1. Unit V Fault Diagnosis

2. Syllabus
Logical Level Diagnosis – Diagnosis by UUT reduction – Fault Diagnosis for Combinational Circuits – Self-checking design – System Level Diagnosis.

3. What is Fault Diagnosis?
A guess as to what’s wrong with a malfunctioning circuit
Narrows the search for physical root cause
Makes inferences based on observed behavior
Usually based on the logical operation of the circuit
©2005 David Lavo

4. VLSI Fault Diagnosis (in One Slide)
Defective Circuit
Observed
Behavior
Tests
Physical Analysis
Diagnosis Algorithm
Location or
Fault
Diagnosis

5. Two Types of Diagnosis Circuit Partitioning (“Effect-Cause” Diagnosis)
Identify fault-free or possibly-faulty portions
Identify suspect components, logic blocks, interconnects
Model-Based Diagnosis (“Cause-Effect” Diagnosis)
Assume one or more specific fault models
Compare behavior to fault simulations
©2005 David Lavo

6. Circuit Partitioning
Separate known-good portions of circuit from likely areas of failure
Simplest method: identify failing flip-flops
Tester can identify failing flops or outputs
Input cone of logic is suspect
Intersection of multiple cones is highly suspect
Single clock pulse with scan can be used for sequential/functional fails
©2005 David Lavo

7. Back-Tracing Failures

8. Effect-Cause Diagnosis
Reasoning based on observed behavior and expected (good-circuit) functions
Commonly used at system and board-levels
Tries to separate good and suspect areas
Advantage: Simple and general
Disadvantage: Not very precise, often gives no indication of defect mechanism
©2005 David Lavo

9. Cause-Effect Diagnosis
Start from possible causes (fault models), compare to observed effects
A simulator is used to predict behavior of the circuit in the presence of various faults
Match prediction(s) against observed behavior
Advantage: Implicates a mechanism as well as a location
Disadvantage: Can be fooled by unmodeled defects
©2005 David Lavo

10. Cause-Effect Diagnosis…
Behavior Signature
Defective Circuit
Diagnosis Algorithm
Comparison &
Conclusion
Tests
Fault Simulator … … … …
Candidate Signatures

11. Fault Diagnosis Overview
Fault Dictionaries
A fault dictionary is a database of the simulated responses for all faults in faultlist
Used by some diagnosis algorithms for convenience:
Fast: no simulation at time of diagnosis
Self-contained: netlist, simulator, and test set not needed after dictionary creation
Can be very large, however!
©2005 David Lavo
Fault Diagnosis Overview

12. Diagnosis by UUT Reduction

13. Self-Checking Circuits
Most important factors in designing a digital system:
Speed, Cost and Correctness.
Some systems used in
1. medical equipment used in ICUs,
2. aircraft control systems,
3. nuclear reactor control systems,
4. military systems and
5. computing systems used in space missions.
High reliability is of the utmost importance.
DSM technology: Signal Integrity problem

14. Self-Checking Circuits:
Def: Error
An incorrect output caused by a stuck-at fault.
Def: Single Error
An error that affects only a single component value
Def: Multiple Error
An error that affects multiple component values.
The component value affected by an error may
change form 0 to 1, or vice versa.
Def: unidirectional errors
When all components affected by a multiple error
change their values monotonically.

15. Self-Checking Circuit

16. Self-checking scheme Self-Checking scheme:
1. a self-checking functional unit.
2. a self-checking checker.
Self-Checking
functional unit
Inputs X
Outputs Y
...
...
...
...
Self-checking
checker
X: input code space
Y: output code space
Error signal

17. Self-Checking Circuits
During the fault-free operation:
a normal input will produce a normal output.
If an incorrect output is produced due to a fault,
the error should be detected by the self-checking
checker.

18. Self-checking scheme Totally self-checking circuit:
1. no erroneous results go undetected and
2. any fault will be eventually detected.
Partially self-checking circuits:
1. This approach is to restrict the set of faults for
which the circuit has to be checked.
2. They are introduced to provide low-cost error
detection.
3. They may be used in non-critical applications.

19. Self-Checking Checkers
Code-disjoint:
TSC Checker:
With the code-disjoint feature, one may be able to
test if the TSC checker is malfunction.

20. Self-checking scheme

21. Self-checking scheme Fault Secure(FS):
code word input to a faulty circuit must not
produce an incorrect code word output.
Self-testing:
a fault in a circuit must be detected by some
input.

22. Self-checking scheme
Fault Secure(FS):
Self-testing:

23. Self-checking scheme
Totally Self-Checking:
Partially Self-Checking:

24. Self-checking scheme Fault-secure-only circuits:
1. No erroneous results go undetected.
2. However, it is possible that some fault can
never be detected.
Self-testing-only circuit:
1. Any fault can produce undetected errors for a
short time.
2. However, there is a code word input that can
detect the fault.