solving exercises.

So next time you see that flash of red in your build log, take a breath. You have the knowledge to decode the message, pinpoint the problem, and get back to what you do best: bringing amazing embedded projects to life.