# Patent

## Title
Test-per-clock based on dynamically-partitioned reconfigurable scan chains

## Abstract

Aspects of the invention relate to a test-per-clock scheme based on dynamically-partitioned reconfigurable scan chains. Every clock cycle, scan chains configured by a control signal to operate in a shifting-launching mode shift in test stimuli one bit and immediately applies the newly formed test pattern to the circuit-under-test; and scan chains configured by the control signal to operate in a capturing-compacting-shifting mode shift out one bit of previously compacted test response data while compacting remaining bits of the previously compacted test response data with a currently-captured test response to form currently compacted test response data. A large number of scan chains may be configured by the control signal to work in a mission mode. After a predetermined number of clock cycles, a different control signal may be applied to reconfigure and partition the scan chains for applying different test stimuli.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 13/919,974, filed Jun. 17, 2013, which is hereby incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to the field of circuit testing technology. Various implementations of the invention may be particularly useful for scan chain-based testing.
BACKGROUND OF THE INVENTION
Since its introduction in the late 1960's, scan-based testing has gained a wide acceptance as a structured design-for-test (DFT) methodology. This methodology connects memory elements such as flip-flops and latches in a circuit to form scan chains and uses them to make internal nodes of the circuit highly controllable and observable. The controllability and observability enables high-quality automated test generation for large industrial circuit designs.
A commonly used scheme of the scan-based testing is test-per-scan. In a test-per-scan system, a test pattern is first shifted into scan chains and subsequently applied to the circuit-under-test. The test responses generated by the circuit-under-test are then captured by and shifted out of the scan chains for analysis. In this testing scheme, the shifting in and out operations require much more clock cycles to perform than the actual testing (i.e. launching and capturing) operation does. Moreover, in part due to power concerns, the shifting clock is usually kept slower than the clock for the circuit's normal operation. This further increases time for the data loading and unloading operations.
Consider, for example, a circuit design with 100,000 scan cells. These scan cells are divided into 500 scan chains, each 200 scan cells long. Assume the shifting and the normal operating clock frequencies of 50 MHz and 500 MHz, respectively. Applying 20,000 double-capture test patterns requires 4,000,000 shift cycles at 50 MHz and 40,000 capture cycles at 500 MHz. As a result, as low as 1% of cycles, or just 0.1% of time, is spent on the actual testing operation—applying test data and capturing test response data. If a BIST (built-in self-test) method is used for the same circuit design, the test time efficiency could be even lower. With 100K single-capture test patterns, 20,000,000 cycles are needed for scan shifting while only 100,000 cycles are needed for launching and capturing. Using the same clock frequencies as the above example, 99.95% of test time is spent on scan shifting. The above two examples demonstrate that the test-per-scan scheme, though well developed and widely adopted, is not very efficient with respect to testing time.
An alternative scheme, test-per-clock, has been developed mainly for BIST. In a conventional test-per-clock BIST system, the outputs of a test pattern generator are directly coupled to the inputs of the circuit-under-test. Accordingly, a new test pattern is applied to the circuit-under-test at every test clock cycle.
A 1979 paper by Konemann et al., “Built-in logic block observation techniques,” 1979 IEEE Test Conference, which is incorporated herein by reference, describes such a system referred to as BILBO (built-in logic block observer). A BILBO is composed of a flipflop register with additional gates for shift and feedback operations. Four different operational modes can be performed by the BILBO: a mission mode (normal circuit functional mode with scan cells working as latches), a linear shift register mode, a feedback mode, and a reset mode for register resetting. In the feedback mode, the BILBO can work either as a multiple-input signature register (MISR) for compacting test responses or as a linear feedback shift register (LFSR) for generating pseudorandom test patterns. The latter is accomplished by keeping constant values at the parallel inputs of the scan chain.
