CSE 522Real-time Embedded Systems

Computer Science & Engineering DepartmentArizona State University

Tempe, AZ 85287

Dr. Yann-Hang Leeyhlee@asu.edu(480) 727-7507

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Real-time Embedded Systems

Embedded system the software and hardware component that is an

essential part of another system Real-time system

provide well-timed computation

deadlines, jitters, periodicity

temporal dependencyPlantsensor actuator

Control-rawcomputation

A/D

A/DD/AReference

input

Controller

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Embedded Systems -- Examples

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Emerging Embedded Systems

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Hardware Platform Organization

buses to connect components – PCI, ISA, PC104+

Package standard chips on PC processor + ASIC SOC

CPU(micro-

processor)

I/O

I/O

I/O

I/O

Timer

memory

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SW Development for RT ES

To write the control software for a smart washer initialize read keypad or control knob read sensors take an action

System current state state transition diagram external triggers via

polling or ISR If there are multiple triggers and

external conditions – single or multiple control loops

initialization

external trigger?

Take actions

Change system state

ISR: to set/clearevents

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Periodic Tasks

Invoke computation periodically Adjust pressure valves at a 20 Hz rate

wait for the interrupt event

Task initialization(set up periodic timer interrupts)

computation

start_time=time( )

Task initialization

computation

Sleep(period - ( time( ) -start_time) )

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SW Development for RT ES

In the example of smart washer Never-ending in a single control loop Single execution threat and one address space Event triggering and state transitions Small memory footprint

What are missing: no concurrency (real-world events occur in parallel) no explicit timing control (let’s add a timer) difficult to develop and maintain large embedded systems –

verifiable, reusable, and maintainable

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RT ES vs. General Software

Multi-tasking for concurrent events Machine dependence and portability Software abstraction, modular design

information hiding, OO, separate compilation, reusable a sorting procedure -- function, input, output specification

Control timing predictable actions in response to external stimuli deadline (absolute or relative), and jitter

Resource constraints and sharing CPU time, stack, memory, and bandwidth

Scheduling

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Timing Constraints and Multi-threading

Given input x1 at time t1, produce output y1 at time t2 Non-deterministic operation, Time-dependent

behavior, and race condition difficult to model, analyze, test, and re-produce.

Example: NASA Pathfinder spacecraft Total system resets in Mars Pathfinder An overrun of data collection task

a priority inversion in mutex semaphore failure of communication task a system reset.

Took 18 hours to reproduce the failure in a lab replica the problem became obvious and a fix was installed

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Trends of RT Embedded Systems Applications

Wide-spreading, distributed, connected, and heterogeneous Mission and safety critical High-end consumer products

cell phone, HDTV, home network, PDA, GPS, appliances Quality of the products

portable/reusable, reliable/dependable, interoperable, predictable (schedulable), and secured

Software extensive A new S-class Mercedes-Benz

over 20 million lines of code nearly as many ECUs as the new Airbus A380 (excluding the plane's

in-flight entertainment system).

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Embedded System Development

Need a real-time (embedded) operating system ? Need a development and test environment ?

Use the host to edit, compile, and build application programs, and configure the target

At the target embedded system, use tools to load, execute, debug, and monitor (performance and timing)

Ethernet

Simulated signal source

(Workstation, embedded system development tools)

(workstation, interface cards), & test harness

Development workstation Embedded systems

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cc

asm

ld

From Source to Executable Compiler, linker, and loader In ELF: executable, relocatable, shared library, and core

information for relocation, symbol, debugging linker resolves symbol reference

Link script or link command file assigns absolute memory addresses (program area, static data,

bss, stack, vector table, etc.) Startup code to disable interrupts, initialize stack, data, zero

uninitialized data area, and call main().

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Real-time Operating System Use the computer hardware efficiently To manage system resource through

system calls -- issued by tasks interrupts -- timer and external events

Typical requirements Support for scheduling of real-time tasks and interrupt handlers Inter-process communication I/O support -- driver User control of system resource -- memory and file system

Thread or process for task execution: smallest execution units that can be scheduled lives in a virtual, insulated environment uses or owns system resources

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Real-time Operating System

Functions: task management,

scheduling, dispatcher communication (pipe, queue) synchronization (semaphore, event)

memory management

time management device driverinterrupt service

Interruptdispatch

Interruptservice

Time service &events

Services (create thread, sleep, notify, send,…)

Scheduling &

dispatcher

System calls(trap)

External interrupt

Timer interrupt

kernel

Task execution

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RTOS Structures Small, fast, proprietary kernels Monolithic kernels that contains all services Component-based kernels or micro-kernel

contain the minimal services small (10K bytes) and modular configurable by selecting components to compose a kernel

