Introduction to Embedded System Design

Embedded System Lab. II

Introduction to Embedded System Design Introduction to Embedded System Design

경희대학교 컴퓨터공학과조 진 성

Embedded System Lab. II 2

주요내용 주요내용

임베디드 시스템 특성 임베디드 시스템 설계 과정 임베디드 시스템 설계 핵심 요소

프로세서 선택 운영체제 선택 소프트웨어 / 하드웨어 분할 개발 툴 / 디버깅 / 테스트


왜 embedded system 인가 ? Embedded systems are key elements of our society today

An adequate supply of competent designers is a rate limiting factor in our economies ability to grow

As processing power increases, we’ll be able to do incredible things that we haven’t begun to imagine.

It is a discipline of Computer Science that has traditionally been: Ignored Conceded to Electrical Engineering Departments Treated as a cast-off because of its intimacy with “hardware” Not well understood because of its special niche

Engineers who are experienced embedded system designers are in short supply and high demand

It’s a really fun topic!


Embedded System 은 무엇이 다른가 ? Dedicated to a specific task or tasks Rich variety of microprocessors ( over 300 types ) Designs are cost-sensitive Have real-time performance constraints Used with Real-Time Operating Systems (RTOS) Software failure can be life-threatening May have constraints on power consumption Operate over a wide-range of environmental conditions Fewer system resources than a desktop system All code might be stored in ROM Require specialized design tools May have on-chip debugging resources


Embedded System 의 특성 In general, there is no architectural link to standard platforms

PC ( Win9X, NT, XP, Linux), MAC, HP, Sun are considered the standard platforms

Almost every embedded design ( hardware and software ) is unique The hardware and software are highly integrated and interdependent

Typically, have moderate to severe real-time constraints Real-time means system must be able to respond to the outside world

May or may not have Operating System ( O/S ) services available No printf() for debugging when there is no terminal!

Tolerance for bugs is 1000X ( or more ) lower in embedded systems than in desktop computers. May be life-threatening consequences if system fails Often engineered for the highest possible performance at the lowest cost Performance may not be an important consideration

Most likely to have power constraints


임베디드 시스템 설계 임베디드 시스템 설계 흐름

소프트웨어 / 하드웨어 설계팀의 활동이 유사 소프트웨어 / 하드웨어 구별이 모호해지고 , Co-Design 의 개념이 더욱 강해짐


임베디드 시스템 설계 전형적인 임베디드 시스템 설계 라이프 사이클

7 개 단계로 구성 제품 사양 작성 설계를 소프트웨어 부분과 하드웨어 부분으로 구분 소프트웨어와 하드웨어 구분의 반복과 미세 조정 독립적인 하드웨어 설계 부분과 소프트웨어 설계 부분 구현 소프트웨어 설계 부분과 하드웨어 설계 부분 통합 테스트와 출시 지속적 유지와 업그레이드


임베디드 시스템 설계 AMD 사의 daniel Mann 의 임베디드 시스템 설계 라이프 사이클

Literature

Compiler Tool Chain

Architectural Simulator Evaluation Board

Customer Benchmarks

Processor Selection Phase Enter Here

Packaged benchmarksPast ExperienceOther Similar DesignsApplications supportInstruction Set SimulatorBenchmarking tools

Design Win

Compiler Tool Chain

Instruction Set Simulator Evaluation Board Out-of-Circuit Emulation

Initial Software

Software Design Team

Logic Simulator Logic Analyzer Oscilloscope

Initial hardware without working memory system

Hardware Designer(s)ASIC

Logic Analyzer In-Circuit Emulator Oscilloscope

Software running onHardware

Design Phase

In-Circuit Emulation Logic Analyzer/Preprocessor JTAG Emulation ROM Emulation MiniMON29K

SW/HW Integration Phase

Software Performance Analysis

Maintenance and upgrade of existing products Courtesy ofDaniel Mann


임베디드 시스템 설계 임베디드 시스템 설계 시 핵심

하드웨어와 소프트웨어가 개발할 기능을 어떤 식으로 분할 할 것인가 ? 적절한 프로세서 선택 적절한 운영체제 선택 적절한 디버깅 / 개발 툴 선택


임베디드 시스템 임베디드 시스템 설계 시 핵심 요소

적절한 프로세서 선택 쉽게 구할 수 있는가 ? 성능은 충분한가 ? 적절한 운영체제가 지원하는가 ? 적절한 툴에 의해서 지원되는가 ?

