#5 Preprocessor and the "volatile" keyword in C

Summary

In this Embedded Systems Programming lesson, Miro Samek demonstrates how to improve a simple 'blinky' program by using the C preprocessor and the 'volatile' keyword. The lesson covers how to enhance code readability with macros for hardware registers, the importance of the 'volatile' qualifier to prevent compiler optimizations for I/O registers, and the practical steps for using external header files provided by microcontroller vendors. The tutorial emphasizes code efficiency and debugging, ultimately helping learners write stable and optimized embedded programs. The next lesson will delve into more advanced GPIO features and bitwise operations.

Takeaways

• 😀 The lesson introduces improvements to the 'blinky' program using the C preprocessor and 'volatile' keyword.
• 😀 You should always make a copy of the previous project and rename it for new lessons to avoid confusion.
• 😀 The C preprocessor allows the use of macros to replace cryptic numbers with more readable names for registers.
• 😀 Macros are defined using the '#define' directive, and they can replace code to improve program readability.
• 😀 The preprocessor only replaces macros that are used in the code, and it performs text substitution before actual compilation.
• 😀 When defining macros, it's important to enclose them in parentheses to avoid unintended behavior in certain contexts.
• 😀 Macros can be defined using other macros, allowing for the creation of more complex register names.
• 😀 Always add comments to your code to improve readability, as comments are ignored by the compiler but are helpful to humans.
• 😀 The 'volatile' keyword is crucial when working with I/O registers, ensuring the compiler does not optimize away critical code.
• 😀 The example shows how to prevent the compiler from optimizing away delay loops by declaring the 'counter' variable as 'volatile'.
• 😀 Using the vendor-provided header file (.h) that contains predefined macros helps avoid manually defining each register macro.
• 😀 Once the vendor header file is included, the program should be tested to ensure the LED blinks as expected, validating the changes.

Q & A

What is the primary goal of this lesson?

The primary goal of the lesson is to show how to improve the 'blinky' program using the C preprocessor and the 'volatile' keyword to enhance readability and efficiency in embedded systems programming.

Why is using macros beneficial for improving code readability?

Using macros allows for the use of descriptive names for registers instead of cryptic numbers, making the code more readable and easier to understand for programmers, especially when dealing with hardware registers.

What role does the C preprocessor play in this lesson?

The C preprocessor handles text substitution before the actual compilation. It replaces macros with their defined values, ensuring that the code is more readable and organized while maintaining efficiency.

What is the function of the 'volatile' keyword in this context?

The 'volatile' keyword tells the compiler that a variable's value can change unexpectedly, such as in hardware registers. This prevents the compiler from optimizing the code in a way that could skip or modify accesses to those variables.

What could happen if the 'volatile' keyword was not used for hardware register access?

Without the 'volatile' keyword, the compiler might optimize the code and skip reading or writing to hardware registers, which would lead to incorrect behavior or functionality, particularly in I/O operations.

How does the preprocessor handle macro definitions?

The preprocessor replaces every occurrence of a macro in the code with its defined value. If the macro is not used, it doesn't affect the compiled code. It also removes lines containing macros before passing the code to the compiler.

Why is it important to enclose macros in parentheses?

Enclosing macros in parentheses ensures that their meaning remains consistent regardless of the context in which they are used. This avoids unexpected behavior due to operator precedence or misinterpretation in expressions.

What does the term 'optimization' refer to in the context of this lesson?

Optimization refers to the compiler's ability to improve performance by eliminating unnecessary instructions, such as removing unused variables or loops. However, this can lead to problems if the compiler optimizes away necessary parts of the code, such as delay loops, which is where 'volatile' becomes important.

How does the program's efficiency compare before and after using the preprocessor and 'volatile' keyword?

The program remains as efficient as before, as the preprocessor and 'volatile' keyword do not introduce runtime overhead. The compiler can still fold constants and avoid unnecessary calculations during execution.

What is the advantage of using the preprocessor header file provided by the microcontroller vendor?

Using the preprocessor header file provided by the microcontroller vendor saves time and effort, as it contains predefined macros for all relevant hardware registers, ensuring that the correct names and offsets are used and reducing the need for manual macro definitions.