Automotive embedded systems part1 v1
- 1. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Automotive Embedded Systems part1 (RTOS) Basics ENG.KEROLES SHENOUDA 1
- 2. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Real-Time Systems Concepts it's time to wake up 2
- 3. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Bare metal “Bare metal" means your application is accessing the silicon chip directly without any intermediary like an OS. The application is the only software that executes on the microprocessor/microcontroller. So we can considered that the Drivers is a bare metal Code and the bios also. 3 Bare-Metal application C - Code
- 4. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system OS Application Write your application on the top of the Operating System. On desktop computers, the selection of an operating system (OS) is largely a matter of taste - Windows vs Apple vs Linux. There is relatively little choice. For an embedded system, the matter is much more complex. The large number of options available reflect the wide diversity of embedded applications. 4 OS Application OS Drivers • Linux Command • Graphical GUI “ex.QT application” • C/C++ app. • RTOS Tasks • Autosar SW Component Embedded Linux • Linux Kernel “Linux.org” • Open embedded YOCTO • Android • Windows IOT Embedded windows RTOS Autosar• Free RTOS • μC/OS-III • RTOS OS + BSW +RTE Embedded Linux Drivers RTOS Drivers MCAl Layer
- 5. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system How to execute two function at the same time on the baremetal application? #THINK IN DEPTH 5
- 6. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Answer //pseudo code Int main() { Switch (get_cpu_ID()) { case core0: //if core0 who is execute the main now FUNC1(); //funch1 will be called by core0 break; case core1: //if core1 who is execute the main now FUNC2(); //funch2 will be called by core1 break; } 6 Bare Metal Application
- 7. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Hypervisors in embedded systems 7 Virtualization you can run operating systems called guests, inside a sandbox contained under the control of another piece of software (the hypervisor) that manages all interaction of the guest toward hardware. The hypervisor runs at a more privileged level of hardware access than the operating system (OS) kernel (supervisor) and user privilege of the guest OS
- 9. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system What is an Operating System 9 A software layer between the application software and the hardware Typical embedded system (ES) solves a problem by decomposing it into smaller pieces called tasks that work together in an organized way System is called multitasking system and design aspects include: Exchanging/sharing data between tasks Synchronizing tasks Scheduling tasks The piece of software that provides the required coordination is called an operating system (OS) When the control must ensure that task execution satisfies a set of specified time constraints, the OS is called a real-time operating system (RTOS)
- 10. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system The Operating System (OS) 10 Also it provides a services: –Process or task management •Creation and deletion of user and system processes –Memory management •Includes tracking and control of which tasks are loaded into memory, monitoring memory, administer dynamic mem –I/O System management •Interaction with devices done through a special piece of software called a device driver •The internal side of that software is called a common calling interface (an application programmer's interface, API) –File system management –System protection –Networking
- 11. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Memory Management In multiprogramming, OS decides which process will get memory when and how much. •Allocates the memory when the process requests it to do so. •De-allocates the memory when the process no longer needs it or has been terminated. Sometimes if the CPU hardware “MMU” provides mapping between virtual and physical addresses, the OS “memory management” will manage the Virtual memory for each task 11
- 12. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Processor Management OS decides which process gets the processor when and how much time. This function is called process scheduling. Operating System does the following activities for processor management. •Keeps tracks of processor and status of process. Program responsible for this task is known as traffic controller. •Allocates the processor (CPU) to a process. •De-allocates processor when processor is no longer required. 12
- 13. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Device Management (I/O System management ) OS manages device communication via their respective drivers. Operating System does the following activities for device management. •Keeps tracks of all devices. Program responsible for this task is known as the I/O controller. •Decides which process gets the device when and for how much time. •Allocates the device in the efficient way. •De-allocates devices. 13
- 14. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system The Kernel The Kernel is the smallest portion of the OS that provides these functions • Scheduler –Determines which task will run and when it will do so •Dispatcher –Performs the necessary operations to start the task •Intertask or interprocess communication –Mechanism for exchanging data and information between tasks and processes on the same machines or different ones 14
- 15. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system What is Real Time? “not only depends upon the logical correctness of the computation but also upon the time at which the result is produced. If the timing constraints of the system are not met, system failure is said A real time system is one in which the correctness of the computations to have occurred.” at specific timeline 15 Real Time: Correct logic With a specific Timeline
- 16. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Real Time System A system is said to be Real Time if it is required to complete it’s work & deliver it’s services on time. Example – Flight Control System All tasks in that system must execute on time. Non Example – PC system There are types of Real Time System Soft real time Hard real time 16