적절한 운영체제 선택 적절한 디버깅 / 개발 툴 선택

컴파일러 툴 – 프로세서 성능에 큰 영향을 미침 하드웨어 / 소프트웨어 디버깅 툴

ROM Emulator 논리분석기 성능분석기 ICE(In Circuit Emulator) 온칩 (on-chip) 하드웨어 디버깅 리소스 인터페이스

성능 측정 툴


적절한 프로세서 선택PowerBudget

Cost ofGoods

Real-timeConstraints Legacy

Code

PerformanceTime toMarket

Landmines

Tool Support


임베디드 프로세서 선택 -1 Issue 1: 성능측면 요구사항

Width of data path Clock speed: RAW MIPS Processor architecture issues Single or multiple processors


임베디드 프로세서 선택 -2 Issue 2: Integration of functions

Microprocessor or micro-controller? Review:

A microprocessor contains the basic CPU functionality, and little more A micro-controller combines the CPU core with peripheral devices

The microprocessor is usually the leading edge of performance Lowest level of integration Highest cost

Higher levels of integration imply Lower system costs Greater reliability

As uP matures the core moves into the uC families


임베디드 프로세서 선택 -3 Higher level of integration ( continued )

Less power Faster Higher processor costs

Issue 3: Use a microcontroller Peripheral choices ( timers, ports, serial comm., A/D, etc. ) On-chip, RAM, ROM, Flash Power requirements Sleep modes Commercially available or build to order ( Motorola 683XX)


임베디드 프로세서 선택 -4 Micro-controller vs System-on-Silicon

Application Specific Integrated Circuit (ASIC) Processor is soft, the processor exists as an encoded HDL

Licensed foundries can fabricate CPU core design into actual Intellectual Property from ARM, MIPS, Motorola Mcore, ARC

Companies do not build ASICs, called Fabless vendors Customizable CPUs

Multiple CPU cores Mix RISC and DSPs Designs out with 64 (and more ) 32-bit RISC and DSP cores


임베디드 프로세서 선택 -5 Issues 4 and up: Software considerations

Legacy code base for existing architecture C code may or may not be portable Assembly code is definitely not portable

Instruction set architecture issues Certain ISAs may be better for certain problems Engineers may be more familiar with certain instruction sets


임베디드 프로세서 선택 -6 Not so obvious considerations:

Compatibility with existing tools Processor vendor’s roadmap for the future and long-term support Design assistance, availability of IP Legacy code

“C is portable”, assembly code is not Pricing and availability Availability of third-party tools ( Emulators, debuggers) Power consumption (power budget)

And… There are lots to choose from! Currently there are over 100 32-bit embedded processors About 1000 different total devices


H/W 및 SW 분할 Recall the beginning phase of the

embedded lifecycle A significant effort usually goes into

partitioning the “algorithm” between the hardware components and the software components

Critical part of the design because mistakes made here can cause an inferior product to result Software team usually tasked with

picking up the pieces

Today, partitioning is becoming an almost seamless process Tools can be used to create a generalized algorithmic solution (UML) and then

partitioned into software and hardware components


H/W 및 SW 분할 The hardware and software in an embedded system work together to

solve a problem ( algorithm ) The decision about how to partition the software components and the

hardware components is usually dictated by speed and cost Dedicated hardware is fast, inflexible and expensive Reconfigurable hardware is fast, flexible and more expensive Software is slower, more flexible and cheaper


H/W 및 SW 분할 There has been a revolution in hardware design called Hardware Descri

ption Languages (HDL) Commonly known as Verilog and VHDL Like C, but includes extensions for real time and other hardware realities Abstracts the design of hardware away from gates and wires to algorithms an

d state machines Hardware design is described in HDL and then compiles to a recipe for th

e silicon FAB to build the circuit Called Silicon Compilation

A single HW designer can now develop an IC that required entire teams several years ago This has led to whole new concept of Systems-on-a-chip, or SOC


