1 mpd 575 design for testability jonathan weaver

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MPD 575MPD 575Design for TestabilityDesign for Testability

Jonathan Weaver

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Development HistoryDevelopment History

• This material was prepared by Cohort 3 students in the Fall of 2002:– Ron Anger– Jim Gregoire– Guillermo Jimenez– Bob Ognjanovski– Rob Spinks

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Need for TestingNeed for Testing

• High Complexity

• Mass Production

• High cost of replacement in the field

• “The earlier the faulty part is rejected, the cheaper it is”

• Testing is no longer viewed as a “no value add” or “hard to justify” expense

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Need for Testing (Cont.)Need for Testing (Cont.)

• Testing is viewed as an integral part of the manufacturing process

• Customer expectations of “0” PPM

• Increase in customer chargeback to recover all costs associated with “faulty” components

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Problems with TestingProblems with Testing

• Testing can comprise as much as 30% of the cost of building a product

• Testing is difficult and time consuming due to the large number of test steps, that must be applied

• Testing is boring and considered not creative• Designs are completed without testing in

mind

“ Testing is painful, Not Testing is suicidal.”

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Why Testing:Why Testing: Relative cost of finding and fixing errorsRelative cost of finding and fixing errors

0

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Cost ($)

Requirements Design Coding Unit Test Acceptance Maintenance

Error Type

Cost vs. Error Type

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Design for TestabilityDesign for Testability

• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness • Examples of DFT techniques• Heuristics• References

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Related DFXsRelated DFXs

• Design for Inspectability

• Design for Dimensional control

• Design for Serviceability

• Design for Diagnostics

• Design for Modularity

• Design for Reliability

• Design for Robustness

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TestabilityTestability

The IEEE Standard Glossary of Software Engineering Terminology (1990) defines testability as:

"(1) the degree to which a system or component facilitates the establishment of test criteria and the performance of tests to determine whether those criteria have been met, and (2) the degree to which a requirement is stated in terms that permit establishment of test criteria and performance of tests to determine whether those criteria have been met."

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Definitions of DFTDefinitions of DFT– Ability to generate, to evaluate and to

apply tests that improve quality and minimizes time-to-profit

– Extent to which a design can be tested for the presence of manufacturing, base component, system, and/or field defects

– Measure of how easy it is to generate test sets that have a high fault coverage

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Design for TestabilityDesign for Testability– An initiative in the computer hardware industry

in the 1980’s– Objectives:

• Lowers the cost of manufacturing • Minimizes the design engineer's involvement in

production set-up • Improves cross-functional communication and

cooperation among design, engineering, and manufacturing

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Design for Testability Design for Testability (Cont.)(Cont.)

• Testing is more expensive in the short-term but cheaper in the longer-term

• Lowers both production and life-cycle costs • Decreases test times and virtually

eliminates harrowing production delays • Guarantees more efficient diagnosis and

repair in the field • Improves fault coverage

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Design for Testability Design for Testability (Cont.)(Cont.)

• Testability must be engineered into the product at the design stage itself, such that optimal compromise is archived between system maintainability and performance.

• To maximize its impacts, DFT must be performed at all stages of the design –from schematics –to design of subsystems – to system integration

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Test Coverage vs. DFTTest Coverage vs. DFT

AB C

Number of Test Steps

Tes

t C

over

age

(%)

100%

A=Design done with testability in mindB= Design made without Testability in mind by a good fault coverage due to large effort in making test steps

C=Design very difficult to test

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Motivation, Goals, and Motivation, Goals, and Benefits of DFTBenefits of DFT

• Better fault coverage and fault isolation

• Shorter testing time

• Higher quality product

• Shorter time-to-market

• Lower life-cycle cost

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• Introduction to DFT

• Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness• Examples of DFT techniques• Heuristics• References

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Key Principles of DFTKey Principles of DFT• Interfaces that are standard, common, and simple• Accessible points• Automated• Self-test with onboard sensors• Integrated (testing multiple components at the

same time)• Testing in parallel (sweep gauges at the same

time)• Testing one thing verifies many (Traction control

switch checks switch, MUX, cluster,…)

