A FreeRTOS distribution for ARM microcontrollers
Hugo Vincent <firstname.lastname@example.org>, 8 March 2011.
This is a real-time operating system for very small devices built around an ARM microcontroller (with typically at least 16 kB of RAM and 64 kB of flash).
A core aim of this project is to provide a ready-to-use distribution of FreeRTOS, in the sense that Linux distributions are much more than just the kernel. As such, we include a clean, consistent build system, a fully functional standard C library with well defined, portable ways of doing hardware abstraction, peripheral access, filesystem access, and so on.
- Support for the ARM Cortex M3 and ARM7TDMI; currently the NXP LPC1768 and LPC2368 ports are working.
- Hard-realtime preemptive multitasking kernel (FreeRTOS v6.1.0).
- Supports protected memory on Cortex M3s that have an MPU (memory protection unit -- included in the LPC17xx series). Tasks that access memory they do no have permission to access are cleanly terminated and debug information is printed to the console.
- Provides exception handlers. Bugs in application code that generate an ARM hardware exception or fault (attempt to access non-existent memory, undefined instruction etc) are trapped and debug output is shown. Shows a stack backtrace and processor state information. In addition, the kernel attempts to recover from such faults by cleanly terminating the responsible task. Similarly for tasks that overflow their application stack.
- Complete C library support (including
malloc, standard file IO etc).
- C++ support, including lightweight STL (www.uSTL.sf.net), and exception handling.
- Ethernet networking with the lightweight uIP TCP/IP stack, including a web server.
- Support for power management (processor is aggressively idled when possible, coming soon: dynamic frequency scaling).
- Well integrated GNU toolchain and build system.
- Build system lists total flash and RAM, and provides an estimate for available heap space for dynamically allocated (malloc'd) memory.
- A UNIX-like filesystem hierarchy. Devices can be accessed via their
/dev/nodes. Filesystems (depending on target hardware) can be added to the root filesystem hierarchy, for example
- A read-only filesystem resident in on-chip flash is supported on all targets.
- mbed (mbed.org) target supports semihosted local filesystem, accessible via the mbed USB interface. Should also work with semihosting-compatible debuggers.
- Read/write FAT filesystem on SD/microSD cards (coming soon).
- One UART is used as the console, which is used for operating system messages,
debug output, and standard IO (
- POSIX APIs for threads, timers, sockets etc. (coming soon). We're aiming for full POSIX 1003.13 Profile 52 support eventually.
- Device drivers for many on-chip and common off-chip peripherals.
- Aims not to drown in excessive configuration options.
- Download and install a compatible arm-none-eabi GNU toolchain, such as the toolchain at http://github.com/hugovincent/arm-eabi-toolchain.
- Edit the configuration options at the top of the Makefile.
- Program the generated binary image to your hardware. The Makefile provides
make installtarget to install to suppoted boards.
To be written.
Tested with the Codesourcery 2010q1-188 arm-non-eabi toolchain, built from source with http://github.com/hugovincent/arm-eabi-toolchain (on Mac OS X 10.6). You will probably have problems with an official Codesourcery toolchain as a number of compiler and C library options had to be changed to suit this project. It is strongly recommended to use this customized toolchain.
This has currently only been tested with mbed (www.mbed.org) hardware, some versions of which use the NXP LPC2368, and some the LPC1768.
This is a summary of roughly how early-boot through to OS running works. This example is for Cortex-M3 - other ARM devices have a slightly different process at the start (the C stack and other C runtime stuff is done in assembler code instead).