- 17. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system There are types of Real Time System Soft real time • Tasks are performed by the system as fast as possible, but tasks don’t have to finish by specific times • Multitask based on Priority scheduling • Ex. Multimedia streaming 25 frames per second on an average Hard real time • Tasks have to be performed correctly and on time • Multitask based on Deadline scheduling • Ex. Aircraft controller, Nuclear reactor controller 17
- 18. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system The Real-Time Operating System (RTOS) An RTOS provides an Abstraction Layer between Application Software and Embedded Hardware Key characteristic of an RTOS is that it has deterministic behavior Basic Services provided by a Real-Time Operating System Kernel “Real-time” means keeping deadlines, not speed Advantages of RTOS in SoC design • Shorter development time • Less porting efforts • Better reusability 18 Task0Task1
- 19. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system When is RTOS necessary? An RTOS may not be necessary in a small-scaled embedded system. An RTOS is necessary when scheduling of multiple processes and devices is important. 19
- 20. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Characteristics of an RTOS Reliability: Available and doesn’t fail. Predictability: Deterministic with small time variance. Performance: Fast enough. Compactness Small and efficient. Scalability Result of modularity and RTOS configurations to be used in many systems. 20
- 21. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Foreground/Background Systems An application consists of an infinite loop that calls modules (that is, functions) to perform the desired operations (background). Interrupt Service Routines (ISRs) handle asynchronous events (foreground). So Foreground is also called interrupt level while background is called task level 21 Background Foreground ISR ISR ISR (more important) Interrupts may be prioritised, so that an ISR may itself be interrupted so that a more important event may be serviced.
- 22. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Handling an Interrupt 22 1. Normal program execution 2. Interrupt occurs 3. Processor state saved 4. Interrupt routine runs 5. Interrupt routine terminates 6. Processor state restored 7. Normal program execution resumes
- 23. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Handling an Interrupt 23
- 24. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Reentrancy A reentrant function is a function that can be used by more than one task without cause of data corruption. A reentrant function can be interrupted at any time and resumed at a later time without loss of data. An error caused by a non- reentrant function may not show up in your application during the testing phase 24 non-reentrant int temp; // global function swap(int *x, int *y) { temp = *x; *x = *y; *y = temp; } reentrant version function swap (int *x, int *y) { int temp; // local temp = *x; *x = *y; *y = temp; }
- 26. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Process or Task A task is usually implement in C by writing a function When a C function is executing, it is called a process or task So Task is not code Two tasks can be executing the same code simultaneously When a process is created, it is allocated a number f resources by the OS, which may include: –Process stack –Memory address space –Registers (through the CPU) –A program counter (PC) –I/O ports, network connections, file descriptors, etc. These resources are generally not shared with other processes 26 int x; int Func() { int y; y = y + 1; x = x + 1; ... } Task 1 Task 2
- 27. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Multitasking Multitasking is the process of scheduling and switching the CPU (Central Processing Unit) between several tasks. Each task is assigned a priority, its own set of CPU registers, and its own stack area 27 Multiple tasks
- 28. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Context Switch (or Task Switch) When a multitasking kernel decides to run a different task, it simply saves the current task's context (CPU registers) in the current task's context storage area – it’s stack. Once this operation is performed, the new task's context is restored from its storage area and then resumes execution of the new task's code. This process is called a context switch or a task switch. Context switching adds overhead to the application. 28
- 29. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Tasks conclusion Each task thinks it has the CPU to itself Each task has its own stack space and its own set of CPU registers Each task has a priority A task may well take the form of a sequence of initialisation code; followed by a loop which iterates forever (or until a flag is set to exit it) Body of loop will contain application code A task calls functions which are preferably reentrant: Can be safely called by more than one function at a time Interrupting them does not corrupt their data Typically implemented using local variables Reusable and reentrant Code 29 Void task1() { // a sequence of //initialisation code While(1) { //application code } }
- 30. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States 30 1.DORMANT 2.READY 3.RUNNING 4.DELAYED 5.PENDING 6.BLOCKED 7.INTERRUPTED A task can be in any of one of states Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 31. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States DORMANT-The DORMANT state corresponds to a task that resides in memory but has not been made available to the multitasking kernel. 31 Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 32. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States READY-A task is READY when it can be executed but its priority is less than that of the task currently being run. In this state, the task actively competes with all other ready tasks for the processor’s execution time. The kernel’s scheduler uses the priority of each task to determine which task to move to the running state. 32 Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 33. