Methods and apparatus for estimating a position of a stuck-at defect in a scan chain of a device under test

1. A method for estimating a position of a stuck-at defect in a scan chain of a device under test, comprising:

as a scan pattern is shifted out of the scan chain, evaluating the scan pattern in real-time for existence of a logic condition;

maintaining a reference to a portion of the scan pattern that is currently being evaluated;

upon identifying the existence of the logic condition when the reference has a predetermined relationship to a stored value, overwriting the stored value using the reference; and

using the stored value to estimate the position of the stuck-at defect in the scan chain.

2. The method of claim 1 , wherein the reference is maintained by updating a count.

3. The method of claim 1 , wherein the logic condition is a logic level.

4. The method of claim 1 , wherein the logic condition is a logic transition.

5. The method of claim 4 , wherein the scan pattern is evaluated for the existence of the logic condition by comparing logic levels of adjacent bits of the scan pattern.

6. The method of claim 4 , further comprising:

reading a value of a final bit of the scan pattern shifted out of the scan chain; and

using the value of the final bit to determine a type of the stuck-at defect.

7. The method of claim 1 , wherein the predetermined relationship is the reference being greater than the stored value.

8. The method of claim 1 , further comprising:

repeating the evaluating, maintaining, identifying and overwriting steps for each of a plurality of scan patterns shifted out of the scan chain; and

initializing a memory holding the stored value, prior to any of the plurality of scan patterns being evaluated for the logic condition, but not between evaluating individual ones of the plurality of scan patterns for the logic condition;

wherein the stored value is used to estimate the position of the stuck-at defect in the scan chain after repeating the evaluating, maintaining, identifying and overwriting steps for each of the plurality of scan patterns.

9. Apparatus for estimating a position of a stuck-at defect in a scan chain of a device under test, comprising:

a control system configured to get or maintain a reference to a portion of a scan pattern that is currently being evaluated by the apparatus;

a memory configured to store a value, the value representing an estimated position of the stuck-at defect;

a comparator configured to assert a control signal when the reference has a predetermined relationship to the value stored in the memory; and

an evaluation circuit configured to i) receive the scan pattern as it is shifted out of the scan chain, ii) evaluate the scan pattern in real-time for existence of a logic condition, and iii) upon identifying the existence of the logic condition when the control signal is asserted, cause the value stored in the memory to be overwritten using the reference.

10. The apparatus of claim 9 , wherein the control system comprises a counter, wherein the reference is a count maintained by the counter, wherein bits in the scan pattern are shifted out of the scan chain in response to a scan clock, and wherein the control system causes the counter to increment the count in response to the scan clock.

11. The apparatus of claim 10 , wherein the control system is configured to reset the counter before each of a plurality of scan patterns is shifted out of the scan chain.

12. The apparatus of claim 9 , wherein the control system is further configured to initialize the memory prior to a plurality of scan patterns being evaluated by the evaluation circuit, but not between the evaluations of individual ones of the plurality of scan patterns.

13. The apparatus of claim 9 , wherein the evaluation circuit comprises a logic AND gate having a first input configured to receive the scan pattern, a second input configured to receive the control signal, and an output coupled to a load input of the memory.

14. The apparatus of claim 9 , wherein the evaluation circuit comprises:

an inverter configured to receive the scan pattern and produce an inverted scan pattern; and

a logic AND having a first input configured to receive the inverted scan pattern, a second input configured to receive the control signal, and an output coupled to a load input of the memory.

15. The apparatus of claim 9 , wherein the evaluation circuit comprises:

a flip-flop configured to receive and output a one-bit delayed version of the scan pattern;

a logic XOR gate having a first input configured to receive the scan pattern, a second input configured to receive the one-bit delayed version of the scan pattern, and an XOR output; and

a logic AND gate having a first input configured to receive the XOR output, a second input configured to receive the control signal, and an output coupled to a load input of the memory.

16. The apparatus of claim 9 , wherein the evaluation circuit comprises:

a delay circuit, configured to receive and output a one-bit delayed version of the scan pattern;

a logic XOR gate having a first input configured to receive the scan pattern, a second input configured to receive the one-bit delayed version of the scan pattern, and an XOR output; and

a logic AND gate having a first input configured to receive the XOR output, a second input configured to receive the control signal, and an output coupled to a load input of the memory.

17. The apparatus of claim 16 , further comprising a control system configured to read a value in the flip-flop after a final bit in the scan pattern has been received by the flip-flop.

18. The apparatus of claim 16 , wherein the counter has a reset input, and wherein the apparatus further comprises a control system configured to i) reset the current bit position, via the reset input, before each of a plurality of scan patterns is shifted out of the scan chain, and ii) read and store a value in the flip-flop after a final bit in the plurality of scan patterns has been received by the flip-flop.

19. The apparatus of claim 9 , wherein the logic condition is a logic level.

20. The apparatus of claim 9 , wherein the logic condition is a logic transition.

21. Apparatus for estimating a position of a stuck-at defect in a scan chain of a device under test, comprising:

means for storing a value representing the position of the stuck-at defect;

means for, as a scan pattern is shifted out of the scan chain, evaluating the scan pattern in real-time for existence of a logic condition;

means for maintaining a reference to a portion of the scan pattern that is currently being evaluated; and

means for, upon identifying the existence of the logic condition when the reference has a predetermined relationship to a stored value, overwriting the stored value using the reference.