
FreeRTOS

FreeRTOS is a real-time operating system kernel for embedded devices that has been ported to 35 microcontroller platforms. It is distributed under the MIT License.
History.
The FreeRTOS kernel was originally developed by Richard Barry around 2003, and was later developed and maintained by Barry's company, Real Time Engineers Ltd. In 2017 Real Time Engineers Ltd. passed stewardship of the FreeRTOS project to Amazon Web Services. Barry continues to work on FreeRTOS as part of an AWS team.
Implementation.
FreeRTOS is designed to be small and simple. The kernel itself consists of only three C files. To make the code readable, easy to port, and maintainable, it is written mostly in C, but there are a few assembly functions included where needed (mostly in architecture-specific scheduler routines).
FreeRTOS provides methods for multiple threads or tasks, mutexes, semaphores and software timers. A tick-less mode is provided for low power applications. Thread priorities are supported. FreeRTOS applications can be completely statically allocated. Alternatively RTOS objects can be dynamically allocated with five schemes of memory allocation provided:
There are none of the more advanced features typically found in operating systems like Linux or Microsoft Windows, such as device drivers, advanced memory management, user accounts, and networking. The emphasis is on compactness and speed of execution. FreeRTOS can be thought of as a 'thread library' rather than an 'operating system', although command line interface and POSIX-like I/O abstraction add-ons are available.
FreeRTOS implements multiple threads by having the host program call a thread tick method at regular short intervals. The thread tick method switches tasks depending on priority and a round-robin scheduling scheme. The usual interval is 1 to 10 milliseconds (1/1000 to 1/100 of a second), via an interrupt from a hardware timer, but this interval is often changed to suit a particular application.
The download contains prepared configurations and demonstrations for every port and compiler, allowing rapid application design. The FreeRTOS.org site also contains a lot of documentation and RTOS tutorials (additional manuals and tutorials are available for a fee), as well as details of the RTOS design.
Derivations.
Amazon FreeRTOS.
Amazon provides an extension of FreeRTOS, referred to as "a:FreeRTOS". This is FreeRTOS with libraries for IOT support, specifically for Amazon Web Services. Since version 10.0.0 in 2017, Amazon has taken stewardship of the FreeRTOS code, including any updates to the original kernel.
SAFERTOS.
SAFERTOS was developed as a complementary version of FreeRTOS, with common functionality but designed specifically for safety-critical implementation. FreeRTOS was subjected to HAZOP, and weaknesses were identified and resolved. The result was put through a full IEC 61508 SIL 3 development life cycle, the highest level for a software-only component.
SAFERTOS was developed by WITTENSTEIN High Integrity Systems, in partnership with Real Time Engineers Ltd, primary developer of the FreeRTOS project. Both SAFERTOS and FreeRTOS share the same scheduling algorithm, have similar APIs, and are otherwise very similar, but they were developed with differing objectives. SAFERTOS was developed solely in the C language to meet requirements for certification to IEC61508.
SAFERTOS can reside solely in the on-chip read only memory of a microcontroller for standards compliance. When implemented in hardware memory, SAFERTOS code can only be utilized in its original, already-certified, configuration. This means certification of systems do not need to re-test the kernel portion of their designs. SAFERTOS is included in the ROM of some Stellaris Microcontrollers from Texas Instruments. SAFERTOS source code does not need to be separately purchased. In this usage scenario, a C header file is used to map SAFERTOS API functions to their location in read-only memory.
OPENRTOS.
OPENRTOS is a commercially-licensed version of Amazon FreeRTOS, sold by WITTENSTEIN High Integrity Systems. This product provides support and allows companies to use the Amazon FreeRTOS kernel and libraries without the a:FreeRTOS MIT license. 