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States RUNNING-A task is RUNNING when it has control of the CPU and it's currently being executed. On a single-processor system, only one task can run at a time. When a task is preempted by a higher priority task, it moves to the ready state. It also can move to the blocked state if: Making a call that requests an unavailable resource Making a call that requests to wait for an event to occur Making a call to delay the task for some duration 33 Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 34. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States WAITING-A task is WAITING when it requires the occurrence of an event (waiting for an I/O operation to complete, a shared resource to be available, a timing pulse to occur, time to expire, etc.). 34 Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 35. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task States BLOCKED CPU starvation occurs when higher priority tasks use all of the CPU execution time and lower priority tasks do not get to run ISR(Interrupt Service Routine)-A task is in ISR state when an interrupt has occurred and the CPU is in the process of servicing the interrupt. 35 Waiting Dormant Ready Running ISR Task Create Task Delete Highest Priority Task Task is Preempted Task Pending EventsTask Gets Event Task Delete Interrupt Int. Exit Task Delete μC/OS-II
- 36. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Exchanging CPU Control 36 void ExampleTask(void *pdata) { for(;;) { /* User Code */ /*System Call */ /* User Code */ } } OSMboxPend(); OSQPend(); OSSemPend(); OSTaskSuspend(); OSTimeDly(); OSTimeDlyHMSM(); More… uC/OS-II Kernel API Control returns from task to OS when Kernel API is called
- 37. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Exchanging CPU Control 37 A B B C A Interrupt Handler OS Task Time Only one of OS, Task, Interrupt Handler gets CPU control at a time Scheduling System Call Interrupt Interrupt Exit
- 38. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Task Scheduling Schedulers are parts of the kernel responsible for determining which task runs next In a Multitasking System, the main objective is to have the CPU utilized at all times. At each opportunity, OS asks: IF N Tasks are ready ? Which one Should I run ? The scheduling can be handled automatically.. Many kernels also provide a set of API calls that allows developers to control the state changes. 38
- 39. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Scheduling - types Non Real -time systems usually use Non-preemptive Scheduling Once a task starts executing, it completes its full execution Most RTOS perform priority-based preemptive task scheduling. Basic rules for priority based preemptive task scheduling The Highest Priority Task that is Ready to Run, will be the Task that Must be Running. 39 Non-preemptivepreemptive
- 40. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Non-preemptive It is also called cooperative multitasking; tasks cooperate with each other to share the CPU. tasks decide for themselves when to give up the CPU; when completed. The new higher priority task will gain control of the CPU only when the current task gives up the CPU. asynchronous events are handled using interrupts tasks still have an assigned priority advantages: - interrupt latency time is low - less need to “guard” shared resources -At the task level, non-preemptive kernels can also use non-reentrant functions. disadvantages: - responsiveness is non-optimal & non-deterministic - one task may lock-up system 40 Time ISR Low-priority Task High-priority Task ISR makes the high-priority task ready low-priority task Relinquishes the CPU (low priority task is finished)
- 41. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Preemptive the highest priority task (ready) is given control of the CPU current running task is suspended after an Interrupt Service Routine (ISR), the new highest priority task is run Most commercial real-time kernels are preemptive advantages: - responsiveness is optimal & deterministic (Execution of the highest-priority task is deterministic) -Task-level response time is minimized disadvantages: - shared resources must be properly guarded -Should not use non-reentrant functions unless exclusive access to these functions is ensured 41 Time ISR Low-priority Task High-priority Task ISR makes the high-priority task ready high-priority task Relinquishes the CPU
- 42. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Scheduler Concepts CPU utilization : keep the CPU as busy as possible. How much of CPU (usually in %) is in use ? Throughput : number of processes (# pf processes) that complete their execution per time unit. Turnaround : total time between submission of a process and its completion. (waiting + executing + IO Time) Waiting time : a mount of time a process has been waiting in the ready queue.(task enters by System initially), then it is placed in “ready state” when the conditions are met. Response time : amount of time it takes from when a request was submitted until the first response is produced. 42
- 44. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Scheduling Algorithms in time-shared System 1. First-Come First-Served 2. Shortest Job First 3. Round Robin 4. Round-robin with priority scheduling 44
- 45. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system First In First Out Algorithm Also known as First Come, First Served (FCFS). It is the simplest scheduling algorithm. FIFO simply queues processes in the order that they arrive in the ready queue It is non-preemptive scheduling overhead is minimal. Throughput can be low. This is considered as a very good algorithm when few small tasks all with small execution times 45
- 46. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Shortest Job First Algorithm Also known as Shortest Job First (SJF). With this strategy the scheduler arranges processes with the least estimated processing time remaining to be next in the queue. If a shorter process arrives during another process 'execution, the currently running process may be interrupted. This algorithm is designed for maximum throughput in most scenarios. Starvation is possible, especially in a busy system with many small processes being run This algorithm can be Preemptive (Running task can be interrupted by another task). Non Preemptive 46 Kindly Note Starvation, in general, means a situation where some process has been denied service for a long time or has been repeatedly denied service.
