The ARM Cortex-M3 is a popular 32-bit processor based on the ARMv7-M architecture. It is designed for low-power embedded applications and includes features like a memory protection unit, single-cycle fast multiply, and low-latency interrupts. Here are some tips and tricks for programming the Cortex-M3 that can help improve performance, reduce code size, and make development easier.
Optimize Code for Size and Speed
The Cortex-M3 includes both a 32-bit Thumb-2 instruction set as well as a 16-bit Thumb instruction set. For optimal code density, use 16-bit Thumb instructions whenever possible. But for performance-critical code sections, use the 32-bit Thumb-2 instructions to maximize speed. The compiler can automatically intermix 16-bit and 32-bit instructions, but you can control this behavior with optimizations.
Enable linker compression to reduce code size. This replaces common symbol references with tokens. Also turn on loop unrolling, function inlining, and unused code elimination to optimize for speed or size.
Use the Hardware Floating Point Unit
The Cortex-M3 processor includes an optional single-precision floating point unit (FPU). Using the FPU can accelerate math-heavy code vs. doing floating point operations in software. Enable the FPU in the compiler settings when building your program.
When using hardware floating point, pass variables by reference instead of value to avoid unnecessary memory copies. Declare variables as float instead of double to match the hardware precision. Use intrinsics like __aeabi_fadd() for better code gen.
Optimize Interrupt Latency
The Cortex-M3 supports low interrupt latency by shadowing key registers onto the stack during exception entries. This avoids having to save and restore registers to memory.
Set up your linker scripts to allocate stack space for each exception handler. Optimize the prologue and epilogue of interrupt routines to save only the registers you need. Minimize stack usage in ISRs to reduce latency.
Use Bit-Banding for Atomic Operations
The Cortex-M3 memory map includes a bit-band region that allows each word in the region to map to a single bit in the bit-band alias memory area. This enables atomic read-modify-write operations on single bits.
Use bit-banding to implement thread-safe flags, mutexes, and other synchronization primitives. Bit-banding provides atomicity without using locks. Accessing bit-band memory does take more cycles than a direct memory write so use judiciously.
Align Buffers for DMA Transfers
The Cortex-M3 includes a DMA controller that can move data between peripherals and memory without CPU intervention. For best performance, make sure DMA buffers are aligned to cache line sizes.
Allocate DMA buffers using linker section directives like .bss to ensure alignment. Turn on cache coherency if DMA and CPU access the same buffer. Use DMA double-buffering and ping-pong transfers to pipeline back-to-back DMA transactions.
Use Cortex-M3 intrinsics
The CMSIS libraries for Cortex-M3 provide intrinsic functions that compile down to single instructions. This can generate tighter code than inline assembly or C library functions.
Use intrinsics like __enable_irq() instead of asm volatile statements. Take advantage of __LDREX() and __STREX() for atomic operations. Utilize __REV(), __RBIT(), and other bit manipulation intrinsics for optimized bitfield access.
Structure Code for Cache Hits
The Cortex-M3 MPU includes optional instruction and data caches to boost performance. But cache misses can cause multi-cycle pipeline stalls. Organize code and data to maximize cache hits.
Place frequently accessed variables, functions, and code regions in contiguous blocks to exploit locality of reference. Use memory barriers and cache maintenance operations to ensure coherence. Turn on cache coloring in the linker to reduce conflicts.
Use Cortex-M3 DSP Instructions
The Cortex-M3 ISA includes optional DSP extensions like single-cycle MAC operations. This can accelerate digital signal processing code for applications like audio processing, filters, and transforms.
Enable the DSP extension in GCC to automatically use MAC and SIMD instructions. Explicitly use Q flag saturation to avoid overflows. Leverage circular buffers and double-buffering to pipeline back-to-back DSP computations.
Optimize Speed vs Energy Usage
The Cortex-M3 processor includes multiple power modes to balance performance vs energy efficiency. Switch between run modes, adjust clock speeds, and gate clocks to inactive logic to save power.
Use Sleep and Deep Sleep low power modes and wake up on interrupts. Modify voltage scaling and clock trees at runtime to scale speed. Shut off peripherals when not active. Cluster code to allow clock gating of unused sections.
Use Cortex-M3 Memory Protection Unit
The optional MPU provides support for protecting regions of memory through permissions and attribute settings. Use this to isolate and sandbox modules and prevent common bugs.
Split memory into privileged and unprivileged regions. Assign read, write, execute permissions to each region. Mark memory as non-cacheable or stack regions as non-executable. Set up MPU regions in system initialization.
Choose the Right Development Tools
The right compilers, debuggers, and IDEs can significantly boost Cortex-M3 programming. GCC tools like arm-none-eabi provide a full-featured open source toolchain.
Use IDEs with debuggers that support SWV tracing and data watchpoints. Verify code with static analysis. Profile performance with tools like gprof and Valgrind. Consider using a RTOS or framework to abstract hardware.
Use Component Libraries
Reusing proven, optimized peripherals and software components can accelerate development. Many open source libraries are available for the Cortex-M3.
Select libraries that align with your OS, RTOS, toolchain, and middleware. Ensure the libraries match Cortex-M3 architecture and hardware features. Modify libraries to utilize Cortex-M3 specific intrinsics and instructions.
Write Reusable, Modular Code
Well-structured, modular code makes Cortex-M3 development more efficient. Break software into independent modules that reduce complexity.
Separate chip-specific, hardware-dependent code from core algorithm implementations to increase portability. Use abstraction layers to isolate hardware interfaces. Write generic interfaces around peripherals and utilities.
