Embedded System

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Embedded System

• A specialized computer system that is part of a larger system or machine. Typically, an embedded system is housed on a single microprocessor board with the programs stored in ROM. Some embedded systems include an operating system, but many are so specialized that the entire logic can be implemented as a single program.

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Embedded System TestingWei-Tek Tsai

Department of Computer Science and Engineering

Arizona State University

Tempe, AZ 85287

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Embedded System(cont’)

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• Differences with desktop computing– The human interface may be as simple as a flashing light

or as complicated as real-time robotic vision. – The diagnostic port may be used for diagnosing the

system that is being controlled -- not just for diagnosing the computer.

– Special-purpose field programmable (FPGA), application specific (ASIC), or even non-digital hardware may be used to increase performance or safety.

– Software often has a fixed function, and is specific to the application.

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• Characteristics of Embedded Systems– Application specific

• Jobs are known a priori• Static scheduling of tasks and allocation of resources

– Real time• Hardware/software tradeoff• Exceptions

– Reactive• Interacts with external environment continuously

– Hierarchy of behaviours• Sequential and concurrent subbehaviours

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• Characteristics of digital systems– Application domain

• General purpose• Dedicated computing and control systems• Emulation and prototyping systems

– Degree of programmability• Application level• Instruction level• Hardware level

– Hardware fabrication technology• Bipolar versus CMOS

– Level of integration• Discrete components versus integrated

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• Definitions– FPGA(Field Programmable Gate Array)

• Programmable HW; configurable gate level interconnection of circuits after manufacturing

• Consists of a matrix of cells: Configurable Logic Blocks(CLBs) and I/O Blocks(IOBs), with programmable switches to provide the desired connections between blocks

• Slower than non-programmable devices, but allows prototype to be designed quickly(circuit design, implementation, verification on desktop workstations)

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– ASIC(Application Specific Integrated Circuit)• Custom designed chip to implement a digital

function/system

• Hardwired(non-programmable) gives the best performance

• Must produce in volume to cover non-recurrent engineering design cost

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– ASIP(Application Specific Instruction Processor)

• A microprocessor with special architecture design, and instruction set chosen for a specific domain of programs

• Easier to cover non-recurrent engineering cost since ASIP has multiple applications

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• HW/SW Co-Design– Meeting system level objectives by exploiting

the synergism of HW and SW through their concurrent design

– Simultaneously design the software architecture of an application and the HW on which that SW is implementation to meet performance, cost, or reliability goals

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– HW/SW Co-Simulation• The joint simulation of HW and SW components

and their interaction

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• Design requirements– Real time/reaction operation– Small size, low weight– Safe and reliable– Harsh environment– Cost sensitivity

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– Real time/reactive operation• Real time system operation: the correctness of a computation

depends, in part, on the time at which it is delivered

• Reactive computation: the software executes in response to external events

• Challenge: Worst case design analyses without undue pessimism in the face of hardware with statistical performance characteristics (e.g., cache memory [Philip Koopman, "Perils of the PC Cache", Embedded Systems Programming, May 1993, 6(5) 26-34]).

http://www-2.cs.cmu.edu/~koopman/pc_cache/index.html

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– Small size, low weight• Physically located within some larger artifact,

therefore, form factors may be dictated

• Weight might be critical in transportation and portable systems for fuel economy or human endurance

• Challenges:– Non-rectangular, non-planar geometries.

– Packaging and integration of digital, analog, and power circuits to reduce size.

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– Safe and reliable• Challenges:

– Low-cost reliability with minimal redundancy

– Harsh environment• Many embedded systems do not operate in a controlled

environment• Additional problems can be caused for embedded computing

by a need for protection from vibration, shock, lightning, power supply fluctuations, water, corrosion, fire, and general physical abuse

• Challenges: accurate thermal modeling and de-rating components differently for each design, depending on operating environment

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– Cost sensitivity• Challenge:Variable "design margin" to permit

tradeoff between product robustness and aggressive cost optimization

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• System level requirements for embedded system– End-product utility– System safety & reliability– Controlling physical systems– Power management

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– End-product utility• Challenge: Software- and I/O-driven hardware

synthesis (as opposed to hardware-driven software compilation/synthesis).

