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  1. 1. RTOS Real Time Operating System By: VIVEK. P.PATKAR. PRAKRUTI. JOSHI. An example of RTOS implementation on Renesas automotive dashboard platform.
  2. 2. What is RTOS?  An RTOS is multitasking operating system for the applications needing meeting of : a) time deadlines and b)functioning in real time constraints
  3. 3. Block Diagram of RTOS
  4. 4. Architecture of RTOS
  5. 5. Architecture of RTOS  An RTOS usually comprises only a kernel. For more complex embedded systems, an RTOS can be a combination of various modules, including the kernel, networking protocol stacks, and other components as illustrated previously.  An operating system generally consists of two parts: kernel space (kernel mode) and user space (user mode).
  6. 6. Kernel The kernel of an RTOS provides an abstraction layer between the application software and hardware.  This abstraction layer comprises of six main types of common services provided by the kernel to the application software. 
  7. 7. KERNEL There are three broad categories of kernel models available Monolithic Kernel E.g.. Examples are Linux and Windows Microkernel E.g. Examples are AmigaOS and QNX Exokernel E.g.. library operating systems (libOSes)
  8. 8. Classification Of RTOS RTOS Hard real time Firm real time Zero degree of tolerance Unacceptable quality reduction E.g. Automobile engine control system and anti lock brake, video transmission, each picture frame and audio must be transferred at fixed rate. Soft real time Reduction in quality reduction is acceptable. E.g. a food processing plant control system. E.g. Mobile phone, digital cameras and orchestra playing robots.
  9. 9. Features Of RTOS Task and Multitask Running Scheduler Timer Function Memory Management Dormant Blocked Ready Pre emptive Non Pre emptive Inter Task Communication
  10. 10. Task and Multitask Context Switch E.g.20 MHz 68000 process or task switch times are roughly 20 microseconds. In contrast, a 100 MHz ARM CPU switches in less than 3 microseconds. Advantages: a)Can concentrate all the talent at a particular task. b)Debugging is simple. c)Easy to understand.
  11. 11. Scheduler Pre emptive Scheduler It allows high priority task to run first and move low priority task which is in running into ready state. E.g. Solaris 2.0/SunOS 5.0, Windows NT, the Linux kernel 2.6 and 3.x, AIX and some BSD systems (NetBSD, since version 5), Anroid. Advantages: High priority task executed immediately. Disadvantages: Starvation.
  12. 12. Scheduler Non Pre emptive Scheduler High priority task co operate with low priority task. E.g. Windows3.1x, Mac OS pre-9. Advantages: No starvation. Easy to implement. Disadvantages: High priority task has to wait for low priority task to complete, thus, not suitable for RTOS.
  13. 13. Timer Function  Task delay. E.g. In μC/OS-II, for a task to delay itself: OSTimeDly(ticks), where ticks is the number of clock ticks from the timer; OSTimeDlyHMSM(H,M,S,M), with Hours, Minutes, Seconds, Microseconds.  Task alert. E.g. µC/OS-II use system clock, Vx Works use watchdog timers
  14. 14. Memory Management Memory allocated for each task.  Memory allocated for each task control block(TCB).  Size of queue.  Size of mailbox  Size of pipe  TCB
  15. 15. Inter task Communication Event Queue Mailbox Pipe Shared Data Protected Disabling Task Switch Semaphore Disabling Interrupts Mutex Counting Semaphore Semaphore Priority Inversion Spinlock Semaphore Dead Lock
  16. 16. Event Event objects are used when task synchronization is required without resource sharing. They allow one or more tasks to keep waiting for a specified event to occur.
  17. 17. Queue Queue : E.g. AVR microcontroller, Linux 2.4; Windows NT/XP/Vista, Mac OS X uses a multilevel feedback queue, uses a Multilevel feedback queue. mailbox: E.g. TI SYS/BIOS v6.33 RTOS, Keil RTX. Pipe: E.g. QNX RTOS, SMX RTOS.
  18. 18. Semaphores E.g. ChibiOS/RT, VxWorks
  19. 19. Binary Semaphore      Whenever it is set, shared data is free. whenever it is reset shared data is busy. Whenever a task access shared data it takes semaphore and the semaphore flag is reset. Whenever a task exit from shared data it releases semaphore and the flag is reset. In this way shared data is protected.
  20. 20. Semaphores E.g. VxWorks, UNIX
  21. 21. Counting Semaphore     It is semaphore in which value can be incremented or decremented. it is unsigned integer. This semaphore can be taken multiple times. Taking semaphore means decrementing the integer. Releasing the semaphore means incrementing the integer.
  22. 22. Semaphores E.g. ChibiOS/RT, VxWorks, CHIP-RTOS-x86 RTX
  23. 23. Spin lock Semaphore       This semaphore will not make running task to blocked immediately. Spin lock is useful in following situation: Suppose low priority task is running and only some time is left for its completion. Now high priority task wants to run Now RTOS provides spin lock semaphore to the high priority task and so it can spin for some time. During that time low priority task is executed and by itself it is moved into blocked state. Now high priority task begins to run.
  24. 24. RTOS Design  Size of ISR: If ISR is long, then there are two drawbacks:   Highest priority task is blocked for more time. ISR is sensitive to error and so it is difficult to debug long ISR. Conclusion: Size of ISR should be small.
  25. 25. RTOS Design  Number of Task: Advantages:  Can concentrate all talent at a particular task.  Debugging is simple.  Very easy to understand. Disadvantages:  More memory needed because each task needs memory.  Speed is less because task switching needs more time. Conclusion: Number of tasks should be moderate.
  26. 26. RTOS Design  Avoid creating and destroying task: It is time consuming and thus speed becomes less. Conclusion: Must avoid this.  Task structure: Task designing should be such that task is blocked at the most only one place.
  27. 27. RTOS Design  Keep time slicing off: Task should not have equal priority.  Techniques    to save memory: For each task provide stack memory, TCB etc. only as much needed. Limit RTOS services. Do not use same function that does nearly same action.
  28. 28. RTOS Design  Techniques to save power:      Idle mode. Power down mode. Sleep mode. Low power mode. Standby mode.
  29. 29. RTOS Design  Encapsulate     hardware with task: This type of task is known as sever task. If hardware is having many tasks, then we must have separate task to encapsulate the hardware. This separate task is known as server task. Whenever task has to access hardware, they give message in the queue to the server task. Then server task makes accessing the hardware in proper manner.
  30. 30. Advantages  Run time facilities i.e. provision of kernel services.  Provision for interrupts.  Task can have priority.  This architecture can suspend low priority task and make high priority task run immediately when need arises.  Scalable.  Portable.
  31. 31. Disadvantages  Cost is more.  License.  Supplier stability/ longevity.  Availability of Development tools.
  32. 32. Examples  LynxOS.  OSE  QNX  RTLinux  Windows  VxWorks CE
  33. 33. Applications  Web server  Microwave oven  MRI(Magnetic Resonance Machine)  Aircraft control  Automotive applications: a) automatic breaking systems. b) fuel injection. c) path tracking.