새로운 HW/SW 설계 흐름

Co-designPhase

•Define the IP •Partition IP between HW and SW

SW Process

HW Process

• Design Algorithm• Write C/C++ Code

• Design Algorithm• Write HDL Code ( Dialect of C )

• Write test vectors• Run Simulations

• Write stub-code to simulate HW

• Compile to Object code

•Compile to Si foundry database

Integrate

Iterate Software

Re-spin the ASIC

Key Point The differences between the process of designing hardware (ASIC ) and the process of designing software are disappearing


Situation Worse in S/W

05

1015202530354045

1980 1982 1984 1986 1988 1990 1992 1994

Hardware

Software

DoD Embedded System CostsB

illio

n $/

Yea

r


On-going Paradigm Shift inEmbedded System Design

Change in business model due to SoCs Currently many IC companies have

a chance to sell devices for a single board

In future, a single vendor will create a System-on-Chip

But, how will it have knowledge of all the domains?

Component-based design Components encapsulate the intell

ectual property Platforms

Integrated HW/SW/IP Application focus Rapid low-cost customization


What is IP? Predesigned, preverified silicon circuit block , usually containing 5000 gat

es, that can be used in building larger application on a semiconductor chip


IP-based Design


UML and Co-design Two new technologies have emerged that address this desire to focus on

the algorithm and not the partition Unified Modeling Language (UML)

iLogix, ObjecTime, Cadence, CoWare HW/SW Codesign

Synopsys, Mentor Graphics, VAST UML products, such as Statemate and Rhapsody (iLogix) and Co-design

products, such as Seamless ( Mentor Graphics ) blur the distinction between hardware and software

Output of UML tool is C++ application code and/or VHDL code Partition decisions are made by which code is generated

Objective: Design the algorithm in an implementation independent way Let the automatic code generator do the grunt work


Unified Modeling Language UML is a way to represent the finite-state behavior of an embedded

system: A finite state machine is an abstract machine that defines a set of conditions

of existence ( called “states” ), a set of behaviors or actions performed in each of these states, and a set of events that cause changes in states according to a finite and well-defined rule set.

Consider some examples of a finite state machine Vending machine Gas pump Flight control system


HW/SW Co-verification After a design is partitioned and separate HW and SW development is

proceeding, how do you know that you’ll get it right? Re-spinning an ASIC is costly Fixing a problem in software will reduce performance

HW/SW Co-verification focuses on validating the correctness of the design across the HW/SW interface


임베디드 시스템 설계 - 디버깅 임베디드 시스템 설계 시간

시스템 설계에 소요되는 시간을 % 로 나타내면 ..( 다음 page) 곡선은 각 단계에서 결함을 고치는데 소요되는 비용 단계가 진행됨에 따라 디버깅에 소요되는 시간 / 비용이 크게 증가 임베디드 시스템 설계자의 60% 가 새 제품을 만들기보다는 기존 제품을

업그레이드하고 유지보수 하는 일을 함


임베디드 시스템 설계 - 디버깅 (2)

SystemSpecification

& Design

HW & SWDesign/Debug

PrototypeDebug

Source: Dataquest Cost to fixa problem

12%37% 20% 31%

51% of Time51% of Time

SystemTest

Maintenance and upgrade


임베디드 시스템 Integration Hardware/Software integration

The separate hardware designers, firmware designers and software designers have completed unit testing their code

The hardware has been built and passes the smoke test The hardware is turned over to the software team

Load the software image Validate the correctness of the hardware performance against the software

design Integrate their software with the hardware Find the bugs Test for performance and reliability


임베디드 시스템 디버깅 Debugging embedded systems is made more difficult because of the

added dimension brought about by untested hardware The process of debugging an embedded system is usually referred to as

Hardware/Software Integration Identifies this as a unique process

New Test? Run Test

PassTest?

Debug

Re-design physical h/w and/or s/w

The Integration “Loop”

Yes

Yes

No

NoStop

Start


하드웨어 디버깅 - 1 Use an oscilloscope to look at the parametric performance of the hardwar

e system Is the power supply voltage stable?

