Ch 1. Introduction to Embedded Systems - KAISTrtcl.kaist.ac.kr/~bkkim/lecture/embedded/EmbSys1B_IntroB_D2.pdf · Ch 1. Introduction to Embedded Systems ... Boolean expr. -> Connection

EE414 Embedded Systems

Ch 1. Introduction to Embedded Systems

Part 2/2

Byung Kook KimSchool of Electrical Engineering

Korea Advanced Institute of Science and Technology

Embedded Systems, KAIST 2

Overview 1.1 Embedded Systems Overview 1.2 Design Challenge – Optimizing Design Metrics

Technologies 1.3 Processor Technologies 1.4 IC Technologies 1.5 Design Technologies

1.6 Trade-offs


1.3 Processor Technology Technology

A manner of accomplishing a task, especially using technical processes, methods, or knowledge.

Three key technologies for embedded systems: 1.3 Processor technology 1.4 IC technology 1.5 Design technology


Processor Technology The architecture of the computation engine used

to implement a system’s desired functionality

Processor Does not have to be programmable Not equal to (contains) general-purpose processor

General-purpose processor Application-specific processor Single-purpose processor


Processor technology Processors vary in their customization for the

problem at hand Summing function

total = 0for i = 1 to N loop

total += M[i]end loop

General-purpose processor

Single-purpose processor

Application-specific processor

Desired functionality


Processor Technology

Application-specific

Registers

CustomALU

DatapathController

Program memory

Assembly code for:

total = 0for i =1 to …

Control logic and State register

Datamemory

IR PC

Single-purpose (“hardware”)

DatapathController

Controllogic

State register

Datamemory

index

total

+

IR PC

Registerfile

GeneralALU

DatapathController

Program memory

Assembly code for:


Control logic and State

register

Datamemory

General-purpose (“software”)


A. General-purpose processors Programmable device used in a variety of

applications Maximize the number of devices sold Also known as “microprocessor”: Central

Processing Unit on a chip Features

Program memory General datapath with large register file and

general ALU User benefits

Programming the functionality (software): Low time-to-market and NRE costs High flexibility

“Pentium” the most well-known, but there are hundreds of others.

IR PC

Registerfile

GeneralALU

DatapathController

Program memory

Assembly code for:


Control logic and

State register

Datamemory


B. Single-purpose processors Digital circuit designed to execute exactly one

program a.k.a. coprocessor, accelerator or peripheral JPEG codec

Features Contains only the components needed to

execute a single program No program memory

Benefits Fast, Low power, and Small size Low unit cost for large volumes

Drawbacks High NRE cost Low flexibility High per-product cost for small volumes.

DatapathController

Control logic

State register

Datamemory

index

total

+


C. Application-specific [instruction-set] processors (ASIP)

Programmable processor optimized for a particular class of applications having common characteristics Compromise between general-purpose and single-

purpose processors Features

Program memory Optimized datapath Special functional units

Benefits Some flexibility, good performance, size and

power Types

Microcontrollers Digital Signal Processors: Multiply-accumulate unit

IR PC

Registers

CustomALU

DatapathController

Program memory

Assembly code for:


Control logic and

State register

Datamemory


1.4 IC technology The manner in which a digital (gate-level) implementation

is mapped onto an IC (Integrated Circuit or chip) IC technologies differ in their customization to a design

Most popular: CMOS IC’s consist of numerous layers (perhaps 10 or more)

IC technologies differ with respect to who builds each layer and when.

Layout Feature size (submicron, nano).

source drainchannel

oxide

gate

Silicon substrate

IC package IC


IC technology (II)

Three types of IC technologies: A. Full-custom/VLSI (Very Large Scale Integration)

B. Semi-custom ASIC (Application Specific Integrated Circuit)

C. PLD (Programmable Logic Device)

Independent from processor technology.


