Within the sketch there are several steps to creating and using the mutex Semaphore to protect the Serial port.įirstly, a Handle for the mutex Semaphore needs to be declared, as a Global Variable. Now either load the AnalogRead_DigitalRead.ino file into your Arduino IDE, or copy and paste it into a new file, which you should name and save appropriately. If you haven't already done this step, please do so now. In a previous ProTip we have described how to install the FreeRTOS Library for the Arduino IDE, and to test that it is working properly. When it has finished with the resource it must 'Give' the token back, allowing other Tasks an opportunity to access the same resource. When a Task wishes to access the resource it must first obtain or 'Take' the token. When used for mutual exclusion the mutex acts like a token that is used to guard a resource. In this example we will use a mutex Semaphore to protect our Serial port. Whereas binary semaphores are the better choice for implementing synchronisation (between tasks or between tasks and an interrupt), mutex semaphores are the better choice for implementing simple mutual exclusion (hence 'MUT'ual 'EX'clusion). Mutex semaphores are binary semaphores that include a priority inheritance mechanism. This situation can be avoided by using a Semaphore (also called a Flag) to protect the hardware resource, and prevent it being used until it is released by the Task using it. When two or more Tasks are sharing a single hardware resource, such as a Serial Port the situation can arise that the scheduler will swap one Task out before it has completed its Serial output, and another Task will commence and may provide some Serial output, effectively scribbling over the output of the first Task.

In FreeRTOS, a thread of execution is called a Task. The scheduler then uses the priority to know which thread of execution to run next. Traditional real time schedulers, such as the scheduler used in FreeRTOS, achieve determinism by allowing the user to assign a priority to each thread of execution.

A part of the operating system called the scheduler is responsible for deciding which program to run when, and provides the illusion of simultaneous execution by rapidly switching between each program. In reality, each processor core can only be running a single program at any given point in time. Most operating systems appear to allow multiple programs or threads to execute at the same time. This article describes the use of mutex semaphores, within an easy to use and robust FreeRTOS implementation that is included in the Arduino IDE as a Library and allows the use of the best parts of both environments. The Arduino IDE and environment has many drivers and libraries available within an arms reach, but the Arduino environment is limited to just setup() and loop() and doesn't support multi-tasking effectively.

Serial port arduino ide