- 47. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_systemRound-robin scheduling Algorithm (time-slicing) Designed especially for time shared systems. Similar to first –come first-served, with preemption added to switch between process. tasks with the same priority are only allowed to run for a predetermined time period, “a quantum” This is also called time slicing. not necessarily supported by all preemptive kernels microC/OS-II does not support round-robin 47 Assumption: Time Slice = 20 time unites
- 48. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Round-robin with priority scheduling The Round Robin Algorithm is slightly modified by assigning priority to all tasks. Highest priority task can interrupt the CPU so it can be executed. 48
- 49. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Scheduling - priorities static the priority of a task is set at compile time and does not change during application execution advantage: all timing constraints can be known at compile time disadvantage: may get priority inversion dynamic each task can change its priority during execution advantage: useful in avoiding priority inversion disadvantage: not easy to guarantee schedulability 49
- 50. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inversions Low-priority process acquires lock, blocks higher-priority process Priority inheritance temporarily raises process priority Difficult to analyze 50
- 51. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inversions Example 51 Task 1 (high) Task 2 (medium) Task 3 (low) 1 2 3 4 5 6 This can happen even in Preemptive kernels. Ex: 3 tasks with low, medium, high priority: Task 3 (low) runs while 1 and 2 wait for some event. At point 1 Task3 acquires an exclusive lock on a resource. Get the resource
- 52. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inversions Example 52 Task 1 (high) Task 2 (medium) Task 3 (low) 1 2 3 4 5 6 At point 2, task 1 becomes ready and preempts task 3 At point 3, task 1 wants the resource but it is still locked by task 3; so task 1 waits and task 3 resumes Get the resource
- 53. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inversions Example 53 Task 1 (high) Task 2 (medium) Task 3 (low) 1 2 3 4 5 6 At point 4, task 2 becomes ready and starts running, preempting task 3. Task 2 runs until done, point 5 and task 3 resumes, not task 1 because the latter is still waiting for the resource locked by task 3 Get the resource
- 54. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inversions Example 54 Task 1 (high) Task 2 (medium) Task 3 (low) 1 2 3 4 5 6 Finally at point 6, task 3 releases the resource and task 1 acquires it and can run. Effectively task 1 has been trapped at the level of task 3 until 3 releases the resource. Get the resource
- 55. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Priority Inheritance Solution 55 Task 1 (high) Task 2 (medium) Task 3 (low) 1 2 3 4 5 6 In a kernel that supports priority inheritance, As soon as task 1 wants the resource (point 3) locked by task 3, task 3 is temporarily promoted to task 1’s priority so that it can continue running until it releases the resource (point 4) Then, task 1 becomes ready because it can acquire the resource released by task 3. Task 1 releases the resource at point 5, but keeps running also task 2 is ready, because it has the highest priority Finally at point 6, task 1 finishes and task 2 can run. Task 3, with the lowest priority, waits. NB Task 2 could have become ready at point 2 or any after thereafter, with no difference in outcome..
