CSE4352

μC/OS-II (read as MicroC/OS-II) is the second generation of μC/OS which is a priority-based, preemptive and real-time multitasking operating system written mainly in the C programming language. It is originally published in a book by Jean J. Labrosse, μC/OS The Real-Time Kernel, which purpose was to describe the internals of a portable operating system with a small footprint. It is now a product which is maintained by Micrium Inc. and licenses are issued per product or royalty free for non-commercial educational uses. Even though the source code of μC/OS is available, it is not by any means considered free or open source software.

μC/OS-II is an extremely detailed and highly readable design study which is particularly useful to the embedded systems student. While documenting the design and implementation of the kernel, the book also walks through the many related development issues such as how to adapt the kernel for a new microprocessor, how to install the kernel, and how to structure the applications that run on the kernel.

Important features of μC/OS-II are ¹⁾:

• Highly portable, scalable and preemptive real-time multitasking kernel that you only build what you need.
• It can manage a predefined maximum number of tasks.
• It can be expanded and connected to addons such as μC/GUI and μC/FS which are GUI and File Systems for μC/OS-II
• It supports all type of processors from 8-bit to 64-bit

μC/OS-II like most modern operating systems has the following components:

• Task Management (i.e. Create, Delete, Change Priority and Suspend/Resume tasks)
• Time and Timer Management
• Fixed Sized Memory Block management.
• Inter-Task Communication (i.e. Message Mailboxes and Message Queues)
• Semaphores, Mutual Exclusion Semaphores
• Many external modules are available as the real-time addons to the core (μC/GUI, μC/FS, μC/CAN, μC/USB, μC/TCP-IP and many more).

μC/OS-II allows one to create new tasks and check the existing status of the tasks stack. Tasks can be deleted or their priority can be changed. Also μC/OS-II provides general information about a specific task and allows one to suspend or resume operation as well on a task.

The current version of μC/OS-II can manage up to 64 tasks. The four highest priority tasks and the four lowest priority tasks are reserved for the OS itself. The lower the value of the priority, the higher the priority of the task. The task priority number also serves as the task identifier μC/OS-II uses rate preemptive monotonic scheduling such that the highest rate of execution is given to the highest priority task which is ready. Tasks are periodic and do not synchronize with one another and

OSInit function is used to initialize the internals of μC/OS-II and MUST be called prior to creating any μC/OS-II object and, prior to calling OSStart().

OSStart function is used to start the multitasking process which lets μC/OS-II manages the task that you have created. Before you can call OSStart(), you MUST have called OSInit() and you MUST have created at least one task.

OSIntEnter function is used to notify μC/OS-II that you are about to service an interrupt service routine (ISR). This allows μC/OS-II to keep track of interrupt nesting and thus only perform rescheduling at the last nested ISR. You are allowed to nest interrupts up to 255 levels deep.

OSIntExit function is used to notify μC/OS-II that you have completed servicing an ISR. When the last nested ISR has completed, μC/OS-II will call the scheduler to determine whether a new, high-priority task, is ready to run.

You MUST invoke OSIntEnter() and OSIntExit() in pairs. In other words, for every call to OSIntEnter() at the beginning of the ISR you MUST have a call to OSIntExit() at the end of the ISR.

Please note that rescheduling is prevented when the scheduler is locked (see OS_SchedLock())

OSTaskCreate function is used to have μC/OS-II manage the execution of a task. Tasks can either be created prior to the start of multitasking or by a running task. A task cannot be created by an ISR.

Signature:

INT8U OSTaskCreate (void (*task)(void *p_arg), void *p_arg, OS_STK *ptos, INT8U prio)

Arguments:

• Task argument is a pointer to the task's code
• P_arg argument is a pointer to an optional data area which can be used to pass parameters to the task when the task first executes. Where the task is concerned it thinks it was invoked and passed the argument 'p_arg' as follows:

void Task (void *p_arg)
{
   for (;;)
   {
      Task code;
   }
}

• Ptos argument is a pointer to the task's top of stack. If the configuration constant OS_STK_GROWTH is set to 1, the stack is assumed to grow downward (i.e. from high memory to low memory). 'pstk' will thus point to the highest (valid) memory location of the stack. If OS_STK_GROWTH is set to 0, 'pstk' will point to the lowest memory location of the stack and the stack will grow with increasing memory locations.
• Prio argument is the task's priority. A unique priority MUST be assigned to each task and the lower the number, the higher the priority.

Returns:

• OS_ERR_NONE: if the function was successful.
• OS_PRIO_EXIT: if the task priority already exist (each task MUST have a unique priority).
• OS_ERR_PRIO_INVALID: if the priority you specify is higher that the maximum allowed (i.e. >= OS_LOWEST_PRIO)
• OS_ERR_TASK_CREATE_ISR: if you tried to create a task from an ISR.

OSTaskDel function allows you to delete a task. The calling task can delete itself by its own priority number. The deleted task is returned to the dormant state and can be re-activated by creating the deleted task again.

Signature:

INT8U OSTaskDel (INT8U prio)

Arguments:

• prio argument is the priority of the task to delete. Note that you can explicitely delete the current task without knowing its priority level by setting 'prio' to OS_PRIO_SELF.

Returns:

• OS_ERR_NONE: if the call is successful
• OS_ERR_TASK_DEL_IDLE: if you attempted to delete μC/OS-II's idle task
• OS_ERR_PRIO_INVALID: if the priority you specify is higher that the maximum allowed (i.e.>= OS_LOWEST_PRIO) or, you have not specified OS_PRIO_SELF.
• OS_ERR_TASK_DEL: if the task is assigned to a Mutex PIP.