A. Full-custom/VLSI All layers are optimized for an embedded system’s

particular digital implementation Designing down to transistor level

Placing transistors Sizing transistors Routing wires

Send mask spec to fabrication plant. Benefits

Excellent performance, small size, and low power. Drawbacks

Very high NRE cost (e.g., $300k), long time-to-market Used for high-volume or extremely performance-critical applications.


B. Semi-custom ASIC Lower layers are fully or partially built

Devices Gate Array Standard Cell: AND gate or AND-OR-INVERT

Designers are left with routing of wires and maybe placing some blocks (Mask design)

The most popular IC technology

Benefits Good performance and size Less NRE cost than a full-custom implementation (perhaps $10k

to $100k)

Drawbacks Still require weeks to months to develop


C. PLD (Programmable Logic Device) All layers already exist

Designers can purchase an IC Programmable circuit: Connections on the IC are either created

or destroyed to implement desired functionality

Devices Programmable Logic Array (PLA)

Programmable array of AND gates and programmable array of OR gates Programmable Array Logic (PAL)

One programmable array Field-Programmable Gate Array (FPGA)

More general connectivity among blocks. Very popular

Benefits Very low NRE costs, almost instant IC availability

Drawbacks Bigger, expensive (perhaps $30 per unit), power hungry, slower.


Moore’s lawTRENDS – Moore’s Law The most important trend in embedded systems

Predicted in 1965 by Intel co-founder Gordon MooreIC transistor capacity has doubled roughly every 18 months

for the past several decades

10,000

1,000

100

10

1

0.1

0.01

0.001

Logic transistors per chip

(in millions)Note:

logarithmic scale


Graphical illustration of Moore’s law

1981 1984 1987 1990 1993 1996 1999 2002

Leading edgechip in 1981

10,000transistors

Leading edgechip in 2002

150,000,000transistors

Something that doubles frequently grows more quickly than most people realize! A 2002 chip can hold about 15,000 1981 chips inside itself

Low-cost high-performance embedded systems proliferate.


1.5 Design Technology The manner in which we convert our concept of desired

system functionality into an implementation

Top-down design process: popular Refines the system through several abstraction levels System level

System specification: in C, English Distributing into several processors.

Behavioral specification Behavior of each processor.

Register Transfer (RT) specification Assembly code level or Register transfer level

Logic specification Boolean equations.

Design Technology (II) Design Approaches – Increased productivity

Compilation/Synthesis Automates exploration and insertion of implementation details

for lower level.

Libraries/IP Incorporates pre-designed implementation from lower

abstraction level into higher level.

Test/Verification Ensures correct functionality at each level, thus reducing

costly iterations between levels.



Design Technology (III) Ideal top-down design process & Design

approachesSystem

specification

Behavioralspecification

RTspecification

Logicspecification

To final implementation

Compilation/Synthesis

Libraries/IP

Test/Verification

Systemsynthesis

Behaviorsynthesis

RTsynthesis

Logicsynthesis

Hw/Sw/OS

Cores

RTcomponents

Gates/Cells

Model simulators/checkers

Hw-Swcosimulators

HDLsimulators

Gatesimulators


A. Compilation/Synthesis Specify desired functionality in an abstract manner Automatically generates lower-level implementation

details. Improve productivity by reducing the amount of details

Tools Logic synthesis tools: Boolean expr. -> Connection of logic gates

(netlist) Register-transfer (RT) synthesis tool: FSM, RT -> Datapath &

controller of Boolean eq. Behavioral synthesis tool: Sequential -> FSM, RT

Software compiler: Sequential program -> Assembly code (RT) System synthesis tool: System spec. -> Sequential programs.


B. Libraries/IP Library: Reuse of preexisting implementations

Improve productivity (Find rather than design)

Libraries Logic level library: Layout of logic gates and cells RT-level library: Layout of RT components Behavioral-level library: Components, such as compression, bus,

processor etc. System level library: Complete systems solving particular

problems.

Intellectual Property (IP) Cores in an intellectual form Copy protected.