- 56. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Scheduling - the “tick” or quantum A hardware Timer generates interrupts at a regular interval 10 -- 1000 Hz; usually derived from power line frequency Used to provide delays and timeouts The ISR for a clock tick calls a kernel service to signal a tick. This may well cause a context switch A higher priority task may have become ready to run in the interim In “round robin” scheduling, tasks of equal priority get a fixed number of ticks’ CPU time than go to the back of the queue The clock interrupt has higher priority than all tasks (because it is an interrupt) but lower priority than other interrupts. 56= special interrupt Periodical = RTOS heartbeat
- 57. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system The RTOS Tick 57 Delaying a task: “n” clock ticks
- 58. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Shared Data between Tasks 58 how does the consumer know that (new) data is available? Data is shared between several tasks and, in order to keep the data in a consistent state, we need to protect it from concurrent access. In other words: access to the data must be serialized. This use case is mainly about task synchronization: the interplay between producer and consumer must work smoothly. How does a consumer know that new data is available? how to serialize access to shared data?
- 59. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Two shared Resource Control 59 the mutex = MUTual EXclusion. the semaphore Resource A resource is any entity used by a task. A resource can thus be an I/O device such as a printer, a keyboard, a display, etc. or a variable, a structure, an array, etc. Shared Resource A shared resource is a resource that can be used by more than one task. Each task should gain exclusive access to the shared resource to prevent data corruption. This is called Mutual Exclusion.
- 60. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system the mutex A critical section is a piece of code (part of a function) from which the access to the shared resource must be atomic. At a certain moment in time, one and only one critical section is allowed to access the shared data The mutex implements two operations: acquire: enter the critical section, or sleep if another task has already acquired the mutex, and release: leave the critical section. Other tasks now have the possibility to acquire the mutex. 60
- 61. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system the mutex The concept of ownership is very important here: the task that acquired the mutex must release it as well. techniques to address mutual exclusion include: disabling/enabling interrupts disabling scheduling performing test-and-set operations (indivisibly) using semaphores 61
- 62. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Mutual Exclusion Disable interrupts Mechanism Demand: exclusive use of a resource Disable interrupts Used on critical sections of code disabling/enabling interrupts used to access a few variables or small data structures used when the process time is minimal affects interrupt latency you should not disable interrupts for any longer than the kernel does in C/OS-II: OS_ENTER_CRITICAL( ); … // you can access shared data here OS_EXIT_CRITICAL( ): 62 Using Test-And-Set to access a resource
- 63. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Mutual Exclusion Disable interrupts Mechanism 63 So if a shared resource is shared between a task and an interrupt service routine, then the task can disable interrupts before entering the shared resource. This will stop the ISR from accessing the shared resource Disadvantages: 1. Disabling interrupt for long time harm the system response And the RTOS Kernel Itself
- 64. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_systemMutual Exclusion Disable preemption (Task Switching) Mechanism 64 disabling scheduling instead of disabling all maskable interrupts, you may just want to disable the scheduler an interrupted task is returned to, not necessarily the highest priority task implies that data is not shared with the ISRs in C/OS-II: OSSchedLock( ); … // you can access shared data here OSSchedUnlock( );
- 65. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system performing test-and-set operations (indivisibly) perform test-and-set operations (indivisibly) two tasks may agree that to use a common resource, they must first check a global variable while it is being checked, and before it is set, by one task, it must be made inaccessible by the other this can be achieved by: disabling interrupts (above) or some processors now support a single TAS instruction Make the following test test-and-set variable Set test-and-set variable to 1 Access resource Set test-and-set variable to 0 65 Kindly Note: DPRAM test-and-set instructions can work in many ways When CPU 1 issues a test-and-set instruction, the DPRAM first makes an "internal note" of this by storing the address of the memory location in a special place. If at this point, CPU 2 happens to issue a test- and-set instruction for the same memory location, the DPRAM first checks its "internal note", recognizes the situation, and issues a BUSY interrupt, which tells CPU 2 that it must wait and retry.