Minimal AC ripple Within limits

Do the bus signals look clean? Ringing on the waveform is within bounds System noise is within acceptable bounds All busses are properly terminated, no reflections

Is the clock(s) distribution clean? Stable waveform without noise, undershoot or overshoot Clock skew within acceptable limits


하드웨어 디버깅 - 2 Verify that the processor to memory interface is stable

Memory set-up times are within acceptable limits Hold times are long enough Proper timing on reads and writes Use an In-Circuit Emulator to force simple memory test programs

No need to load a program to test memory Use a Logic Analyzer to look at bus activity to verify that all signals are

understood Verify that the memory decoding logic is working properly


하드웨어 디버깅 - 3 Use emulator to force accesses to peripheral devices

Memory access conflicts All glue logic PAL equations are correct Read and Write to ASIC registers Read and write to registers of other peripheral devices

Use an ICE (In Circuit Emulator) to load simple HW test programs (diagnostics) Diagnostics will exercise hardware and verify reliability Will not verify functionality


운영체제 선택 An RTOS enables a design team to create a complex system that

responds to every event with the right action at the right time, every time. Provides mechanisms to precisely synchronize a large number of tasks Reliable

An RTOS provides mechanisms to handle faults, bugs, and other unexpected events that inevitably affect any system of embedded software

When the level of complexity of an embedded system is such that partitioning the software into smaller, independently executing modules significantly lowers the overall design complexity and leads to greater system reliability Time-to-market pressure balances cost of the RTOS


RTOS - Pre-emptive schedule General purpose operating systems are very democratic

Every task or process has equal access to the CPU Scheduling decisions are based upon time sharing

Tasks get to execute without interruption until their time slice is up Problem: If a less important task is running and a more important, deadline depend

ent task is pending, then the higher priority task must wait its turn RTOSes are different!

Pre-emptive scheduling allows the higher priority task to take over the CPU from lower priority tasks

Extends from application level code to drivers and interrupts


RTOS - Predictability RTOSes must provide the designers with predictable performance data f

or system latencies Need to know best case and worst case scenarios for

Task switching times Elapsed time from the completion of the last instruction of the prior task to the begi

nning of execution of the first task of the replacement process Measure of the overhead caused by the RTOS

Interrupt handling Elapsed time from the arrival of the interrupt signal to the beginning of execution of

the first instruction of the interrupt handler Worst case timing for a lower priority interrupt must take into account the time requ

ired to process all higher priority interrupts


RTOS 선택 - economic factors Generally, we view RTOSes as intellectual property

Pay a royalty to RTOS vendor for each copy of the RTOS that you deliver in your application ( annuity ) Example: WinCE royalty ~$15 per copy volumes > 10000

Cost of tools Some RTOS vendors supply complete development and debug environments f

or their customers One-stop shopping Products are well-integrated and work together

Board support packages (BSP) Integrating an RTOS with your hardware platform ( target system ) is a non-triv

ial exercise Requires expert knowledge of the O/S and hardware Is there an existing BSP for your hardware?

Pay the O/S vendor to do it? ( $50K - $1M )


RTOS 선택 - CPU issues Has an RTOS been written and fine-tuned to your CPU choice?

Take advantage of specialized performance hardware on CPU? Example:

May contain shadow registers for rapid task switching May contain special registers for storing task tables context switching.

Key kernel functions written in assembly language for maximum performance Has performance of RTOS been fully characterized with your CPU? Does the RTOS support the peripheral register set on your microcontrolle

r? Example: PowerPC 860 contains 200 registers for peripheral devices ( timer

s, ethernet ports, other O/S relevant I/O devices) require support Support for virtual and protected memory management?


RTOS 선택 - Application support How many simultaneous tasks can the RTOS support? How many interrupts can be supported? Does RTOS vendor provide support services for specialized applications?

FAA and FDA certification of O/S for mission critical applications Protocol stacks for telecomm and datacomm ( TCP/IP )

Architecture of RTOS Flat memory model

Applications and RTOS are built and linked as a single executable Entire address space of processor is available to O/S and all applications Generally fast, but very susceptible to crashes due to buggy code

Errant pointers can overwrite critical kernel code

Documents

Introduction to Embedded System Design