《FreeRTOS on Freescale MCU》.docx

FreeRTOS on Freescale MCU Introduction: FreeRTOS is a popular open source real-time operating system for embedded devices that is designed to be small and simple. Nearly all the code is written in C, with only a few assembler functions where completely unavoidable, which make the code readable, maintainable and easy to port. Multitasking is a major feature of FreeRTOS, which makes it more like a task scheduler than an “operating system”. A conventional processor (single core processor) can only execute a single task at a time, by the effort of switching between tasks rapidly, a multitasking operating system makes it appear as if each task is executing concurrently. The beauty of multitasking is allowing the operation of other tasks while a specific task is waiting for respond instead of keep wasting time on one task, which can be thought of as a “thread library”. FreeRTOS implements multiple threads (tasks) 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. At (1) task 1 is executing. At (2) the kernel suspends tasks 1 … … and at (3) resumes task 2. While task 2 is executing (4), it locks a processor peripheral for its own exclusive access. At (5) the kernel suspends task 2 as its job finished … … and at (6) resumes task 3. Task 3 tries to access the same processor peripheral, finding it locked task 3 cannot continue so suspends itself at (7). At (8) the kernel resumes task 1. Etc. The next time task 2 is executing (9) it finishes with the processor peripheral and unlocks it. The next time task 3 is executing (10) it finds it can now access the processor peripheral and this time executes until suspended by the kernel. Files needed to port FreeRTOS to Processors: FreeRTOS has been ported to 31 microcontrollers and demos are free downloading from its official website that make it easy to learn. Since it is distributed under the GPL with a