- 66. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system SEMAPHORES | Operating System (Simplified) 66 https://www.youtube.com/watch?v=PQ5aK5wLCQE
- 67. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Mutual Exclusion - semaphores a protocol mechanism to control access to shared resources, signal the occurrence of an event or allow two tasks to synchronize one analogy is that in order for a task to use a resource it must first draw a key for it if the key(s) is in use, the task suspends wait on (pend) & signal (post) semaphore two types of semaphores: binary and counting (general) 67 task 2 task 1 1. pend 4. post Printer 2. pend 6. post 5. “my name is task 2” 3. “my name is task 1”
- 68. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Mutual Exclusion - semaphores 68 accessing shared data in C/OS-II OS_EVENT *SharedDataSem; SharedDataSem = OSSemCreate(1); ... void Function (void) { INT8U err; //read as “char *err” (string) ... OSSemPend(SharedDataSem, 0, &err); … //access shared data OSSemPost(SharedDataSem); } binary semaphore timeout value {wait forever in this case} error message code
- 69. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Encapsulated semaphores Principle: task does not have to control semaphore Resource controls the semaphore Typical: driver of the resource solves semaphores Release of a semaphore is done through a timeout eg. Network card, serial port, ... Advantages: tasks just use the driver programmer does not have to deal with the semaphores 69
- 70. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Deadlock Two tasks waiting for each others semaphore most kernels allow you to specify a timeout when acquiring a semaphore (with error code) 70
- 71. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Synchronisation Synchronisation between tasks Synchronisation between tasks and ISR How ? Use of semaphores "unilateral rendezvous" semaphore signals an event (flag, NOT a key) 71
- 72. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Synchronization : Use of semaphores one task can be synchronized with another (no data transferred) by using a semaphore the semaphore is a flag signaling the occurrence of an event (vice a key for mutex) in this case, the semaphore is initialized to 0 72 Task 2 Task 1 2. Post 1. Pend
- 73. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system "bilateral rendezvous" bilateral rendezvous - two tasks can be synchronized with each other by using two separate semaphores 73 Task 2 Task 1 1. Signal Task 2 2. wait for signal from task 2 4. wait for signal from task 1 3. Signal Task 1
- 74. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Event Flags When ? A task has to synchronise with multiple events Disjunctive synchronisation logical OR synchronisation if one of the events occurred Conjunctive synchronisation logical AND synchronisation if all events occurred 74
- 76. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system Intertask communication What ? ISR or task sends information to other task How ? Global variables sending messages Global variables Make sure you have exclusive access by disabling and enabling interrupts by use of semaphores more commonly, messages are sent between tasks usually achieved via a message mailbox or message queue 76
- 77. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system message mailbox message mailbox messages are typically pointer-sized variables a task or an ISR can deposit a message (pointer) into a mailbox. similarly, one or more tasks can retrieve the message from the mailbox the sending and receiving tasks must agree on what type of data the message points to 77 Task 1 Task 2 1. Post 2. Pend 100msec
- 78. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system message mailbox message mailbox (cont’d) C/OS-II provides the following type of typical mailbox services: initialization OSMboxCreate(...); deposit (post) OSMboxPost(…); wait (pend) OSMboxPend(…); //includes timeout get (accept) OSMboxAccept(…); query OSMboxQuery(…); *pend will wait on message (for some timeout period, while accept will get the message if it is there but not wait for it) 78
- 79. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system message queues message queues used to send one or more messages to tasks it is basically an array of mailboxes messages can be retrieved either FIFO or LIFO a kernel will normally provide the same mechanisms for a message queue as for a message mailbox 79
- 80. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system References Embedded Microcomputer Systems Real Time Interfacing Third Edition Jonathan W. Valvano University of Texas at Austin. MicroC/OS-II the real-time kernel second edition jean j.labrosse. RTOS Concepts http://www.embeddedcraft.org. Task Synchronization Prof. Chung-Ta King. Study of an operating system: FreeRTOS Nicolas Melot Real-Time Operating Systems Prof. Stephen A. Edwards Real Time Operating Systems for Networked Embedded Systems Subhashis Banerjee Indian Institute of Technology Delhi USING THE FREERTOS REAL TIME KERNEL A Practical Guide Richard Barry http://www.rtos.be/2013/05/mutexes-and-semaphores-two-concepts- for-two-different-use-cases/ 80
- 81. https://www.facebook.com/groups/embedded.system.KS/ Follow us Press here #LEARN_IN DEPTH #Be_professional_in embedded_system References https://www.embedded.com/design/operating- systems/4436454/Selecting-an-operating-system-for-an-embedded- application- http://free-electrons.com/training/ CENG-336 Introduction to Embedded Systems Development https://nawid2012.wordpress.com/2012/11/13/real-time-systems/ Embedded Real-Time Systems Memory management Unit 4 Real Time Operating System Real Time Operating Systems Real Time Operating Systems Johan Dams 81