RT extensions (to extend Unix or Windows) Why -- richer environment, more functionality, familiar interfaces Compliant kernel (LynxOS) -- Takes an existing RTOS and make it

execute other UNIX binaries Dual kernels– add an RTOS kernel between the hardware and the

OS. (RTLinux) OS kernel modifications – use patches to make Linux kernel more

deterministic (Real-time Linux distributions)

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RTOS vs. OS

Often used as a control device in a dedicated application

Well-defined fixed-time constraints The system allows access to sensitive resources with defined

response times. interrupt latency and time for context switch

worst-case and average response times Requirements of RTOS

predictable (??) upper bounds for system calls and memory usage configuration of memory layout and kernel data structures fine grain interrupt control

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Task Management in vxWorks

Task structure: task control block --

priority(initial and inherited), stack frame, task current state, entry point, processor states (program counter, registers) callback function (hook) pointers for OS events

Create and initialization taskInit, taskActivate, taskSpawn task_id = taskSpawn (name, priority, options, stacksize, main, arg1,…,

arg10); Task control and deletion: taskDelay (nanosleep), taskSuspend,

taskResume, taskRestart, exit, taskDelete

Execution

Ready Blocked

executing

pending ready delayed

suspendedtaskInit()

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Scheduling Mechanism

Priority-driven and round-robin with timeslicing taskPrioritySet (tid, priority), kernelTimeSlice taskLock, taskUnlock -- disable and enable scheduling (preemption) 0 (highest) to 255 (lowest) in vxWorks

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Shared Code and Reentrancy A single copy of code is invoked by different concurrent

tasks must reentrant pure code variables in task stack (parameters) guarded global and static variables (with semaphore or

taskLock) variables in task content (taskVarAdd)

taskOne ( ) { ..... myFunc ( ); ..... }

taskTwo ( ) { ..... myFunc ( ); ..... }

myFunc ( ) { ..... ..... }

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Inter-task Communication Shared memory

vxWorks has all tasks in a single address space simple and unprotected direct access as long as the address is known

Message queue -- multiple senders and receivers msgQCreate( ), msgQDelete( ), msgQReceive( )

status = msgQSend(msgQId, buffer, nBytes, timeout, priority ) queue size, message size, timeout parameters msg_pri_normal -- to be added to the queue tail msg_pri_urgent -- to be added to the queue head ISR cannot read a message queue mq_notify( ) in POSIX -- notification to a single task when a new

message arrives at an empty queue

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Inter-task Communication

Pipe -- virtual I/O, built on top of msgQ pipeDevCreate (name, nMessages, nBytes) -- create a named

pipe in the global file descriptor table open, read, write, and ioctl routines for device control select( ) -- wait for input from multiple pipes (with a timeout)

Network Inter-task communication Socket and RPC of vxWorks -- compatible with BSD 4.3 Unix

Signals (for asynchronous events) between tasks and ISR -- to execute signal handler of a task bind a handler to a signal, send a signal (kill(tid, signo)), signal

masks sigqueue( ) in POSIX sigInit( ), sigqueueInit( )

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Signals

To register a handler -- signal (signo, sigHandler)void sigHandler ( int sig, int code, struct sigcontext * pSigCtx);

Exception: issues a signal to the running task if no signal handler, suspend the task hardware dependent return with exit(), taskRestart(), longjump()

normal program { . . . . }

sigHandler { . . . . }

signal

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Synchronization and Mutual Exclusion Semaphores in vxWorks:

binary mutual exclusion -- addresses inheritance, deletion safety and recursion counting

Routines: semBCreate( ), semMCreate( ), …, semDelete( ) semTake( ), semGive( ), semFlush( ) (broadcasting)

Semaphore id, queue type (SEM_Q_PRIORITY or SEM_Q_FIFO), and timeout on waiting

SEM_ID sem = semBCreate (SEM_Q_PRIORITY, SEM_FULL); semTake(sem, WAIT_FOREVER); . . . . . semGive(sem);

Synchronozation: ISR calls semGive( ) to signal an event task calls semTake( ) to wait for the event

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Synchronization and Mutual Exclusion Mutual Exclusive -- restriction:

can be given by the task took it (owns it) cannot be given from an ISR no flush

Priority inheritance: set flag = SEM_Q_PRIORITY | SEM_INVERSION_SAFE

Deletion Safety: delete a task while it is holding a semaphore SEM_DELETE_SAFE: protect from deletion

Recursion: (task ownership count) take a semaphore more than once by the task that owns it released when it is given the same number of times

POSIX semaphore (counting) unnamed -- malloc a semaphore struct and use * to operate named --open a semaphore in OS, shared among processes

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Interrupt Service Routines (1) Interrupt service routines (ISRs) run outside of any task’s

context. Thus they involves no task context switch. intConnect ( ) -- to install user-defined ISR_routine

If intConnect() is used for PowerPC, it is not needed to explicitly disable the current interrupt source and do interrupt acknowledgement.