The 1979 paper describes an example of a test-per-clock architecture with BILBOs working in pairs. One BILBO in a BILBO pair is configured to operate in the feedback mode functioning as a linear feedback shift register. This LFSR BILBO generates and launches a test pattern every test clock cycle. In the meantime, the other BILBO in the pair is configured to operate also in the feedback mode but functioning as a multiple-input signature generator. This MISR BILBO captures a test response every test clock cycle and compacts it with its previously compacted test response to form a new compacted test response. After a number of test clock cycles, a test response signature is eventually shifted out of the MISR BILBO by switching its operational mode from the feedback mode to the linear shift register mode. The unloaded test response signature can then be analyzed.
Another test-per-clock BIST system is described in a 1989 paper by Krasniewski et al., “Circular self-test path: a low cost BIST technique for VLSI circuits,” IEEE Trans. CAD, vol. 8, pp. 46-55, 1989, which is incorporated herein by reference. This system uses a feedback shift register with the last flipflop being supplied to the first flipflop. This shift register serves simultaneously for test pattern generation and test response compaction.
Compared to the test-per-scan scheme, the test-per-clock scheme is more time-efficient because no slow shifting operation is needed for every test pattern. However, the conventional test-per-clock BIST scheme may have a problem on power consumption. In the traditional test-per-clock BIST scheme, all scan chains change their contents every clock cycle no matter whether they are used for pattern generation or for test response compaction. This can lead to excessive circuit toggling and thus power dissipation. Moreover, the traditional test-per-clock scheme has been developed mainly for BIST. It is desirable to develop a new test-per-clock scheme that not only is more time-efficient than the test-per-scan scheme but also addresses the limitations of the existing test-per-clock scheme.
BRIEF SUMMARY OF THE INVENTION
Aspects of the invention relate to a test-per-clock scheme based on dynamically-partitioned reconfigurable scan chains. A test architecture according to various embodiments of the invention comprises scan chains, each of which is reconfigurable to operate, based on a control signal, in one of at least three modes, a shifting-launching mode, a capturing-compacting-shifting mode and a mission mode.
Every clock cycle, scan chains in the shifting-launching mode shift in test stimuli one bit per scan chain and immediately applies the newly formed test pattern to the circuit-under-test. Scan chains in the capturing-compacting-shifting mode shift out one bit of previously compacted test response data every clock cycle while compacting remaining bits of the previously compacted test response data with a currently-captured test response to form currently compacted test response data. The capturing-compacting-shifting mode may keep scan chains from driving the integrated circuit or allow scan chains to drive the integrated circuit. After a predetermined number of clock cycles, a different control signal may be applied to reconfigure and partition the scan chains for new test stimuli.
The test stimuli may be supplied by an ATE (automated test equipment), a test data decompressor driven by an ATE, a PRPG (pseudorandom pattern generator) or any device that can supply test patterns in some form. The compacted test response data that are being shifted out may be collected directly by the same ATE used as the test stimuli source, a test response compactor, or any device that can collect and perhaps analyze test responses.
The test architecture may further comprise a control signal register configured to store the control signal.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example of a test architecture for test-per-clock based on dynamically partitioned and reconfigurable scan chains that may be employed by various embodiments of the invention.
FIG. 2 illustrates a flow chart describing methods of test-per-clock based on dynamically-partitioned configurable scan chains that may be employed by various embodiments of the invention.
FIG. 3 illustrates an example of how the scan chains 121-125 in FIG. 1 interact with each other during testing.
FIG. 4 illustrates an example of a scan cell design that may be used to implement the disclosed test-per-clock scheme.
FIG. 5 illustrates another example of a scan cell design that may be used to implement the disclosed test-per-clock scheme.
FIG. 6 illustrates another example of a scan cell design that may be used to implement the disclosed test-per-clock scheme.



