1. Design for Testability
Sungho Kang
Yonsei University

2. Outline Introduction Testability Measure Design for Testability Ad-Hoc
Testable Design
Conclusion

3. Merging Design and Test
Introduction
Design and Test become closer

4. Design and Test
Introduction
Design process

5. Testability Measures Testability Objective
Inherent property of the circuit based on the circuit topology
Probability that a fault is detected by a random vector
Function of controllability and observability
Controllability
Ability to establish a specific signal value at each node in the circuit by setting values on the circuit inputs
Observability
Ability to determine the signal value at any node in a circuit by controlling primary inputs and observing its outputs
Objective
Estimate testability
Provide guidance for redesign
Provide guidance in search process of test generation
Measures should be computationally simple

6. What Designers Want to Know
Testability
Will this device require an inordinate amount of time, level of effort, and/or test length in order to provide acceptable testing?
Require general information about design
What are the problem areas in the design where a modification can ease the testing problem?
Require detailed information
Testability measures might be useful
Detailed information may be useful to the above question

7. SCOAP Sandia Controllability and Observability Analysis Program
Testability
Sandia Controllability and Observability Analysis Program
Compute relative complexity to control and observe
Combinational Node
Primary input or a combinational standard cell output node
Sequential Node
Output node of a sequential standard cell

8. SCOAP 6 Numbers for Each Node, N
Testability
6 Numbers for Each Node, N
CC0(N): combinational 0-controllability
minimum # of combinational nodes to set node N to 0
CC1(N): combinational 1-controllability
minimum # of combinational nodes to set node N to 1
SC0(N): sequential 0-controllability
minimum # of sequential nodes to justify 0 to node N
SC1(N): sequential 1-controllability
minimum # of sequential nodes to justify 1 to node N
CO(N): combinational observability
# of combinational nodes between N and PO and minimum # of combinational nodes to propagate a signal value on N to PO
SO(N): sequential observability
# of sequential nodes between N and PO and minimum # of sequential nodes to propagate a signal value on N to PO

9. SCOAP Initial Values for PIs and POs Buffers and Inverters PI PO
Testability
Initial Values for PIs and POs
PI PO
CC0(X) 
CC1(X) 
SC0(X) 
SC1(X) 
CO(X)  0
SO(X)  0
Buffers and Inverters
Buffers Inverters
CC0(Y) CC0(X)+1 CC1(X)+1
CC1(Y) CC1(X)+1 CC0(X)+1
SC0(Y) SC0(X) SC1(X)
SC1(Y) SC1(X) SC0(X)
CO(X) CO(Y) CO(Y)+1
SO(X) SO(Y) SO(Y)

10. SCOAP Y = AND (X1, X2) CC0(Y) = min[CC0(X1), CC0(X2)] + 1
Testability
Y = AND (X1, X2)
CC0(Y) = min[CC0(X1), CC0(X2)] + 1
CC1(Y) = CC1(X1) + CC1(X2) + 1
SC0(Y) = min[SC0(X1), SC0(X2)]
SC1(Y) = SC1(X1) + SC1(X2)
CO(X1) = CO(Y) + CC1(X2) + 1
SO(X1) = SO(Y) + SC1(X2)

11. SCOAP Fanout CC1(Y) = CC1(X) CC1(Z) = CC1(X)
Testability
Fanout
CC1(Y) = CC1(X)
CC1(Z) = CC1(X)
CO(X) = min [CO(Y), CO(Z)]

12. SCOAP Example CC0(A) = CC1(A) = CC0(B) = CC1(B) = CC0(C) = CC1(C) = 1
Testability
Example
CC0(A) = CC1(A) = CC0(B) = CC1(B) = CC0(C) = CC1(C) = 1
CC0(D) = min [CC0(A), CC0(B)] + 1 = 2
CC1(D) = CC1(A) + CC1(B) + 1 = 3
CC0(E) = CC0(D) + CC0(C) + 1 = 4
CC1(E) = min [CC1(C), CC1(D)] + 1 = 2
CO(E) = 0
CO(D) = CO(E) + CC0(C) + 1 = 2
CO(C) = CO(E) + CC0(D) + 1 = 3
CO(A) = CO(D) + CC1(B) + 1 = 4
CO(B) = CO(D) + CC1(A) + 1 = 4

