Unit III Design for Testability

Syllabus

Design for Testability – Ad-hoc design –

generic scan based design – classical scan based

design – system level DFT approaches.

Validation and Test of Manufactured Circuits

Components of DFT strategy• Provide circuitry to enable test• Provide test patterns that guarantee reasonablecoverage

Goals of Design-for-Test (DFT)Make testing of manufactured part swift andcomprehensive

DFT MantraProvide controllability and observability

Design for Testability

M state regs

N inputs K outputs

K outputsN inputs

Combinational

Logic

Module

Combinational

Logic

Module

(a) Combinational function (b) Sequential engine

2N patterns 2N+M patterns

Exhaustive test is impossible or unpractical

Test Approaches

• Ad-hoc testing• Generic Scan-based Design• Classical scan DesignsProblem is getting harder – increasing complexity and heterogeneous combination of

modules in system-on-a-chip.– Advanced packaging and assembly techniques extend

problem to the board level

Ad Hoc Design for Testability Techniques

• Test points• Initialization• Monostable Multivibrator• Oscillators and clocks• Partitioning of counters and shift registers • Partitioning of large combinational circuits• Logical Redundancy• Global feedback paths

Test PointsMethod of Test Points:

Block 1 Block 2Block 1 is not observable,Block 2 is not controllable

Block 1 Block 2

1- controllability: CP = 0 - normal working mode CP = 1 - controlling Block 2 with signal 1

1

CP

Improving controllability and observability:

Block 1 Block 2

0- controllability: CP = 1 - normal working mode CP = 0 - controlling Block 2

with signal 0

&

CP

OP

OP

Test Points (contd.)Method of Test Points:

Block 1 Block 2Block 1 is not observable,Block 2 is not controllable

Block 1 Block 21

CP1

Improving controllability:

Block 1 Block 2

Normal working mode:CP1 = 0, CP2 = 1

Controlling Block 2 with 1:CP1 = 1, CP2 = 1Controlling Block 2 with 0:CP2 = 0

MUX

CP1

&

CP2

CP2

Normal working mode:CP2 = 0

Controlling Block 2 with 1:CP1 = 1, CP2 = 1Controlling Block 2 with 0:CP1 = 0, CP2 = 1

Monostable Multivibrator

Oscillators and Clocks

Partitioning of Registers

Partitioning of Large Combinational circuits

Partitioning of Large Combinational circuits (contd.)

Generic scan-based Design

• Full Serial Integrated Scan• Isolated Serial Scan• Nonserial Scan

Full Serial Integrated Scan

Isolated Serial Scan

Isolated Serial Scan (contd.)

Non-serial Scan

Level-Sensitive Scan Design (LSSD)

System-Level DFT Approaches

• System-level Busses• System-level Scan paths

System-level Busses

System-level Scan paths