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Key Principles of DFTKey Principles of DFT• Identification of opportunities• Standardization• Simplification of interfaces• Adjustable• Tunable• Diagnostics• Indicators• Procedures• Location

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Key Principles of DFTKey Principles of DFT• Accessibility• Obstruction• Orientation• Visibility• Intuitive• Tools (not specialized)• Ergonomic• Non-destructive• Models/CAE

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• Introduction to DFT • Key Principles in DFT

• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness • Examples of DFT techniques• Heuristics• References

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Perspective of DFTPerspective of DFT

• Keywords in Testability:– Understandability (The more information we have,

the smarter we test)

– Predictability– Observability and Traceability (What we see is

what we test ) – Controllability (The better we can control it , the more

the testing can be optimized)

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Perspective of DFTPerspective of DFT

• Keywords in Testability:– Understandability (The more information we have,

the smarter we test)

– Predictability– Observability and Traceability (What we see is

what we test ) – Controllability (The better we can control it , the more

the testing can be optimized)

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Perspective of DFT (Cont.)Perspective of DFT (Cont.)

• DFT involves modifying the design in such way that maximum controllability and observability are attained.

• DFT is an approach in which the component (SW or HW) is designed from the start such that testing problems do not arise during the product life-cycle

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Evaluation of Component Evaluation of Component Testing CapabilityTesting Capability

Four Levels of testing

• Level 1: Initial– Constructed with ad-hoc testing

mechanism, testing format, and testing functions

– More time in understanding behaviors, debugging, and testing

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Evaluation of Component Evaluation of Component Testing Capability Testing Capability (Cont.)(Cont.)

• Level 2: Standardize– Built to support pre-defined testing

mechanism & testing format– Reduces cost of debugging and testing– Extra programming overhead

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• Level 3: Systematic– Design with a set of systematic testing

mechanics– Easy to monitor and to test the

components– Reduce programming overhead

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• Level 4: Customizable– Design to facilitate the support of the

testing functions & customization– Help to set-up testing for components

based software

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Mechanisms to Increase Mechanisms to Increase Component TestabilityComponent Testability

• Framework-based Testing facility– Well-defined framework (such as class

library) to add test code– Simple and flexible to use– Need component source code

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Mechanisms to Increase Mechanisms to Increase Component Testability Component Testability (Cont.)(Cont.)

• Built-in testing– Need well-defined built-in mechanisms to

add test code– High programming overhead during

component development– No external support needed

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Mechanisms to Increase Mechanisms to Increase Component Testability Component Testability (Cont.)(Cont.)

• Automatic component wrapping for testing

– Component wrapped inside program for testing

– Low programming overhead– Well-defined testing framework to interact

with testing tools

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness • Examples• Heuristics• References

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DFT ProcessDFT Process

1. Evaluate testability of system architecture

2. Define testability requirements and targets

3. Describe testability context

4. Perform testability reviews

5. Define required design changes

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DFT Process (cont.)DFT Process (cont.)

6. Collect experience

7. Define General testing strategy and standards

8. The design is not finished until final testing requirements are defined and accounted for

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process

• DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness• Examples• Heuristics• References

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DFT in Hardware DFT in Hardware DevelopmentDevelopment

• Test generation for large circuits is very time consuming. One way to get around this problem is to constrain or to modify the design in order to make test generation easier.

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• Most DFT techniques are targeted to sequential circuits where test generation is usually a difficult problem

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• If testing is not considered during the design phase, then very low fault coverage and high test generation times can result.

