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‎.gitignore

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_build

‎CMakeLists.txt

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cmake_minimum_required(VERSION 3.0)
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# project name
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project(bare_metal)
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# select linker script
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set(LINKER_SCRIPT "src/startup/stm32f031x6.ld")
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add_compile_options(
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-Wall
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-pedantic
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-Wextra
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-fstrict-volatile-bitfields
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-ffunction-sections
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)
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# define compiler options more info in: tools/gcc-arm/share/doc/gcc-arm-none-eabi/readme.txt
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set(CPU_OPTIONS "${CPU_OPTIONS} -mthumb -mcpu=cortex-m0")
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set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CPU_OPTIONS}")
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set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CPU_OPTIONS}")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fms-extensions")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-use-cxa-atexit")
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-threadsafe-statics")
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set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -T ${LINKER_SCRIPT}")
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set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -nostartfiles")
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# set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -nostdlib")
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# optimizations (-O0 -O1 -O2 -O3 -Os -Ofast -Og -flto)
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set(OPTIMIZATION_DEBUG "-Og")
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set(OPTIMIZATION_RELEASE "-O2 -flto")
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set(CMAKE_C_FLAGS_DEBUG "${OPTIMIZATION_DEBUG} -g -DDEBUG")
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set(CMAKE_CXX_FLAGS_DEBUG "${OPTIMIZATION_DEBUG} -g -DDEBUG")
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set(CMAKE_C_FLAGS_RELEASE "${OPTIMIZATION_RELEASE}")
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set(CMAKE_CXX_FLAGS_RELEASE "${OPTIMIZATION_RELEASE}")
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# select build type, can be Debug, Release
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set(CMAKE_BUILD_TYPE Debug)
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include_directories(
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${CMAKE_SOURCE_DIR}
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)
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link_directories(
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${CMAKE_SOURCE_DIR}
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)
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# list of modules to build final firmware (without extension .c or .cpp)
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add_executable(${PROJECT_NAME}.elf
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src/main
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src/startup/startup
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src/startup/stack
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src/startup/handlers_cm
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)
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include("cmake/flash.cmake")

‎README.md

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# Bare metal example for ARM cortex-M using C++
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This example show how to write simple startup code in C++ and use Cmake for building.
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Presented example is very simple "Hello blinky" program for STM32F0xx which blink LED connected to GPIO A15 and blink on maximum speed (no delay or timers are used).
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## C++ advantages for embedded
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- Possible to use latest standards (C++17)
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- Very small compiled code (comparable with C)
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### Limitations, or not recommended C++ features in embedded
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- Do not use new and delete (unless you define your own on heap or in static array - memory pool)
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- No exceptions are allowed and also no components which using exceptions (like most of `std::`, ...)
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- no RTTI, ..
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*Everything from this is also possible to use, but then is necessary to implement some stub functions for heap management and basic IO for `newlib`, after program can be much bigger in memory (often more than 80KB).*
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## Code structure
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- `CmakeLists.txt` is Cmake building script
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- `src` folder contain application code
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- `src/startup` folder contain all startup code + linker script
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- `cmake/arm-none-eabi.cmake` is platform file which define compilers and configuration of cmake for this project
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## Dependencies
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- arm-gcc for embedded [download here](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads)
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- [cmake](https://cmake.org/download/)
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- no need any dependencies like CMSIS library for building of this example.
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## Building
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```
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mkdir _build
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cd _build
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cmake -D CMAKE_TOOLCHAIN_FILE=../cmake/arm-none-eabi.cmake ..
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make
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```
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or use `make bin`, `srec`, `hex`, `flash`, ...

