an overview of hardware design methodology ian mitchelle de vera

An Overview of Hardware Design Methodology

Ian Mitchelle De Vera

Outline of the Presentation

• Design Classifications• Typical Design Flow• Post Design • Design Data Hierarchy• Testbenches• Design Flow Overview• Challenges of Hardware Design

Design Classifications

• ASIC Design

• Processor Design

• Memory

• Embedded Design

ASIC Design• Application Specific IC e.g. cruise controller, etc.• Typical Characteristics specific applications short development time mass market low cost slow performance via older manufacture facilities• Today’s ASIC designs may contain millions of transistors

Processor Design• Designs that follow certain architectural standards e.g. microprocessors, signal processors, graphic processors, etc.• Typical characteristics architectural specification long development time expensive fixed market high performance via state-of-the-art manufacture facilities

Memory• On-chip storage device e.g. RAM, Video RAM, etc.• Typical Characteristics high density low-high performance regular structure special design methodology special manufacture facilities

Embedded Designs• Core-based design or intellectual property (IP) design• Emerging market• New challenges in tools/methodologies• System-on-a-chip

Video Core

Audio Core

uControllerCore

RAM

ASIC

Typical Design Flow• Obtain product requirements• Derive design specifications• Choosing technology and tools• Verification

a. Simulationb. Design Reviewc. Synthesisd. Physical Implementation

e. Formal Verification• System Integration and Test

Post Design

• Setup manufacture line• Obtain first silicon• Diagnosis of defects / Yield of improvement• System bring-up test• Silicon debugging / On-chip repair• Mass production / Speed Grading• Ship to customers

Design Data Hierarchy

There are four levels of abstraction in the design process:

A. Behavior Level Like a C program Behavioral code consists of a process containing constructs like loops, conditions, array and variable declarations. Focuses on functionality and the system level architecture but less on defining architectural details (e.g. sequential or parallel structure) Simulation on this level is faster because the description is more abstract than RTL.

B. RTL (Register Transfer Level)

explicitly define all state-holding elements specifies the architecture in detail

C. Gate Level All high-level constructs (if, case, assign) are converted to gates (and, or, not).D. Transistor Level

All are converted to nmos, pmos, etc.E. Layout (final picture)

Data Hierarchy

Behavior

RTL

Gate Level

Transistor/Layout

FunctionalSimulation

Synthesis

Test GenerationDesign for Test

Physical Prop.Area/Timing Optimization

Specification

Implementation

LessComplex

VeryComplex

if (x = 0) A = T1else A = T2

T0 T2

T1

x 0

x = 0

C Program (if, else)

Verilog, VHDL

AND, OR, NOT

CMOS, PMOS, NMOS

TestbenchesDue to increases in design size and complexity, digital designverification has become an increasingly difficult and laborious task. To meet this challenge, verification engineers rely on several verification tools and methods.

Testbenches have become the standard method to verify HLL(High Level Language) designs.

Testbench Tasks

Typically, testbenches perform the following tasks:

• Instantiate the design under test (DUT)• Stimulate the DUT by applying test vectors to the model.• Output results to a terminal or waveform window for visual inspection.• Optionally compare actual results to expected results.

Typically, testbenches are written in the industry-standard VHDL or Verilog hardware description languages.

Testbenches invoke the functional design, then stimulate it.

Complex testbenches perform additional functions—for example, they contain logic to determine the proper design stimulus for the design or to compare actual to expected results.

Standard HDL Verification Flow

Constructs used in Testbenches• The following are some constructs used frequently in testbenches:

1. Generating Clock Signals designs that use system clocksto sequence logic must generate a clock. Iterative clocks can easily be implemented in both VHDL and Verilog source code

2. Providing Stimulus to obtain testbench verification resultsstimulus must be provided to the DUT. Concurrent stimulusblocks are used in testbenches to provide the stimuli. Twomethods are employed: absolute-time stimulus and relative-timestimulus.

A. absolute time stimulus simulation values arespecified relative to simulation time zero.B. relative time stimulus supplies initial values,then waits for an event before retriggering the stimulus.

Both methods can be combined in a testbench, according tothe designer’s needs.

3. Displaying Results displaying results is facilitated by the$display and $monitor keywords in Verilog.

4. Automatic Verification automating the verification oftestbench results is recommended, particularly for largerdesigns. Several methods can be used:

A. Database comparisons: first, a database file containing expected output (‘golden vector’ file) is created. Then, simu-lation outputs are captured and compared to the reference vectors in the golden vector file.

B. Waveform comparison: waveform comparisons can beperformed automatically or manually. The automatic method employs a testbench comparator to compare a golden waveformagainst a testbench output waveform. C. Self-checking Testbenches: a self-checking testbenchchecks expected results against actual results at run time not atthe end of the simulation.

Testbench Environment

Verilog Example Verification

If the simulation succeeds, the following information is shownon the terminal screen:

Design Flow Overview

ArchitectureSpecification

Block

ImplementationSpecification Blk

BehaviorModel

RTL Model

Gate LevelModel

TransistorLevel

Schematic

DesignLibrary

Layout

Concept!

Manufacturing Chip!

synthesis

custom design

Validation of fcn

Validationof Timing, Power, etc.

Validation of Test/Implementation

Challenges of Hardware Design

• Functional Verification What functions do we need? How do we know the design performs all functions we want.

• Equivalence checking Specification = Implementation?

• Test Preparation Is the chip manufactured correctly?

• Performance constraints Timing, power, noise are ok?