DETAILED DESCRIPTION OF THE INVENTION
Various aspects of the present invention relate to test-per-clock techniques based on dynamically partitioned and reconfigurable scan chains. In the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention may be practiced without the use of these specific details. In other instances, well-known features have not been described in details to avoid obscuring the present invention.
Some of the techniques described herein can be implemented in software instructions stored on a computer-readable medium, software instructions executed on a computer, or some combination of both. Some of the disclosed techniques, for example, can be implemented as part of an electronic design automation (EDA) tool. Such methods can be executed on a single computer or on networked computers.
The detailed description of a method or a device sometimes uses terms like “shift,” “compact,” and “apply” to describe the disclosed method or the device function/structure. Such terms are high-level abstractions. The actual operations or functions/structures that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. It should also be appreciated by one of ordinary skill in the art that the term “coupled” means “connected directly or indirectly.”
Although the operations of the disclosed methods are described in a particular sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangements, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the disclosed flow charts and block diagrams typically do not show the various ways in which particular methods can be used in conjunction with other methods.
FIG. 1 illustrates an example of a test architecture for test-per-clock that may be employed by various embodiments of the invention. The test architecture 100 comprises a test stimuli source 110, scan chains 121-125, a test response collector 130, a configuration register 140 and a configuration source 150. The test stimuli source 110 may be an ATE (automated test equipment), a test data decompressor driven by an ATE, a PRPG (pseudorandom pattern generator), or any device that can supply test patterns in some form. The ATE is commonly used for deterministic testing. The deterministic testing usually employs test patterns generated by ATPG (automatic test pattern generation). To test a large circuit, compressed test patterns are often used. Before being shifted into scan chains, the compressed test patterns are decompressed by the test data decompressor. For non-deterministic testing such as BIST, the PRPG can serve as the test stimuli source.
The test response collector 130 in the test architecture 100 may be the same ATE used as the test stimuli source 110, a test response compactor, or any device that can collect and perhaps analyze test responses shifted out of the scan chains. The test response compactor may compact test responses spatially, temporally, or both. The spatial compaction may be achieved by using a spatial compactor constructed with elementary gates such as AND, OR, NAND and NOR gates. The temporal compaction may be accomplished by using, for example, a multiple-input signature register (MISR).
The test stimuli source 110 and the test response collector 130 described above are similar to those used in a conventional test architecture. The scan chains 121-125 in the test architecture 100, however, are configured differently from conventional scan chains. Unlike conventional scan chains, scan chains used in various embodiments of the invention can operate, depending on a control signal, in one of at least three modes: a shifting-launching mode, a capturing-compacting-shifting mode and a mission mode. In the mission mode, scan chains perform regular circuit functions; in the shifting-launching and capturing-compacting-shifting modes, scan chains are used for testing the circuit. Specifically, scan chains in the shifting-launching mode are responsible for controlling the internal states of the circuit-under-test. They shift test data in and applying them to the circuit-under-test every clock cycle. Moreover, they do not capture any test responses. Scan chains in the capturing-compacting-shifting mode, on the other hand, are responsible for collecting test response data generated by the circuit-under-test. They shift out one bit of a previously compacted test response signal while compacting remaining bits of the previously compacted test response signal with a currently-captured test response signal to form a currently compacted test response signal.
In FIG. 1, the scan chains 121, 123 and 125 are shown to work in the shifting-launching mode, the capturing-compacting-shifting mode and the mission mode, respectively. The control signal for mode switching is generated by the configuration source 150 and applied to the scan chains 121, 123 and 125 by the configuration register 140. FIG. 1 only illustrates one configuration (partition) of the scan chains. Different partitions of the scan chains can be obtained by varying the control signal. The content of the configuration register 140 can be reloaded during a test session, depending on various requirements. In particular, the control signal may be varied with test patterns applied. The configuration source 150 may be an on-chip device or an external device.
As seen in the figure, a majority of the scan chains operate in the mission mode. This arrangement can alleviate power issues because logic states associated with these scan chains closely resemble those when the circuit works in its designed functional mode. Only a small portion of the scan chains may cause extrinsic circuit toggling. In a conventional at-speed scan test, by contrast, a capture clock burst is applied to all scan chains which can result in a sudden current change within a few nanoseconds and thereby circuit failures.
FIG. 2 illustrates a flow chart 200 describing methods of test-per-clock based on dynamically-partitioned configurable scan chains that may be employed by various embodiments of the invention. FIG. 3 illustrates an example of how the scan chains 121-125 in FIG. 1 interact with each other during testing. To simplify the figure, only one scan chain is used to represent scan chains in each operation mode. For ease of understanding, methods of test-per-clock based on dynamically partitioned and reconfigurable scan chains that may be employed according to various embodiments of the invention will be described with reference to the flow chart 200 in FIG. 2 and the test architecture 100 illustrated in FIG. 3. It should be appreciated, however, that alternate implementations of a test architecture may be used to perform the methods of test-per-clock based on dynamically-partitioned configurable scan chains illustrated by the flow chart 200 according to various embodiments of the invention. Likewise, the test architecture 100 may be employed to perform other methods of test-per-clock based dynamically-partitioned configurable scan chains according to various embodiments of the invention.