Learn ARM Assembly Basics
Knowing basic ARM Assembly language can help optimize performance-critical routines and boot code. Inline assembly allows mixing C and Assembly.
Learn the Cortex-M3 register conventions and instruction syntax. Use Assembly for low-level functions like context switching. Implement ISRs and fault handlers in Assembly. Call Assembly routines from C using __asm() directives.
Use Linker Scripts Efficiently
Linker scripts control memory mapping and placement. They allow optimizing layout for performance, size, and hardware addressing.
Place code and constants in flash, variables in RAM sections. Arrange objects for cache locality. Set alignment constraints. Use linker macros to customize scripts. Learn to tweak sections, symbols, regions for efficiency.
Profile and Benchmark Code
Profiling Cortex-M3 code can identify optimization opportunities. Use tools like gprof to find hotspots and bottlenecks to improve.
Profile entire tasks as well as fragment hotspots. Inspect generated assembly to verify optimizations. Use timing APIs and hardware performance counters to measure jitter and runtime. Prioritize fixes that provide maximum speedup.
Design for Low Power
Optimizing for low power utilization allows Cortex-M3 systems to run on batteries or energy harvesting. Minimize switching activity through efficient coding.
Minimize memory accesses which consume power. Reduce unnecessary computation and looping. Use sleep modes and wake on interrupt. Dim peripherals such as LEDs. Follow coding guidelines to maximize power savings.
Use an RTOS or scheduler
Using a real-time operating system or scheduler can greatly simplify Cortex-M3 programming. An RTOS provides preemption, synchronization, and timing services.
Select a small footprint RTOS that fits Cortex-M3 memory constraints. Ensure Cortex-M3 specific RTOS ports exist. Use RTOS services for task management, queues, mutexes, and time delays. Write code as RTOS-aware threaded tasks.
Handle Compiler Errors and Warnings
The compiler can detect bugs and issues through warnings and errors. Always compile with all warnings enabled and do not ignore errors.
Understand warning causes and fix the root issue. Increase warning verbosity for more extensive checking. Use static analysis tools to find additional bugs. Enable as many compiler diagnostics as possible to catch problems early.
Utilize Compiler Extensions
Compiler extensions provide useful C language features not in the C standard. These enhance coding flexibility, safety, and efficiency.
Use attributes to customize code generation. Enable type generics for type-safe containers. Leverage compile time recursion through constexpr. Consider using code-checking tools like Kconfig and sparse.
Document Effectively
Detailed documentation helps new users get started and aids long-term maintenance. Comments should explain rationale and constraints.
Document hardware dependencies, interfaces, and toolchain assumptions. Use a documentation generator like Doxygen. Follow marking standards like Javadoc. Keep comments concise but descriptive. Maintain a coding standard for consistency.
Adopt a Coding Standard
Using a consistent coding standard makes code easier to understand and maintain. Standards cover formatting, naming, and structure.
Follow industry standards like MISRA C or JSF AV. Use commonsense conventions for files, variables, functions. Consistent use of braces, line length, and indentation aids readability. Enforce with lint tools.
Learn from Open Source Projects
Studying open source Cortex-M3 code can provide good examples of best practices. Reusing code can accelerate projects.
Find projects on GitHub and other repositories. Reuse generic drivers and utilities. Adapt code to fit toolchain and OS constraints. Give back by contributing improvements to open source.
Build Safety-Critical Systems
Safety-critical systems have stringent fault tolerance and reliability requirements. Cortex-M3 supports features to help.
Use memory protection and privilege modes. Enable independent watchdog timers. Leverage lockstep cores. Adhere to coding standards like MISRA C. Strive for high test coverage and run static analysis.
Know the ARM Toolchain
Having in-depth knowledge of the ARM compiler and assembler options aids optimization. The toolchain affects generated code quality.
Understand compiler optimization levels. Use assembler directives to fine-tune placement. Pass specific command line options for desired behavior. Compile small test cases with different settings. Generate assembly listings to verify output.
Write Portable Code
Writing portable C code allows reuse across Cortex-M3 projects and other ARMv7-M chips. Isolate hardware dependencies.
Use CMSIS libraries to abstract hardware. Avoid non-standard language extensions. Do not hardcode memory addresses. Parameterize timing constants. Follow standards for types, naming, and formats.
Validate with Simulation Models
Simulate Cortex-M3 code on instruction set simulators before running on hardware. ModelSim and QEMU can simulate execution.
Find ARMv7-M ISS models for Cortex-M3. Link code with ISS libraries to run. Compare trace logs and peripheral accesses between hardware and simulation runs. Fix any functional or timing issues.
Use Revision Control
Use version control systems like Git or SVN to maintain code history. This enables source control, collaboration, and release management.
Commit related changes with descriptive messages. Store repositories on servers for collaboration and backups. Maintain separate branches for experimental work. Use tags to label releases. Automate builds and testing.
Design for Testability
Writing testable code with high coverage ensures functionality and prevents regressions. Unit testing allows exhaustive tests.
Isolate dependencies to mock them during testing. Refactor code to increase modularity. Add assertions to validate state during execution. Use dependency injection to simplify testing. Automate tests as part of build process.
Conclusion
Using these Cortex-M3 programming tips and best practices can help developers write high-quality embedded software. Efficient use of the Cortex-M3 architecture through the compiler, libraries and writing clean and portable code results in better performance, smaller size, and quicker time to market. As with any architecture, fully understanding the hardware capabilities is key to unlocking its potential.