13. SCOAP Tree Problem in reconvergent fanout Optimistic Error
Testability
Tree
Good estimate
Problem in reconvergent fanout
Optimistic Error
Want 3/
Pessimistic Error
Want 2/2

14. Limitations of Testability Measures
All testability measure have similar simplifying assumptions so that all results are estimates
Involves the restricted information source (only circuit topology)
Faults which are difficult to test cause problems
Testability data provide a relatively poor indication of whether or not an individual fault will be detected by a given test

15. Design for Testability
Difficulty in ATPG
Not effective for large sequential circuits
Advantages
Test generation is easy
High quality testing
Disadvantages
Area overhead
Timing overhead

16. Classification of DFT Ad-Hoc Design Structured Design Initialization
Design for Testability
Ad-Hoc Design
Initialization
Adding extra test points
Circuit partitioning
Structured Design
Scan design
Scan Path
Level Sensitive Scan Design
Random Access Scan
Boundary Scan
Built-in Self Test

17. Ad Hoc Techniques
Ad-Hoc
Techniques which can be applied to a given product, but are not directed at solving the general problem
Cost is lower than that of structured approaches (Scan, BIST, etc.)
The job of doing test generation and fault simulation are usually not as simple or as straightforward

18. Test Points
Ad-Hoc
Employ test points to enhance controllability and observability
Large demand on extra I/O pins
Example

19. Test Points
Ad-Hoc
Multiplexing monitor points

20. Test Points
Ad-Hoc
Use demultiplexer and latch register to implement control points

21. Test Points
Ad-Hoc
Time sharing I/O ports

22. Test Points Candidates for control points
Ad-Hoc
Candidates for control points
Control, address, and data bus lines on bus structured designs
Enable/hold inputs to microprocessors
Enable and read/write inputs to memory devices
Clock and preset/clear inputs to memory devices
Data select lines to multiplexers and demultiplexers
Control lines on tristate devices
Candidates for observation points
Stem lines associated with signals having lots of fanouts
Global feedback paths
Redundant signal lines
Outputs of logic devices having many inputs
Outputs from state devices
Address, control, and data buses

23. Initialization Design circuits to be easily initializable
Ad-Hoc
Design circuits to be easily initializable
Initialization
Process bringing a sequential circuit into a known state at some known time
Circuits requiring some clever initialization sequence should be avoided
Flip-flop with explicit clear
Use explicit clear to all FFs

24. Oscillators and Clocks
Ad-Hoc
Disable internal oscillators and clocks during test
Example
A = 0 and B : Test input

25. Partitioning
Ad-Hoc
Partitioning shift registers into smaller units

26. Partitioning
Ad-Hoc
Split large counters

27. Partitioning
Ad-Hoc
Partitioning large circuits into small subcircuits to reduce test generation cost
Example
T1=0 T2=0 : Normal Mode
T1=0 T2=1 : Test C1
T1=1 T2=0 : Test C2

28. Avoid Use of Redundant Logic
Design Rule
If a redundant fault occurs, it may invalidate some test for non-redundant faults
Such faults cause difficulty in calculating fault coverage
Much test generation time can be spent

29. Avoid Global Feedback Paths
Design Rule
Provide logic to break global feedback paths
Asynchronous circuits other than latches should be avoided when possible
Avoid combinational feedback loop

30. Avoid Gated Clock Make sure EN settles before CLK changes
Design Rule
Make sure EN settles before CLK changes
Or redesign the circuit as follows

31. Bypass Counters
Design Rule

32. Avoid Internal Pulse Generator
Design Rule
All internal pulse or clock generators should be isolated during test

33. Avoid Cross-coupled NAND/NOR
Design Rule
Add logic to make each cross-coupled NAND/ NOR gate behave as a transparent buffer during test

34. Avoid Bus Floating
Design Rule
Make sure each tristate bus has one pullup register, pulldown register or bus holder

35. Avoid Potential Bus Contention
Design Rule
Make sure only one tristate gate is selected at a time

36. Easily Testable Circuits
Testable Design
Aimed at developing design techniques that start with a functional specification and results that are easy to test
Properties
Small test sets
No redundancy
Tests can be found without much extra work
Tests can be easily generated
Faults should be locatable to the desire degree