In operation 210, test stimuli are shifted from a stimuli source (test stimuli source 110) into a first portion of a plurality of scan chains in a circuit (scan chains 121) one bit per scan chain to form a new test pattern as illustrated by an arrow 350. The connections between the test stimuli source 110 and the scan chains 123 and 125 are blocked as illustrated by arrows 370 with broken lines.
Immediately after being formed, in operation 220, the new test pattern is applied to the circuit. The new test pattern propagates through the combinational part of the circuit until a test response reaches a second portion of the plurality of scan chains (scan chains 123) as illustrated by an arrow 310. A circuit response caused by the new test pattern also reaches a third portion of the plurality of scan chains (scan chains 125) as illustrated by an arrow 320. This circuit response will circulate within the circuit and eventually reach the scan chains 123 as illustrated by arrows 330.
In operation 230, the scan chains 123 shift out previously compacted test response data one bit per scan chain to the test response collector 130 as illustrated by an arrow 360. The connections between the test response collector 130 and the scan chains 121 and 125 are blocked as illustrated by arrows 380 with broken lines.
At about the same time as the operation 230, in operation 240, the test response corresponding to the new test pattern is compacted with the previously compacted test response data to generate newly compacted test response data in the scan chains 123. As shown in FIG. 3, the scan chains 125 are also employed to drive the circuit as illustrated by arrows 340. This functionality can be disabled if needed which will be discussed in detail later.
As noted previously, the scan chains 121, 123 and 125 are configured to operate in the shifting-launching mode, the capturing-compacting-shifting mode and the mission mode, respectively, based on the control signal stored in the configuration register 140. The above four operations may be performed once for every clock cycle for a predetermined number of times. At the end, a different control signal may be loaded into the configuration register 140 to reconfigure the scan chains. The reconfigured scan chains are then employed for the next test pattern(s). This dynamic partitioning and reconfiguring approach contrasts not only the conventional scan method but also the BILBO approach discussed in the background section.
Another difference between various embodiments of the present invention and the BILBO-based techniques may lie in the operation of test response compaction. As previously noted, a BILBO scan chain in the feedback mode works as a multiple-input signature generator for compacting test responses: Test responses are captured and compacted for a number of clock cycles (corresponding to the same number of test patterns) to generate a signature. No bit of the compacted test response signal is shifted out during that time period. Instead, the signature is shifted out after a number of test patterns are applied. By contrast, a scan chain in the capturing-compacting-shifting mode according to various embodiments of the invention does not have a feedback loop. Moreover, one bit of previously compacted test response data is shifted out every clock cycle.
By adopting the test-per-clock scheme, various embodiments of the invention remove the lengthy scan shift-in phase used in the test-per-scan scheme and perform launch-capture testing every clock cycle. This allows more test patterns to be applied within a certain period of time and may improve the fault coverage. Alternatively, one can choose to apply the same number of test patterns as that of a conventional scan test, yet in a much shorter period of time, thereby reducing the test cost.
By dynamically partitioning and reconfiguring the scan chains, the disclosed test-per-clock scheme can allow the majority of scan chains to operate in the mission mode to alleviate power issues without significant impact on test generation and fault coverage. The control signal remains static after a given configuration is established. It can be, therefore, placed and routed with no rigid timing constrains similar to those of scan enable signals whose distribution and delivery, especially for the at-speed test purpose, must meet non-flexible timing closure conditions. The low-power capabilities may enable applying test patterns at higher, close to the functional, frequency, which can further increase fault coverage metrics.
Various scan cell designs can be used to implement the disclosed test-per-clock scheme. FIGS. 4-6 are just three examples. The scan cell in FIG. 4 comprises a D-type flip-flop, a 4-input multiplexer 410, an AND gate 430, an NAND gate 440 and an XOR gate 450. Table 1 below defines the four modes of operation controlled by M1 and M2. When the control inputs M1M2 are set to 00, the scan cell enters the emission mode and receives input from the circuit. In the mission mode, scan cells on the same scan chain do not communicate with each other. When the control inputs M1M2 are set to 10, the scan cell is connected with other scan cells in series to work in the shift-launching mode. The output signal of the scan cell drives the circuit while being shifted. When M2 is set to 1, the scan cell enters the capturing-compacting-shifting mode. The inputs from the circuit and the scan cell located before the present scan cell on the scan chain are combined by the XOR gate 450 to serve as the input for the scan cell. Depending on the value of M1, the output of the present scan cell is either blocked (M1=1) from propagating back to the circuit or unblocked (M1=0). FIG. 5 and FIG. 6 illustrate two different scan cell designs that can also be used to implement the disclosed test-per-clock scheme. The corresponding modes of operation are listed in tables 2 and 3, respectively. Among the three examples, the scan cell shown in F the most silicon area friendly.