<Reset> Hardware set's up a basic C stack with predefined stack pointer. Hardware jumps to Reset_Handler - these addresses are defined in the .vectors section. [Reset_Handler] - this code does what is traditionally called crt0 (C run time). - can't assume all C features are working - initialises C features like data (pre-initialized variables) and bss (zero-initialized variables) - copy initial vector table from Flash to RAM and atomically perform relocation of it - set up any faults and so forth (generally a good idea to attempt to handle faults than to just ignore them, which will trigger reset) - optionally initialise a different stack (the "process stack") - pass control to Boot_Init() [Boot_Init] - call System_Init() [System_Init] - set up clocks and PLLs if applicable - enable clocks and if applicable switch core clock source (normally to something faster) - enable power/clocks to core peripherals - if applicable, set up memory management/remapping/acceleration etc - call Board_EarlyInit() [Board_EarlyInit] - set NVIC vectors for low-level interrupts (supervisor call, system timer tick (part of the Cortex M3 complex, not a SoC level timer), etc - if used, set up any debug communications channels, and - set up minimal pin multiplexing etc and peripheral settings so that the debug UART works (the early/late dichotomy is so that debugging or error messages during the late init/boot process can be seen, and that certain boot operations can assume the presence and functionality of certain OS functionality) - set any "unsafe" GPIOs to a safe value (things that might be left floating at boot, but need to be in a defined state for safe operation - initialise the watch dog timer (optional) - call Console_EarlyInit() [Console_EarlyInit] - set up buffers etc so that printf or other IO machinery works as intended. This might be done in a safe-but-slow manner - do any other C/C++ initialisation required (e.g. call C++ constructors) - call OS_Init() [OS_Init] - initialise core OS data structures like the task lists or the device manager - these structures allow operating system functionality to be used from here down (e.g. buffered, device-oriented stream IO; atexit(); POSIX-like signals etc.) - call Console_LateInit() [Console_LateInit] - re-initialise IO machinery to work in an efficient and thread-safe manner - call Board_LateInit() [Board_LateInit] - initialise other IO/peripherals e.g EEPROM/flash where configuration data might be stored, real-time clock/timer, other ("safe") GPIO etc - call main() [main] - initialise application-level data structures/objects - start application threads - call OS function to start the scheduler (this function does not return) - if main() ever returns, teardown or disable any "unsafe" things, then power down while we wait for the Watch dog timer to reset us.
Portions copyright Richard Barry, Real Time Engineers Ltd:
FreeRTOS V6.1.0 - Copyright (C) 2010 Real Time Engineers Ltd. FreeRTOS is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License (version 3) as published by the Free Software Foundation. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
Portions copyright Hugo Vincent:
Copyright (C) 2010 Hugo Vincent <email@example.com> This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License (version 3) as published by the Free Software Foundation. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
The FreeRTOS source code is licensed by the modified GNU General Public License (GPL) text provided below.
This is a list of files for which Real Time Engineers Ltd or Hugo Vincent are not the copyright owner and are NOT COVERED BY THE GPL.
Various header files provided by silicon manufacturers and tool vendors that define processor specific memory addresses and utility macros. Permission has been granted by the various copyright holders for these files to be included in the FreeRTOS download. Users must ensure license conditions are adhered to for any use other than compilation of the FreeRTOS demo applications.
The uIP TCP/IP stack the copyright of which is held by Adam Dunkels. Users must ensure the open source license conditions stated at the top of each uIP source file is understood and adhered to.
Miscellaneous drivers, e.g. for Digi Xbee radios, SD cards etc. License text is in those files.
The GPL license text follows:
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Any FreeRTOS source code, whether modified or in it's original release form, or whether in whole or in part, can only be distributed by you under the terms of the GNU General Public License plus this exception. An independent module is a module which is not derived from or based on FreeRTOS.
EXCEPTION TEXT: Clause 1 Linking FreeRTOS statically or dynamically with other modules is making a combined work based on FreeRTOS. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holder of FreeRTOS gives you permission to link FreeRTOS with independent modules that communicate with FreeRTOS solely through the FreeRTOS API interface, regardless of the license terms of these independent modules, and to copy and distribute the resulting combined work under terms of your choice, provided that: 1) every copy of the combined work is accompanied by a written statement that details to the recipient the version of FreeRTOS used and an offer by yourself to provide the FreeRTOS source code (including any modifications you may have made) should the recipient request it. 2) The combined work is not itself an RTOS, scheduler, kernel or related product. 3) The independent modules add significant and primary functionality to FreeRTOS and do not merely extend the existing functionality already present in FreeRTOS. Clause 2 FreeRTOS may not be used for any competitive or comparative purpose, including the publication of any form of run time or compile time metric, without the express permission of Real Time Engineers Ltd. (this is the norm within the industry and is intended to ensure information accuracy).