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• The objective of DFT is to improve the controllability and observability of internal circuit nodes so that the circuit can be tested more efficiently and effectively

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Controllability:• Ability to set or to reset internal nodes

from the primary inputs

Observability: • Ability to observe the value of an

internal node at the primary outputs

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• DFT attempts to improve circuit testability by making the internal nodes more controllable and observable

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• Benefits in implementing DFT in HW development:

– Shorter time-to-market– Reduced test time– Less expensive testing equipment– Yield learning, which is often overlooked

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• Sacrifices in implementing DFT in HW development:

– Increased area of components– More pins on printed circuit boards(PCB)– Increased PCB area– Degraded performance on the circuits

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development

• DFT in Software development• DFT, Reliability, and Robustness• Examples of DFT techniques• Heuristics• References

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DFT in Software developmentDFT in Software development

• Most complex modern systems are a blend of Software and Hardware

• Testability analysis of a system is incomplete without adequately accounting for the effect of software

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Challenging Problems in Challenging Problems in Software TestingSoftware Testing

– Software is usually much more complicated than hardware

– Typically, about 40 to 50% of the overall development budget is spent on testing

– Absence of “Known good” response– Lack of testing models, adequate testing

criteria, and testing methods– Software flaws are design flaws

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Software VerificationSoftware Verification

The IEEE Standard Glossary of Software Engineering Terminology (1990) defines software verification to be the “Process of evaluating a system or component to determine whether the products of a given development phase satisfy the conditions imposed at the start of that phase."

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Software TestabilitySoftware Testability

Software testability can be defined as “the probability that a piece of software will fail on its next execution during testing (with a particular assumed input distribution) if the software includes a fault.”

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True ReliabilityTrue Reliability

Software Testability

Software Testing

Formal Verification

Software Code

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development

• DFT, Reliability, and Robustness• Examples of DFT Techniques• Heuristics• References

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DFT, Reliability, and DFT, Reliability, and RobustnessRobustness

• Testability: “A design characteristic allowing the following to be determined with a given confidence, in specified time and condition (noise): location of any faults, whether an item is inoperable, is operable but degraded, and/or is operable”.

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• Reliability - the probability that the System will perform its intended function over time under specific operating conditions

• Reliability - Targets may be set on the commodity or by specific tests used to age the commodity and account for the noise factors.


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• Key Life Testing – A method to demonstrate Reliability and Robustness by combining the primary stresses into one test or a series of tests on the same System.

• Noise Factors – All noise factors should be accounted for in the appropriate testing (ex. DVP)


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• The component/subsystem/system MUST consistently perform its ideal function in the presence of uncontrollable influences (NOISE FACTORS).

• Noise factors MUST be included in testing plans used to demonstrate testability and reliability

• Noise Factors must be identified and linked to Potential Failure Modes and Design Verification Testing Plans to achieve an appropriate robustness using reliability metric(s) to assess the consistent performance of the System design.


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• Functional performance targets should be established during the development of program-specific System Design Specification, P-diagrams, and FMEAs

• Where individual component targets are not available or appropriate, the subsystem or system target will be referenced


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• Targets for both the SOFT and the HARD reliability failures are to be established and to be documented in the component Design Verification Plan (DVP). These testing targets and criteria are to reflect customer expectations for the useful life of the component/subsystem/ system ideal function(s). Use of generic “failure levels” are not acceptable, as they may not sufficiently represent the customer expectations for product reliability

SOFT (degraded performance to an unacceptable level)HARD (product function ceases)

DFT, ReliabilityDFT, Reliability,, and and RobustnessRobustness

Design ValidationDesign Validation

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Any product testing plan MUST include: The range of critical Noise Factors that the

component/subsystem/system will be exposed to during the System Useful Life

Compounded noise factors to create worst-case noise scenarios (i.e. min/max levels of part tolerance (dimension, strength, smoothness) against an extreme set of external noises (i.e. temperature, humidity, user conditions).


