Skip to content

Latest commit

 

History

History
59 lines (49 loc) · 2.37 KB

README.md

File metadata and controls

59 lines (49 loc) · 2.37 KB
Raw

Learning Objectives

  • Resource Management.

Concepts

In a multitasking system there is potential for error if one task starts to access a shared resource, but does not complete its access before being preempted. If the task leaves the resource in an inconsistent state, then access to the same resource by any other task or interrupt could result in data corruption, or other similar issue.

To ensure data consistency is maintained at all times, access to a shared resource, must be managed using a 'mutual exclusion' technique. The goal is to ensure that, once a task starts to access a shared resource, the same task has exclusive access to the resource until the resource has been returned to a consistent state.

FreeRTOS provides several features that can be used to implement mutual exclusion, but the best mutual exclusion method is to (whenever possible, as it is often not practical) design the application in such a way that resources are not shared, and each resource is accessed only from a single task.

Critical Sections

Critical sections are regions of code that are surrounded by calls to the macros taskENTER_CRITICAL() and taskEXIT_CRITICAL(), respectively. Critical sections work by disabling interrupts and therefore, must be kept very short, otherwise they will adversely affect interrupt response times.

Suspending (or Locking) the Scheduler

If a shared resource is only accessed from multiple tasks and not from any interrupt, the access to it can be synchronized by suspending (or locking) the scheduler. Suspending the scheduler prevents a context switch from occurring, but leaves interrupts enabled. If an interrupt requests a context switch while the scheduler is suspended, then the request is held pending, and is performed only when the scheduler is resumed (un-suspended).

The scheduler is suspended by calling vTaskSuspendAll() and is resumed (un-suspended) by calling xTaskResumeAll().

Mutex

Mutex, short for Mutual Exclusion, is a synchronization primitive used to synchronize access to a shared resource that is shared between two or more tasks.

Steps

  1. Execute the following commands from the root of the repository to build:

    rm -rf build
mkdir build
cd build
cmake -B . -S ../source/ -DTUTORIAL=21
make

  2. Execute the following command to run the binary:

    ./freertos_example

  3. Understand the output.