‎cmake/arm-none-eabi.cmake

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set(CMAKE_SYSTEM_NAME Generic)
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set(CMAKE_SYSTEM_PROCESSOR arm)
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set(CMAKE_C_COMPILER arm-none-eabi-gcc)
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set(CMAKE_CXX_COMPILER arm-none-eabi-g++)
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set(CMAKE_C_COMPILER_ID GNU)
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set(CMAKE_CXX_COMPILER_ID GNU)
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set(CMAKE_C_COMPILER_FORCED TRUE)
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set(CMAKE_CXX_COMPILER_FORCED TRUE)

‎cmake/flash.cmake

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# cmake script for generating binary and hex format and for flashing
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add_custom_command(OUTPUT ${PROJECT_NAME}.srec
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DEPENDS ${PROJECT_NAME}.elf
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COMMAND ${CMAKE_OBJCOPY} -Osrec ${PROJECT_NAME}.elf ${PROJECT_NAME}.srec
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)
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add_custom_command(OUTPUT ${PROJECT_NAME}.hex
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DEPENDS ${PROJECT_NAME}.elf
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COMMAND ${CMAKE_OBJCOPY} -Oihex ${PROJECT_NAME}.elf ${PROJECT_NAME}.hex
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)
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add_custom_command(OUTPUT ${PROJECT_NAME}.bin
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DEPENDS ${PROJECT_NAME}.elf
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COMMAND ${CMAKE_OBJCOPY} -Obinary ${PROJECT_NAME}.elf ${PROJECT_NAME}.bin
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)
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add_custom_command(OUTPUT ${PROJECT_NAME}.lst
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DEPENDS ${PROJECT_NAME}.elf
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COMMAND ${CMAKE_OBJDUMP} -S ${PROJECT_NAME}.elf > ${PROJECT_NAME}.lst
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)
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add_custom_command(OUTPUT ${PROJECT_NAME}.sym
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DEPENDS ${PROJECT_NAME}.elf
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COMMAND ${CMAKE_NM} -C -l -n -S ${PROJECT_NAME}.elf > ${PROJECT_NAME}.sym
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)
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add_custom_target(srec
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DEPENDS ${PROJECT_NAME}.srec
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)
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add_custom_target(hex
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DEPENDS ${PROJECT_NAME}.hex
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)
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add_custom_target(bin
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DEPENDS ${PROJECT_NAME}.bin
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)
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add_custom_target(lst
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DEPENDS ${PROJECT_NAME}.lst
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)
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add_custom_target(sym
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DEPENDS ${PROJECT_NAME}.sym
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)
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add_custom_target(flash
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pystlink flash:erase:verify:${PROJECT_NAME}.srec
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DEPENDS ${PROJECT_NAME}.srec
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)

‎src/main.cpp

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// define used registers
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#define RCC_AHB1 *(volatile unsigned *)(0x40021014)
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#define GPIOA_MODER *(volatile unsigned *)(0x48000000)
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#define GPIOA_BSRR *(volatile unsigned *)(0x48000018)
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// application
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void main_app() {
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RCC_AHB1 |= 1 << 17; // enable clock for GPIOA
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GPIOA_MODER |= 1 << (15 * 2); // set output on GPIOA.P15
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while (true) {
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GPIOA_BSRR = 1 << 15; // set output on GPIOA.P15
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GPIOA_BSRR = 1 << (15 + 16); // clear output on GPIOA.P15
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}
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}

‎src/startup/handlers_cm.cpp

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// handlers definition code
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// undefined handler point to this function, this stop MCU
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// this function name must by not mangled, so must be C
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extern "C" void __stop() { while (true); }
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// handlers for cortex-M core
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// these handler are 'weak' and can be overwritten by non-week function
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// default is __stop() function
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__attribute__((weak, alias("__stop"))) void RESET_handler();
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__attribute__((weak, alias("__stop"))) void NMI_handler();
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__attribute__((weak, alias("__stop"))) void HARDFAULT_handler();
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__attribute__((weak, alias("__stop"))) void MEMMANAGE_handler();
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__attribute__((weak, alias("__stop"))) void BUSFAULT_handler();
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__attribute__((weak, alias("__stop"))) void USAGEFAULT_handler();
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__attribute__((weak, alias("__stop"))) void SVCALL_handler();
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__attribute__((weak, alias("__stop"))) void DEBUGMONITOR_handler();
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__attribute__((weak, alias("__stop"))) void PENDSV_handler();
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__attribute__((weak, alias("__stop"))) void SYSTICK_handler();
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// dummy handler
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__attribute__((weak, alias("__stop"))) void DUMMY_handler();
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// vector table for handlers
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__attribute__((section(".vectors"), used)) void (*isr_vectors[])() = {
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RESET_handler,
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NMI_handler,
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HARDFAULT_handler,
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MEMMANAGE_handler,
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BUSFAULT_handler,
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USAGEFAULT_handler,
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DUMMY_handler,
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DUMMY_handler,
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DUMMY_handler,
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DUMMY_handler,
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SVCALL_handler,
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DEBUGMONITOR_handler,
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DUMMY_handler,
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PENDSV_handler,
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SYSTICK_handler,
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};