Design ValidationDesign Validation

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Testing metrics must include component, subsystem, and system test samples in either:

Key Life Testing (KLT) Test-to-failure (Weibull analysis) Signal-to-Noise Ratio (Taguchi methods for

robustness) Comparative testing (testing against either a

competitors’ or surrogate component) Component, subsystem, and system level testing

(a weak test)

DFT, Reliability, and DFT, Reliability, and Robustness Robustness

Testing MatrixTesting Matrix

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness and Robustness

• Examples of DFT Techniques• Heuristics• References

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Example of DFT Technique:Example of DFT Technique: In-circuit TestIn-circuit Test

• Highly cost-effective test approach• Testing access made with bed-of-nail-fixture• Highly automated• Total nodal access (Test Points) through

devices (i.e. pins, test pads, connectors or vias)

• Verifies the electrical characteristics of each component

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Example of DFT Technique:Example of DFT Technique:Built-In Self-Test (BIST)Built-In Self-Test (BIST)

• Implementation of a different kind of logic in the design so it can test itself

• BIST can be categorized in:– Online BIST: Testing is done while the

system is in normal operation or during idle mode

– Offline BIST: System is brought into a testing mode at predetermined regular intervals

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Example of DFT Technique:Example of DFT Technique:Logical Built-In Self-Test (LBIST)Logical Built-In Self-Test (LBIST)

• Used to test standard cell logic

• A State machine is used to drive pseudo-random vectors into scan chains and then the output of the chain is compressed into a signature value to be scanned out at the end of test

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Example of DFT Technique:Example of DFT Technique:Boundary-ScanBoundary-Scan

• New practical testability tool.• Initiated by Joint European Test Action Group• Provides the ability to develop a test to

exercise all devices pins with a limited amount of effort

• Extra control lines must be added to the device to support the boundary-scan function

• It is intended to check for shorts or open connections between ICs mounted on a circuit board

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Example of DFT Technique: Example of DFT Technique: Automatic Test Pattern Generation (ATPG)Automatic Test Pattern Generation (ATPG)

• Reduce the volume of data needed to test each device to the highest possible coverage

• Unlike functional test vector, ATPG specifically targets structural defects or faults

• Includes Advance Pattern Compression and optimization techniques

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• Advance Pattern Compression: 1) Static Compression Technique (eliminate redundant test from a given pattern-it does not detect new faults) and 2) Dynamic Compression Technique (multiple faults are targeted during test pattern generation itself)

• Pattern Optimization Capability: Order pattern sets from the most effective (highest test coverage) to least effective pattern

Example of DFT Technique:Example of DFT Technique:ATPG (Cont.)ATPG (Cont.)

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Example of DFT Technique:Example of DFT Technique:Full-Scan DesignFull-Scan Design

• All circuits are placed in a Scan chain, and values are scanned before and after each test vector

• Straightforward ATPG problems

• Guarantee high coverage

• High-speed testing

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Example of DFT Technique:Example of DFT Technique:Full-Scan Design (Cont.)Full-Scan Design (Cont.)

• Disadvantages:– Some designs are not able to abide by

design rules in all cases– Area overhead (10-20% additional area

dedicated to testing), routing difficulties– Timing impact– Many testing cycles required on testers

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Example of DFT Technique:Example of DFT Technique:Static Fault AnalysisStatic Fault Analysis

• Used as a rapid means to assess the inherent testability of a system

• Identifies undetectable faults, ambiguity groups, and redundant tests

• Identifies the topological testability limitations of the system, and makes DFT recommendations to overcome them

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Example of DFT Technique:Example of DFT Technique:Testability Engineering and Maintenance System (TEAMS)Testability Engineering and Maintenance System (TEAMS)

• Graphical software tools for diagnostic model development and analysis

• Integrates a unique multi-signal flow graph modeling methodology

• Integrate various analysis techniques for performing testability analysis and design for testability

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Example of DFT Technique:Example of DFT Technique:TEAMS (Cont.)TEAMS (Cont.)

• Examples of problems that TEAMS can solve:– With a given set of tests, can all failures be

detected?– What testing should be used and where

should they be located, so all the faults can be isolated in minimal time and/or cost?

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Example of DFT Technique:Example of DFT Technique:TEAMS (Cont.)TEAMS (Cont.)

• Examples of problems that TEAMS can solve (Cont.):– What is the most efficient sequence of

testing that will isolate all the failures?– What percent of modules, pulled as

“faulty”, are actually OK?– Are all the components within the system

reliable enough to survive the entire mission?