Save registersset up stackcheck interrupt source (vector)invoke routinerestore registers and stackexit

ISR_routine ( ) { . . . . . . . . }

handler wrapper

user ISR

intConnect(INUM_TOIVEC(someIntNum), ISR_routine, someVal);

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Interrupt Service Routines (2) Interrupt stack -- a suitable size of maximum interrupt

nesting depth Whenever the architecture allows it, all ISRs use the

same interrupt stack The stack is allocated as system starts up Must be large enough to handle the worst case

Limitations to ISRs ISR should not be blocked

don’t take a semaphore (malloc() and free() takes a semaphore), giving semaphore however is permitted

don’t perform I/O via vxWorks drivers that can get blocked ISRs cannot call printf() to output message to console, please

use logMsg() and other functions defined in logLib

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Interrupt Service Routines (3)

Interrupt-to-task communications Interrupt events usually propagate to task-level code. The following techniques can be used to communicate from

ISRs to task-level code Shared memory and ring buffers. ISRs can share variables,

buffers, and ring buffers with task level code Semaphores. Giving semaphores is permitted Message queues. ISR can send messages to message queues

for tasks to receive Pipes. ISRs can write messages to pipes that tasks can read Signals. ISRs can signal tasks, causing asynchronous scheduling

of their signal handlers

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Timer and Clock Clock tick Watchdog timer in vxWorks -- mantained by the

system clock ISR wdCreate( ), wdDelete( ), wdStart( ), wdCancel( ) Typically invokes a callback function when the delay is

expired (at the interrupt level of the system clock) real-time clock -- POSIX timer

create, set and delete a timer that signals tasks when goes off Delay a period:

taskDelay( ) in vxWork nanosleep( ) in POSIX

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Device Driver Purpose:

a well defined and consistent interface to handle requests for device operations

isolate device-specific code in the drivers A software interface to hardware devices

resides in kernel or user spaces Classification

character device (terminal) block (disk) -- with buffer cache network pseudodevice

When to call a device driver configuration, I/O operations, and interrupt

OS specificcode

I/O classspecific code

Hardwarespecific code

I/O adapters

devicedriver

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Interaction with I/O Devices Polling --

Read status until the device is ready, and then read/write data

Check flags which are set by ISR – Interrupts when a new input arrives or the device completes an output

operation If the flag is set, proceed to do the I/O operation

Use driver – Build input/output buffers in the driver If an new input arrives, ISR reads the data and saves in the buffer (if a

write is completed, ISR triggers the next write if the output buffer is not empty)

Application reads from (or write to) the corresponding buffer

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Structure of Device Driver

“/tty0/”1

“/xx1/”3

“/pty0/”1

Device-dependent

data

Device list(of device descriptors)

drvnum create remove open close read write ioctl 0 ** ** 1 2 3

Driver table(function pointers)

drvnum value 2 2 1 *dev 3

File descriptortable

0123

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Board Support Package Most of RTOS’s are independent of the particular target board. The board-specific code for initializing and managing a board’s

hardware is called the BSP. The BSP provides RTOS with standard hardware interface functions which allow it to run on a board.

Tools - ApplicationsHardware-Independent Software

I/O System vxWorks Libraries TCP/IP

File Systems

Hardware-Dependent Softwarewind Kernel

BSP

HardwareSCSI Controller

SCSI Driver Network Driver

Serial Controller Clock Timer Ethernet Controller

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Remote Target Boot Process

Basic initialization code exits in ROM. This code can initialize the serial port or ethernet controller allows the user to set the target parameters like the name of

kernel image , target IP address, host IP address etc needed initially to pull over the kernel image into target .

It can perform TFTP and other serial port transfers.

VxWorks

TargetRS-232

Ethernet

Host

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Target’s boot ROM code executes on power up. The boot ROM code containing a bootloader:

Allows setting of boot parameter. Downloading & executing kernel image.

Default Boot ROM’s does not contain the VxWorks system. Replace board manufacturer’s ROMs with vxWorks

bootloader Downloads VxWorks into target memory via the network

(TFTP) or serial port Starts executing VxWorks.

Boot ROM

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Boot Sequence

vxWorks boot sequenceromInit.s : romInit()

bootInit.c : romStart()

usrConfig.c and bootConfig.c : usrInit()

Libcuptoolvx.a : kernelInit()

usrCofig.c and bootConfig.c : usrRoot()

Disables interrupts, puts the boot type on the stack, clears caches

The text and data segments are copied from ROM to RAMCache initialization, zeroing out the system bss segment, initializing interrupt vectors and initializing system hardware to a quiescent state

Initiates the multitasking environment: disables round-robin mode, creates an interrupt stack, creates root stack and TCB

Initializes the I/O system, installs drivers, creates devices, and then sets up the network as configured in configAll.h and config.h