– System safety & reliability• Challenges:

– Reliable software

– Cheap, available systems using unreliable components

– Electronic vs. non-electronic design tradeoffs

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– Controlling physical systems• Challenge: Distributed system tradeoffs among

analog, power, mechanical, network, and digital hardware plus software

– Power management• Challenge: Ultra-low power design for long-term

battery operation

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Embedded System(cont’)• Embedded system lifecycle

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Major Embedded OS

• QNX 4 RIOS

• Embedded Linux

• Windows CE

• VxWorks

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Major Embedded OS (cont’)• Supported processor

• QNX: all generic x86 based processors(386+)• Linux: virtually on every general purpose micro-

processor(ARM, StrongARM, MIPS, Hitachi SH, PowerPC, x86)

• WindowsCE: (x86, MIPS, Hitachi SH3 and SH4, PowerPC and StrongArm processors)

• VxWorks: (PowerPc, 68K, CPU32, ColdFire, MCORE, 80x86 and Pentium, i960, ARM and StrongARM, MIPS, SH, SPARC, NECV8xx, M32 R/D, RAD6000, ST 20, TriCore)

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Major Embedded OS(cont’)

• Memory constraints– QNX is the smallest– Windows CE needs 350KB for a minimal

system– Linux needs 125 – 256 KB fro a reasonable

configured kernel– VxWorks: a few kilobytes for a deeply

embedded system

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• Architecture Comparison– QNX: A very small microkernel surrounded by a team

of cooperating processes that provide higher level OS services.

– Linux: a layering structure and comprised of modules.

– WindowsCE: The operating system architecture of Windows CE is a hierarchical one.

– VxWork: Individual modules may be used in development and omitted in production systems.

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Major Embedded OS(Cont’)

• Process Management– QNX:

• Process manager is not in micro kernel

• Use message passing primitives to communicate with other processes

• Scheduling is managed by the micro kernel scheduler

• Scheduling methods: FIFO, RR, adaptive

• Fully preemptible

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– Linux:• Implements threads in kernel

• Three classes of threads– Real-time FIFO: having highest priority and not

preemptable

– Real-time RR: same as real-time FIFO but preemptable

– Time sharing: lowest prority

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– WindowsCE:• Support both processes and threads

• Full memory protection applied to application processes

• Thread scheduling is preemptive, using 8 different priority levels

• A maximum of 32 simultaneous processes

• A process contains one or more threads

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– VxWorks:• A multitasking kernel

• Transparently interleave task execution

• Uses Task Control Blocks(TCB’s) to keep track of tasks

• Priority scheduling and priority based preemption

• RR scheduling only applies to the tasks of the same priority

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• Interprocess Communication– QNX:

• Message passing facilities

• a message delivered from one process to another need not occupy a single, contiguous area in the memory

• All the system services within QNX are built upon message passing primitives

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Major Embeded OS(cont’)

– Linux:• Uses original Linux IPC mechanisms: signals, wait

queues, file locks, pipes and named pipes, system V IPC, Unix Domain Sockets.

• The embedded linux can choose any one of the IPC methods for its particular application.

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– WindowsCE:• Supports message passing between processes

• Supports memory mapping between processes

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– VxWorks:• Shared memory

• Message passing queues

• Pipes

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• Memory management Clinux :

• Without a MMU, running on a flat memory model(virtual = physical)

• No paging, no protection, no memory sharing

• No fork() since no copy-on-write, only limited version of vfork()

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– WindowsCE:• More elaborate memory management

• Supports paged virtual memory partially

• Requires CPU to support a TLB but not a full page model

• Full memory protection applies to application processes

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• Network support– QNX: contains low level network communication in its

microkernel

– Linux: automatically get the most current Internet protocols.

– WindowsCE:has communication stacks of various kinds at the same level as kernel. Supports IP, HTTP, FTP and so on

– VxWorks: very good network support of almost all internet protocols.

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• Factors impact embedded operating system– Application requirement impact– Hardware impact

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H/S interaction• Today’s embedded system designers face the difficult task

of integrating and verifying application-specific software and hardware with intellectual property (IP) such as protocol stacks and commercial real-time operating systems (RTOSs). When the system design includes developing application-specific integrated circuits (ASICS), engineers often postpone the integration and verification task until a hardware prototype is available. Waiting until this stage to debug adds unnecessary costs and delays. However, new methodologies allow integration teams to verify their embedded systems applications while meeting design and time-to-market goals.

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H/S interaction(cont’)

• Semiconductior manufactures should not ignore embedded software

• Software experts are unlikely to solve the embedded software problem on their own

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H/S interaction(cont’)• Software/Hardware Codesign

– Simultaneous design of both hardware and software to implement in a desired function

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• Why is embedded software an issue for semiconductor manufactures?– Silicon without software getting rare– Time-to-volume is often dominated by SW

development– Software requirements affect hardware design– Embedded software design is getting

harder(networking, complexity)– Mainstream SW engineering is not addressing

embedded SW well

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• Why is embedded SW not just SW on small computers?– Interaction with physical processes(sensors, actutors,

etc.)

