Microcontrollers are small, low-power computers that are used to control electronic devices. They are found in a wide range of products including remote controls, toys, home appliances, cars, and industrial automation systems. Programming microcontrollers requires knowledge of C programming as it is the most common language used.
Getting Started with C and Microcontrollers
To program microcontrollers with C, you will need a microcontroller development board, a C compiler, and an IDE (integrated development environment). Popular options include:
- Microcontroller boards: Arduino, STM32, PIC, AVR
- Compilers: GCC, Keil, IAR
- IDEs: Arduino IDE, MPLAB X, Atmel Studio, Keil uVision
Start by choosing a development board and learning how to upload code examples. Then install the compiler and IDE. Read the documentation to learn the toolchain workflow from code to compiling to uploading to the microcontroller.
C Programming Basics
C is a procedural programming language. It has constructs like variables, data types, loops, conditional statements, and functions. Here are some basics you’ll need to know:
- Data types: int, float, char, pointers
- Variables and constants
- Operators: arithmetic, relational, logical
- Control flow: if-else, switch, for loop, while loop
- Functions: declarations, calls, arguments
- Arrays and strings
- Structures and unions
Study C programming tutorials and books to gain a solid grasp of syntax and constructs. Write some small practice programs for your desktop computer before moving to microcontroller code.
Microcontroller Specifics
There are some key differences when programming microcontrollers in C compared to desktops:
- Limited resources – less memory, storage, and CPU power
- Real-time operation – code needs to execute within precise time contexts
- I/O peripherals – interacting with ports, timers, ADCs, communication buses
- Lack of OS – no operating system, you manage the bare metal hardware
- Specialized compilers/debugging – compilers optimize for size, debuggers allow flash breakpoints
Your C code must be tightly optimized. You have direct access to registers and hardware. Real-time response is critical. Resource constraints force you to minimize RAM usage.
General Coding Guidelines
Here are some best practices to follow when writing C programs for microcontrollers:
- Use static and global variables sparingly
- Split code into reusable functions
- Minimize RAM usage with constants not variables
- Avoid recursive functions and dynamic memory
- Use efficient data types like bitfields
- Keep real-time code fast and deterministic
- Properly handle peripheral initialization and interrupts
- Implement error handling and recovery
- Test rigorously on actual hardware
- Validate timing, power draw, and resource usage
Well-written microcontroller C code is optimized for space, performance, and reliability. Balance tradeoffs between size, speed, and maintainability.
Peripheral Programming
A microcontroller’s peripherals allow it to interface with sensors, displays, networks, motors, and other electronics. Here’s how to use them in C:
GPIO and Ports
General purpose I/O ports connect to LEDs, buttons, and more. Set pin modes, read pin values, and write to output pins. Handle external interrupts.
Timers and Counters
Timers generate precise delays and provide timebases for PWM signals. Use capture and compare modes to measure signals. Handle timer interrupts.
ADCs
Read analog voltages with analog-to-digital converters. Configure resolution and sampling rates. Use interrupts or poll ADC status. Calibrate for accuracy.
Communication Interfaces
Interfaces like I2C, SPI, UARTs allow communicating with other ICs. Master protocols, handle bus collisions, manage synchronization.
Other Peripherals
Pulse width modulation, direct memory access controllers, watchdog timers, and more. Refer to your microcontroller’s reference manual.
Optimizing C Code
Microcontrollers have very limited resources. Below are tips for optimizing C code to minimize size and maximize speed:
- Use compiler optimizations like -Os to shorten code
- Avoid floating point numbers and math
- Replace multiplication with shifts and addition
- Use bitwise operations instead of conditionals
- Minimize function calls through inlining
- Leverage C preprocessor macros for code reuse
- Simplify expressions and remove unneeded variables
- Write time-critical code sections in assembly
Well-optimized C code makes efficient use of the microcontroller hardware. Balance tradeoffs when improving performance.
Testing and Debugging
Thoroughly test C programs on real microcontroller hardware. Here are useful practices:
- Unit test individual functions and modules
- Make use of simulators and emulators for controlled testing
- Verify edge cases and failure modes
- Stress test limits of timing, I/O, and resources
- Enable asserts and watchdogs to catch errors
- Use debuggers and probes to trace execution
- Monitor variables and peripheral states
- Implement logging and print debugging
Catching bugs early prevents problems in the field. Validate timing, memory safety, peripherals, and integration with external components.
Example C Microcontroller Programs
Here are examples of common C programs for microcontrollers:
- Blink an LED – basic GPIO output toggle
- Button debouncing – clean signal input from a mechanical button
- DC motor control – PWM and H-bridges
- Read a temperature sensor – analog voltage conversion
- Generate a clock signal – timer interrupts at fixed frequency
- SPI peripheral driver – library for communicating over SPI
- Real-time PID control loop – maintain precise closed-loop control
Start simple and build up to more complex programs. Refer to datasheets and application notes.
Conclusion
C programming allows full control over microcontroller hardware. With some basic C knowledge and microcontroller specifics, you can write optimized and robust code for embedded systems. Make use of all the development tools available to code, test, and debug your programs.