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness and Robustness• Examples of DFT Techniques

• Heuristics• References

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HeuristicsHeuristics

• Prototype designs work, the problems show up later.

• Diagnostics are highly efficient in finding solved problems.

• Murphy’s law applies 95% of the time. The other 5% we are on coffee breaks.

• When all but one wire, in a group, switches, the one will switch too.

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• Worst Case tolerances never add – but when they do they are in our best customer’s machine

• Map your testing strategy and your design approach with respect to inheritance hierarchies

• Make control structures explicit• Don’t squeeze the code

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• The percent of errors (bugs) left after software validation is proportional to the percent of errors found during validation

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Testability Challenges... the Testability Challenges... the Management IssueManagement Issue

• Because DFT is essentially a management issue and not a technology issue, any testability effort must have management's full commitment and support if it is to succeed

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The Testability ChallengeThe Testability Challenge

• Regardless of the trends in system testing capability, the basic challenge for test engineers is not to change the design, but rather to make the designer a believer in testability.

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• Introduction to DFT • Key Principles in DFT• DFT Considerations• DFT Process • DFT in Hardware development• DFT in Software development• DFT, Reliability, and Robustness and Robustness• Examples of DFT Techniques• Heuristics

• References

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ReferencesReferences

http://www.teamqsi.com/: Qualtech System Inc WEB Page.Electronic News: Decreasing the Cost of Testing with Automatic Test Pattern Generation

Integrated Diagnostics Toolset. IEEE Autotest Conference (1997).

Integrated Process for Fault Diagnosis. IEEE Aerospace Conference (1999).

http://www.teamqsi.com/

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ReferencesReferencesS. Deb, K.R. Pattipati, V. Raghavan, M. Shakeri, and R. Shrestha, “Multi-signal Flow Graphs: A Novel

Approach for System Testability Analysis and Fault Diagnosis,” IEEE Aerospace and Electronics Magazine, May 1995, pp. 14-25. (Winner of the Best Technical Paper Award at the 1994 IEEE AUTOTEST Conference, Anaheim, CA, September 1994).

IEEE Standard Glossary of Software Engineering Terminology (1990) http://www.cigitallabs.com/resources/definitions/testability.html

http://www.ate.agilent.com/emt/industry/testabilityguidelines/index.shtml

http://www.cigitallabs.com/resources/definitions/testability.html

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ReferencesReferencesPhillips, Jeffery C., “Essential Testability Guidelines for current Technology.” IEEE

computer society press reprint, Los Alamitos, CA 90720 1993

Pettichord, Bret., “Design for Testability”,

http://www.io.com/~wazmo/papers/design_for_testability_PNSQC.pdf 2002

Illlman, Richard., “Design for testability: separating the myths from reality.http://www.eetimes.com/in_focus/silicon_engineering/OEG20020718S002

5 18-July-2002

Olausson, Mikeal, and Wiklund, Daniel. “Introduction to Design for Testability.”http://www.ida.liu.se/~zebpe/teaching/test/lec6.pdf2001

http://www.io.com/~wazmo/papers/design_for_testability_PNSQC.pdf

http://www.eetimes.com/in_focus/silicon_engineering/OEG20020718S002

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ReferencesReferencesGao, Jerry. “Component Testability and Component Testing Challenges”,

Technical Report http://www.sei.cmu.edu/pacc/cbse2000/papers/18/18.pdf in San Jose State University, in 2000.

Neal, Bob. “Test for Designability.”

Technical Report Agilent Technology

15 February 2003

http://www.home.agilent.com/upload/cmc_upload/All/Bneal_dft_dfd.pdf

http://www.sei.cmu.edu/pacc/cbse2000/papers/18/18.pdf

http://www.home.agilent.com/upload/cmc_upload/All/Bneal_dft_dfd.pdf

1 mpd 575 design for testability jonathan weaver

Documents

testing cont

design stage

testability cont

mind testing

establishment of test

design engineers involvement

test times

test sets