‎src/startup/stack.cpp

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// stack definition code
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// top of stack
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extern unsigned _stacktop;
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// initial stack pointer is first address of program
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__attribute__((section(".stack"), used)) unsigned *stack_init = &_stacktop;

‎src/startup/startup.cpp

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// very simple startup code with definition of handlers for all cortex-m cores
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// location of these variables is defined in linker script
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extern unsigned _data_load;
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extern unsigned _data_start;
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extern unsigned _data_end;
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extern unsigned _bss_start;
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extern unsigned _bss_end;
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extern unsigned _init_array_start;
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extern unsigned _init_array_end;
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extern unsigned _fini_array_start;
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extern unsigned _fini_array_end;
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// main application
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extern void main_app();
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// reset handler
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void RESET_handler() {
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unsigned *src, *dst;
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// copy data
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src = &_data_load;
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dst = &_data_start;
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while (dst < &_data_end) {
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*dst++ = *src++;
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}
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// zero bss
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dst = &_bss_start;
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while (dst < &_bss_end) {
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*dst++ = 0;
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}
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// get current stack pointer
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register unsigned *msp_reg;
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__asm__("mrs %0, msp\n" : "=r" (msp_reg) );
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// fill unused SRAM
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dst = &_bss_end;
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while (dst < msp_reg) {
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*dst++ = 0x55555555;
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}
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// call constructors for static instances
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dst = &_init_array_start;
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while (dst < &_init_array_end) {
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((void (*)())*dst++)();
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}
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// run application
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main_app();
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// call destructors for static instances
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dst = &_fini_array_start;
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while (dst < &_fini_array_end) {
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((void (*)())*dst++)();
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}
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// stop
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while (true);
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}

‎src/startup/stm32f031x6.ld

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MEMORY {
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FLASH(rx) : ORIGIN = 0x08000000, LENGTH = 32K
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SRAM(rwx) : ORIGIN = 0x20000000, LENGTH = 4K
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}
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SECTIONS {
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. = ORIGIN(FLASH);
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.text : {
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KEEP(*(.stack))
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KEEP(*(.vectors))
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KEEP(*(.vectors*))
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KEEP(*(.text))
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*(.text*)
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KEEP(*(.rodata))
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*(.rodata*)
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} >FLASH
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.init_array ALIGN(4): {
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_init_array_start = .;
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KEEP(*(.init_array))
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_init_array_end = .;
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} >FLASH
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.fini_array ALIGN(4): {
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_fini_array_start = .;
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KEEP(*(.fini_array))
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_fini_array_end = .;
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} >FLASH
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}
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SECTIONS {
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_stacktop = ORIGIN(SRAM) + LENGTH(SRAM);
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_data_load = LOADADDR(.data);
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. = ORIGIN(SRAM);
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.data ALIGN(4) : {
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_data_start = .;
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*(.data)
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*(.data*)
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. = ALIGN(4);
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_data_end = .;
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} >SRAM AT >FLASH
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.bss ALIGN(4) (NOLOAD) : {
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_bss_start = .;
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*(.bss)
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*(.bss*)
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. = ALIGN(4);
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_bss_end = .;
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*(.noinit)
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*(.noinit*)
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} >SRAM
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. = ALIGN(4);
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_heap_start = .;
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}

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