– Critical properties are not all functional(real-time, fault recover, power, security, robustness)

– Heterogeneous(hareware/software, mixed architectures)

– Concurrent(interact with multi processes)

– Reactive(operating at the speed of environment)

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• Hardware experts have sth to teach to the SW world– Concurrency– Reusability– Reliability– Heterogeneity

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H/S interaction(cont’)• What an embedded program might look like

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H/S interaction(cont’)• Simple example: controlling an inverted

pendulum with embedded SW

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• Metaphor for:– Disk drive controllers– Manufacturing equipment– Automotive:

• Drive-by-wire devices

• Engine control

• Antilock braking systems, traction control

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– Avionics• Fly-by-wire devices

• Navigation

• Flight control

– Certain “software radio” functions– Printing and paper handling– Signal processing(audio, video, radio)– …

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• System HW/SW design methodology– Specification capture

• Create model

• Select language for specification

– Exploration• Allocate architectural components

• Partition the specification to the architectural components

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– Refinement of specification– Hardware and software design

• Synthesis

• Simulation

– Physical design• Generate manufacture data for hardware

• Compile code for instruction sequence

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H/S interaction(cont’)• Co-design, particularly important when designing

embedded systems or systems-on-a-chip• There are many areas in which the co-design principle can

bring product enhancements • Massive parallelism, distributed algorithms and special

architectures • Efficient interfaces are required • Low-Power Processors • Reconfigurable Systems are capable of adapting to

changing environments or to incomplete specifications• Parallel I/O on multiple PC's

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• Hardware/software codesign– the cooperative design of hardware and software

components;

– the unification of currently separate hardware and software paths;

– the movement of functionality between hardware and software;

– the meeting of system-level objectives by exploiting the synergism of hardware and software through their concurrent design.

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H/W interaction(cont’)

• Why is it important?– Reconfiguration: exploiting the synergy

between hardware and software

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– Embedded systems are application specific systems which contain both hardware and software tailored for a particular task and generally part of a larger system

– Reusability: to provide design approaches that scale up, without a total redesign for a legacy product

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• Existing Problems– Model Continuity Problem

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• Importance of Model Continuity– many complex systems do not perform as

expected in their operational environment; – continuity allows the validation of system level

models at all levels of hardware/software implementation;

– trade-offs are easier to evaluate at several stages

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• Consequences of losing such model continuity– cost increases and schedule over-runs (due to

modifications late in phases);

– the ability to explore hardware/software trade-offs is restricted (e.g. movement of functionality between, modification of interfaces);

– state of the art applications require a case-by-case analysis;

– different design cultures hamper integration

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• Solution– Unified Design Environment: it is emphasized

that hardware design and software design use the same integrated infrastructure, resulting in an improvement of overall system performance, reliability, and cost effectiveness.

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• Typical context for co-design process

An “ideal” process flow

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Memory Constraints

• Memory is usually a critical resource and the memory size is often very restricted

• Both static and dynamic memory usage within a task and the dynamic memory usage due to communication should be considered

• Mapping is also a problem

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Fault-tolerance

• Allowable system failure probability is 10-10 per hour

• Software fault tolerance

• Hardware fault tolerance

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Fault-tolerance(cont’)

• Software fault tolerance– Timeouts– Audits– Exception handling– Task roll back– Incremental reboot– Voting

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• Hardware fault tolerance– Redundancy Schemes

• One for one redundancy: each hardware module has a redundant hardware module

• N + X redundancy: if N hardware modules are required to perform system functions, the system is configured with N + X hardware modules; typically X is much smaller than N

• Load sharing: under zero fault conditions, all the hardware modules that are equipped to perform system functions, share the load

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– Standby synchronization• Bus cycle level synchronization

• Memory mirroring

• Message level synchronization

• Checkpoint level synchronization

• Reconciliation on takeover

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• Fault handling techniques– Fault handling lifecycle– Fault detection – Fault isolation

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• Fault-handling lifecycle

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• Fault detection– Sanity monitoring– Watchdog monitoring– Protocol faults– In-service diagnostics– Transient leaky bucket counters

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Fault-tolerance

• Fault isolation If a unit is actually faulty, many fault triggers will

be generated for that unit. The main objective of fault isolation is to correlate the fault triggers and identify the faulty unit. If fault triggers are fuzzy in nature, the isolation procedure involves interrogating the health of several units. For example, if protocol fault is the only fault reported, all the units in the path from source to destination are probed for health

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Timing • Real time: A real-time system provides specified system

services with known timing and latency characteristics, so that applications can be designed and developed that meet perscribed timing constraints

• Hard real time: In a hard real-time system, the timing constraints have an upper worst-case value, which if exceeded, cause the application to fundamentally fail

• Soft real time:In a soft real-time system, the timing constraints do not have an upper worst-case value, but meet an acceptable statistical distribution of timings. In this case, occasional longer latencies either do not really cause failures, or the failure rates are acceptable

Embedded System

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