RTOS Overview

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From the course by EIT Digital

Development of Real-Time Systems

207 ratings

EIT Digital

Development of Real-Time Systems

207 ratings

Course 3 of 4 in the Specialization Development of Secure Embedded Systems

This course is intended for the Master's student and computer engineer who likes practical programming and problem solving! After completing this course, you will have the knowledge to plan and set-up a real-time system both on paper and in practice. The course centers around the problem of achieving timing correctness in embedded systems, which means to guarantee that the system reacts within the real-time requirements. Examples of such systems include airbags, emergency breaks, avionics, and also multi-media systems like video playback and QoS in web servers. The course teaches how to plan real-time systems in theory using established mathematical proofs and how to implement them in practice by using the most common scheduling methods. We also learn and how to program the system in the C language using the FreeRTOS real-time kernel. Finally we have a look at the future of real-time systems namely multi-core real-time systems! This course focus on the learn-by-doing approach with many examples and real-world programming assignments. We have 5 modules, each with a gentle graded quiz in the end and one peer reviewed programming assignment. In case you have no experience with C programming, please check you a practical course like: https://www.coursera.org/learn/arduino-platform The course is actually quite fun! -Simon Holmbacka / Åbo Akademi University Check out our whole curriculum: https://research.it.abo.fi/

From the lesson

Real-Time Operating Systems

This week is what we all have been waiting for! We will deepen our learning of FreeRTOS, its kernel and the functionalities. We demonstrate the importance of predictable computer architectures for example when determining the context switch and factors influencing this overhead. As we head towards the future, we finish this course by introducing you to multi-core real-time systems and scheduling methods for multi-core real-time systems. Concretely, you will learn: (1) The internal mechanisms of FreeRTOS, for example mutexes/semaphores and message queues. (2) Multi-core computer architectures for real-time systems. (3) Multi-core scheduling methods.

RTOS Overview6:53

The FreeRTOS Kernel6:15

Meet the Instructors

Simon Holmbacka
Dr
Åbo Akademi University, Faculty of Science and Engineering

[MUSIC]

In this module we want to have deeper look in to what actually happens inside OS when

scheduling real time tasks.

We therefore going to dedicate part of this module the real-time operating

systems.

Real-time operating systems can be categorized into different types

depending on functionality and complexity.

The most basic OS type is the thread package

which contains only a simple task based execution model.

Most other OS level functionalities like file handling, networking, and sometimes

dynamic memory allocation is only available as an option outside of the OS.

Thread packages use, in general, no virtual memory so

tasks can operate in the global memory space and

the developers must ensure that memory violations do not occur.

Microkernels are a step up from the thread packages, but

it's still a very small system with only the basic functionality inside the kernel.

Microkernels use message passing instead of kernel calls.

Permit communication between tasks and the kernel.

One upgrade from thread packages is also the use of virtual memory and this

simplifies the life for the programmer since the task can declare a virtual

memory space and a run time system will ensure that no memory violations happen.

Monolithic kernels have also been used in real time operating systems.

The idea behind the monolithic kernel is completely the opposite

to the one of micro kernels.

They have here one big kernel containing most of the functionality and

all kernel resources are called synchronously by kernel calls.

There's a strong separation between kernel space, in which for example

device drivers run and use the space where the real-time applications run.

The OS we focus on in this course is FreeRTOS.

FreeRTOS is a small real time kernel ported over 35 different architectures.

From tiny 8 bit micro controllers to our grade mobile phone multi core CPUs.

It is a very popular choice both for industry and

academia because of its light weightness and low learning threshold.

FreeRTOS is also open source.

So no functionality in the kernel is hidden from the programmer.

It supports both preemptive and cooperative scheduling, and

FreeRTOS in itself is not very big.

It uses about 500 bytes of data memory for the kernel, and

then additional memory for each task, depending on the stack sizes.

And the whole OS uses about 10k of program memory.

Depending on the compiler and its options.

So it fits in almost all small micro-controllers without a problem.

FreeRTOS is up and running in about one millisecond.

So compared to a normal desktop OS this is quite fast.

RTOS uses task as a definition of work instead of the traditional process.

Tasks are created in FreeRTOS either during system initialization or

during run time in another task.

So when a task is created in FreeRTOS,

the task gets a C structured called a Task handle.

The task handles away the control interface between the task and the OS.

So for example, if the OS wants to change the priority of task.

The task handle is first as a reference to the system call.

The task also needs a stack to store local variables in.

And this stack is allocated in the memory heap and

its size is given as an input when creating the task.

The information about the task is stored in a structure called a Task Control Block

or TCB.

Here we store information about the references to the task handle,

the name of the task, and its priority.

They also store a point that their task function

which tells the OS where to jump when this task is selected for scheduling.

The stack size of the task is defined here and

it is possible to pass some parameters to the task in form of a void pointer.

We will show this in practice by defining the functionality of two tasks.

So here we have the first task called the SensorTask.

And this task is use to simply read a value from a physical sensor and

send it a message que.

The task is written as a C function.

With an infinite loop which never ends unless the task is deleted by the system.

Inside the loop we have one read function

which gets the measurement value from the ad converter connected to the sensor.

And the other task is called the LCDTask.

It is also a C function which contains code to display the measurement value

on an LCD display.

So in the same fashion this task is also an infinite loop

with continuously reads the message queue for incoming data.

Once data arrives it's res variable gets a value and

the function for writing the LCD display is called with res as the argument.

You will see to end both tasks a function call to vTaskDelay.

This is a functionality in FreeRTOS to place a task in the blocking state and

it will stay there as long as the parameter indicates in milliseconds.

When the functionality of our tasks are defined,

we must include the tasks in our system, and start scheduling them.

This is done in FreeRTOS,

by firstly including the FreeRTOS.h header in the project.

In the main function, we create one TaskHandle per task.

And we have one called sensorHandle and one called LCDHandle.

We then create the tasks by using the vTaskCreate function called in FreeRTOS.

And pass the handle, the priority, the stacksize and

a function pointer as a reference to the system call.

We then start the scheduler by calling the vTaskStartScheduler.

And after this we go into an infinite loop because after this point

only the tasks should execute, not the main function.

In conclusion,

we have chosen FreeRTOS as the reference OS because it is very easy to learn.

It is ported to almost all popular embedded processors, and

it is open-source.

In FreeRTOS we defined a task in a single style function.

And the FreeRTOS kernel switches between these different functions,

as it schedules the tasks.

In other lessons in this course we will also have a look at some more advanced

concepts of FreeRTOS.

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