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Real-Time and EmbeddedSystems

Nam Ho

Dr. Anh-Vu Dinh-Duc


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8/20/2010 Embedded Systems Course: Real-Time Operating Systems - 2010 2

Module Outline

Introduction to Real-Time Systems

Real-Time Task Scheduling

Real-Time Operating Systems (RTOS)

Measuring Execution Time and Real-TimePerformance

Development of Embedded Systems Our Research Results in Embedded

Systems


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What is a Real-Time

Operating Systems (RTOS)?

OS provides Task management (scheduling, communication, synchronization) Hardware abstract (device drivers) Resource allocation (memory management)

Time control Error control

RTOS Provides some basic functionalities like regular OS But, involves real-time task scheduling The need for "deterministic" timing behavior A ideal platform for developing applications that require precise

timing (time bonded) Allows the designer to manage both soft and/or hard real time

requirements


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Components in RTOS


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RTOS supports Multitasking

CodeData

Stack

Heap

Shared Data

Shared Code

TASK1s

Segments

FLASH

SRAM

kernelCodeData

Stack

Heap

Shared Data

Shared Code

TASK2s

Segments

kernel

system call

Memory Map


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RTOS supports Multitasking

Task uses Task ControlBlock (TCB) to remember itscontext Task ID Task Name Status Priority (if part of a preemptive

scheduling plan)

Task Stack

Task Program Counter

RTOS updates TCB whentask is switched


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RTOS supports MultitaskingStack

status

pc

sp

priority

TCBTask_Rountine()

{

while(1)

{

..

}

}

sp

pc

CPU register

Context

Stack

status

pc

sp

priority

TCBTask_Rountine()

{

while(1)

{

..

}

}

Memory

CPU

TASK 1 TASK n


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Task Context Switch

( 2. )max min . / 1,1{1 } { }

where S { . | 1, ..., ; 1, ..., / }i

The entire task set is feasible by RMS if and only if

ejiL t pjji n t Si t

k p j i k p pj i j

Affect the schedulability test


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Non preemptive RTOS

Higher prioritytasks only gaincontrol of the CPUwhen the currenttasks gives upcontrol of the CPU

Most importantdrawback is lowresponsiveness


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Preemptive RTOS

Used when systemresponsiveness isimportant

The highest priority taskready to run is alwaysgiven control of theCPU. The lower prioritytask is suspended

Most commercial real-time kernels arepreemptive


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Task State Clock-Driven

Based

k Task(T(tk))

Start time(tk)

0 T1 1

1 T2 2

2 T1 4

3 I 5

4 T2 6

5 T1 7

6 T2 8

9 I 9

10 T1 10

11 T2 11


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Task State Event-Driven

Based

Blocked state: The task is waiting for an event to occur. A semaphore/mutex token for which a task is waiting is released An Arriving message from message queue A time delay imposed on the task expires


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Scheduler Implementation Table-driven scheduler

Input: stored schedule (tk, T(tk)) for k= 0, 1, n 1.Task SCHEDULER:

set the next scheduling point i= 0 and table entry k= 0;set the timer to expire at tk;do forever:

accept timer interrupt;if an aperiodic task is executing, preempt the task;current task T= T(tk);increment iby 1;compute the next table entry k= imod n;set the timer to expire at [i/n] * M + tk;if the current task Tis I,

let the task at the head of the aperiodic queueexecute;

elselet the task Texecute;

sleep;end do.

End SCHEDULER.

k Task(T(tk))

Start time(tk)

0 T1 1

1 T2 2

2 T1 4

3 I 5

4 T2 6

5 T1 7

6 T2 8

9 I 9

10 T1 10

11 T2 11

pre-computed schedule

IdleTimer

interrupt


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Scheduler Implementation

Cyclic Scheduler ?

The concept implementation is the same

as table-driven schedulers

Priority-Based Scheduler ?

Complex

Using several queues or link lists in eachof task states

Case study: cOS-II


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Scheduler Implementation

Priority Scheduler


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Inter-Task Communication and

Synchronization

Semaphore Controls access to a

shared resource

Signal the occurrence ofan event

Allows two tasks tosynchronize their

activities Mutex

Event


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Inter-Task Communication and

Synchronization

Message Queue A message queue is

used to send one or

more messages to atask

Reader getsmessage in FIFO or

LIFO order Mailbox

Pipe


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Interrupt Timing Issues

Interrupt An interrupt is a hardware mechanism used to inform the

CPU that an asynchronous event has occurred. ISR involve in disabling interruptsand enabling interrupts.

In a real-time environment, interrupts should be disabledas little as possible

Interrupt Latency Maximum amount of time interrupts are disabled + time to

start executing the first instruction in the ISR

Lower latency is desired Interrupt Response Interrupt Recovery ISR Processing Time


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Interrupt Timing Issues


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Timer Services

In developing real-time applications, clearly it is desirable to havefacilities to set and get the time

Time services are among some of the basic facilities provided toprogrammers by every RTOS

Embedded processors are usually equipped with a timer circuit (on-chip timer)

RTOS maintains a number of ticks or system clocks based on theinterrupt received from hardware timers

The finer the resolution of the clock, the more frequent need to be thetime service interrupts and larger is the amount of processor time thekernel spends in responding to these interrupts (low performance)

Timer services supported by cOS-II OSTimeDly() OSTimeDlyHMSM() OSTimeDlyResume() OSTimeGet() OSTimeSet()


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Requirement for a good RTOS

Short Interrupt Latency

Fast Context Switch

Proper scheduling Fine granularity Timer Services

Rich set of Inter-Task Communication

Mechanism Small size (Footprint)


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