METHODS FOR ANALYZING SCAN CHAINS, AND FOR DETERMINING NUMBERS OR LOCATIONS OF HOLD TIME FAULTS IN SCAN CHAINS
First Claim
1. A method for analyzing a scan chain of a device under test, comprising:
- setting an environmental variable of the scan chain to a first value, and then shifting a non-constant sequence pattern through the scan chain to determine whether an unexpected toggle pattern is output from the scan chain;
setting the environmental variable to a second value, and then shifting the non-constant sequence pattern through the scan chain to determine whether the non-constant sequence pattern is output from the scan chain at an expected time; and
at least when the scan chain both i) outputs an unexpected toggle pattern at the first value of the environmental variable, and ii) fails to output the non-constant sequence pattern at the second value of the environmental variable, at the expected time, determining a number of possible hold time faults in the scan chain based on results of a first number of scan tests.
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Accused Products
Abstract
In a method for determining a number of possible hold time faults in a scan chain of a DUT, an environmental variable of the scan chain is set to a value believed to cause a hold time fault in the scan chain, and then a pattern is shifted through the scan chain. The pattern has a background pattern of at least n contiguous bits of a first logic state, followed by at least one bit of a second logic state, where n is a length of the scan chain. The number of possible hold time faults in the scan chain can be determined as a difference between i) a clock cycle when the at least one bit is expected to cause a transition at an output of the scan chain, and ii) a clock cycle when the at least one bit actually causes a transition at the output of the scan chain. If a value of the environmental variable at which the scan chain operates correctly can be determined, the location of one or more hold time faults can also be determined.
34 Citations
28 Claims
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1. A method for analyzing a scan chain of a device under test, comprising:
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setting an environmental variable of the scan chain to a first value, and then shifting a non-constant sequence pattern through the scan chain to determine whether an unexpected toggle pattern is output from the scan chain; setting the environmental variable to a second value, and then shifting the non-constant sequence pattern through the scan chain to determine whether the non-constant sequence pattern is output from the scan chain at an expected time; and at least when the scan chain both i) outputs an unexpected toggle pattern at the first value of the environmental variable, and ii) fails to output the non-constant sequence pattern at the second value of the environmental variable, at the expected time, determining a number of possible hold time faults in the scan chain based on results of a first number of scan tests. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18)
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19. A method for analyzing a scan chain of a device under test, comprising:
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setting an environmental variable of the scan chain to a first value, and then shifting a non-constant sequence pattern through the scan chain to determine whether an unexpected toggle pattern is output from the scan chain; setting the environmental variable to a second value, and then shifting the non-constant sequence pattern through the scan chain to determine whether the non-constant sequence pattern is output from the scan chain at an expected time; and when the scan chain both i) outputs an unexpected toggle pattern at the first value of the environmental variable, and ii) outputs the non-constant sequence pattern at the second value of the environmental variable, at the expected time, determining a location of at least one possible hold time fault in the scan chain based on results of a number of scan tests, wherein the second number of scan tests are performed using the first and second values of the environmental variable. - View Dependent Claims (20)
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21. A method for determining a number of possible hold time faults in a scan chain of a device under test, comprising:
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setting an environmental variable of the scan chain to a value believed to cause a hold time fault in the scan chain; and
thenshifting a pattern through the scan chain, the pattern having a background pattern of at least n contiguous bits of a first logic state, followed by at least one bit of a second logic state, where n is a length of the scan chain; and determining the number of possible hold time faults in the scan chain as a difference between i) a clock cycle when the at least one bit is expected to cause a transition at an output of the scan chain, and ii) a clock cycle when the at least one bit actually causes a transition at the output of the scan chain. - View Dependent Claims (22, 23)
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24. A method for determining a location of at least one possible hold time fault in a scan chain of a device under test, comprising:
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alternately setting an environmental variable of the scan chain to a first value and a second value, the first value believed to cause a hold time fault in the scan chain, and the second value believed not to cause a hold time fault in the scan chain; shifting at least one scan pattern into the scan chain while the environmental variable is set to the second value; for each scan pattern shifted into the scan chain, and after shifting a particular scan pattern into the scan chain, temporarily setting the environmental variable to the first value; and
thenclocking the scan chain at least once; and
thenshifting the particular scan pattern out of the scan chain while the environmental variable is set to the second value; on a per-bit basis, comparing each particular scan pattern shifted out of the scan chain to an expected scan pattern, to identify at least one location of a difference in bits; and determining, based on the at least one location of the difference in bits, a location of the at least one possible hold time fault in the scan chain. - View Dependent Claims (25, 26, 27, 28)
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Specification