C. Test/Verification Ensure that functionality to be correct.

Tools Simulation

Gate level simulator RT level (HDL: Hardware Description Language)

Provide output waveforms, given input waveforms. Processor level: HW & SW co-verification. System level: model simulator, model checker.

Formal verification Growing in popularity.


Design productivity exponential increase

Exponential increase over the past few decades. 1981: 100 TRs/month 2002: 5000 TRs/month.

100,000

10,000

1,000

100

10

1

0.1

0.01

1983

1987

1989

1991

1993

1985

1995

1997

1999

2001

2003

2005

2007

2009

Prod

uctiv

ity(K

) Tra

ns./S

taff

–M

o.


1.6 Trade-offs In the past:

Hardware and software design technologies were very different

Recent maturation of synthesis enables a unified view of hardware and software

Hardware/software “codesign” The co-design ladder -> Unified view

Implementation

Assembly instructions

Machine instructions

Register transfers

Compilers(1960's,1970's)

Assemblers, linkers(1950's, 1960's)

Behavioral synthesis(1990's)

RT synthesis(1980's, 1990's)

Logic synthesis(1970's, 1980's)

Microprocessor plus program bits: “software”

VLSI, ASIC, or PLD implementation: “hardware”

Logic gates

Logic equations / FSM's

Sequential program code (e.g., C, VHDL)

The choice of hardware versus software for a particular function is simply a tradeoff among various design metrics, like performance, power, size, NRE cost, and especially flexibility;

there is no fundamental difference between what hardware or software can implement.


Independence of processor and IC technologies Basic tradeoff

General vs. custom With respect to processor technology or IC technology The two technologies are independent

General-purpose

processorASIP

Single-purpose

processor

Semi-customPLD Full-custom

General,providing improved:

Customized, providing improved:

Power efficiencyPerformance

SizeCost (high volume)

FlexibilityMaintainability

NRE costTime- to-prototype

Time-to-marketCost (low volume)


Design productivity gap While designer productivity has grown at an impressive rate over the

past decades, the rate of improvement has not kept pace with chip capacity

10,000

1,000

100

10

1

0.1

0.01

0.001


(in millions)

100,000

10,000

1000

100

10

1

0.1

0.01

Productivity(K) Trans./Staff-Mo.IC capacity

productivity

Gap


Design productivity gap (II) 1981 leading edge chip required 100 designer months

10,000 transistors / 100 transistors/month 2002 leading edge chip requires 30,000 designer months

150,000,000 / 5000 transistors/month Designer cost increase from $1M to $300M

10,0001,000

100101

0.10.01

0.001


(in millions)

100,00010,00010001001010.10.01

Productivity(K) Trans./Staff-Mo.IC capacity

productivity

Gap


The mythical man-month The situation is even worse than the productivity gap indicates In theory, adding designers to team reduces project completion time

In reality, productivity per designer decreases due to complexities of team management and communication

In the software community, known as “the mythical man-month” (Brooks 1975) At some point, can actually lengthen project completion time! (“Too many cooks”)

10 20 30 400

10000

20000

30000

40000

50000

60000

43

24

1916 15 16

18

23

Team

Individual

Months until completion

Number of designers

1M transistors, 1 designer=5000 trans/month

Each additional designer reduces for 100 trans/month

So 2 designers produce 4900 trans/month each

…


Summary Three key technologies

Processor: general-purpose, application-specific, single-purpose

IC: Full-custom, semi-custom, PLD Design: Compilation/synthesis, libraries/IP,

test/verification

A unified view of hardware and software is necessary to improve productivity Hardware and software codesign.

References [1] Frank Vahid, “Embedded system design: A

unified hardware/software introduction”, John Wiley & Sons, 2002.


Ch 1. Introduction to Embedded Systems - KAISTrtcl.kaist.ac.kr/~bkkim/lecture/embedded/EmbSys1B_IntroB_D2.pdf · Ch 1. Introduction to Embedded Systems ... Boolean expr. -> Connection

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