TABLE 1









Mode











M1
M2















Mission
0
0



Shifting-Launching
1
0



Capturing-Compacting-Shifting W/O Blocking
0
1



Capturing-Compacting-Shifting W/ Blocking
1
1




















TABLE 2









Mode











CE
SE















Mission
1
0



Shifting-Launching
0
1



Resetting
0
0



Capturing-Compacting-Shifting W/ Blocking
1
1




















TABLE 3









Mode











M1
M2















Mission
x
0



Shifting-Launching
1
1



Capturing-Compacting-Shifting W/ Blocking
0
1










CONCLUSION
While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. For example, while specific terminology has been employed above to refer to electronic design automation processes, it should be appreciated that various examples of the invention may be implemented using any desired combination of electronic design automation processes.

## Claims

1. A method, comprising:
shifting test stimuli into a first portion of a plurality of scan chains in a circuit one bit per scan chain to form a new test pattern;
applying the new test pattern to the circuit;
shifting out previously compacted test response data stored in a second portion of the plurality of scan chains one bit per scan chain;
compacting a test response corresponding to the new test pattern with the previously compacted test response data to generate newly compacted test response data in the second portion of the plurality of scan chains; and
repeating the above operations for a predetermined number of times.

2. The method recited in claim 1, further comprising:
applying a control signal to configure some scan chains in the plurality of scan chains to operate in a shifting-launching mode as the first portion of the plurality of scan chains, some other scan chains in the plurality of scan chains to operate in a capturing-compacting-shifting mode as the second portion of the plurality of scan chains, rest of the plurality of scan chains to operate in a mission mode as a third portion of the plurality of scan chains.

3. The method recited in claim 2, wherein the control signal is stored in a register in the circuit.

4. The method of claim 2, wherein the control signal is a first control signal, wherein the applying operation is performed before any of the shifting and compacting operations, and further comprising:
applying a second control signal different from the first control signal to reconfigure at least some of the plurality of scan chains after completion of the repeating operations.

5. The method recited in claim 1, wherein parallel outputs of the second portion of the plurality of scan chains are blocked from driving the circuit.

6. The method of claim 1, wherein the first portion of the plurality of scan chains is blocked from capturing test responses.

7. The method of claim 1, wherein each of the second plurality of scan chains is configured to be free of feedback loops.

8. A non-transitory computer-readable medium storing instructions which, when executed by one or more computers, cause the one or more computers to perform a method comprising:
updating a test pattern by shifting a respective stimulus bit into each of a first group of scan chains of a circuit;
applying the updated test pattern to the circuit;
shifting out a respective bit of previously compacted test response data from each of a second group of scan chains of the circuit;
compacting an updated test response corresponding to the updated test pattern with the previously compacted test response data to produce updated compacted test response data in each of the second group of scan chains; and
repeating the above operations for a predetermined number of clock cycles.

9. The non-transitory computer-readable medium of claim 8, wherein the method further comprises:
by application of a control signal:
configuring the first group of scan chains to operate in a shifting-launching mode;
configuring the second group of scan chains to operate in a capturing-compacting- shifting mode; and
configuring a third group of scan chains to operate in a mission mode, wherein the third group of scan chains comprises all scan chains of the circuit excluding the first and second groups of scan chains.

10. The non-transitory computer-readable medium of claim 9, wherein the method further comprises storing the control signal in a register of the circuit.

11. The non-transitory computer-readable medium of claim 9, wherein the control signal is a first control signal, wherein application of the first control signal is performed before any of the updating and shifting operations, and wherein the method further comprises:
by applying a second control signal different from the first control signal, reconfiguring at least some of the scan chains of the circuit, after completion of the repeating operations.

12. The non-transitory computer-readable medium of claim 8, wherein the method further comprises blocking parallel outputs of scan cells of the second group of scan chains from driving the circuit.

13. The non-transitory computer-readable medium of claim 8, wherein the method further comprises blocking scan cells of the first group of scan chains from capturing test responses.

14. The non-transitory computer-readable medium of claim 8, wherein each of the second group of scan chains is configured to be free of feedback loops.

15. A test-per-clock circuit test system, comprising:
a test stimuli source;
a test response collector;
a non-transitory computer-readable medium storing instructions, which when executed by a processor, cause performance of a method comprising:
updating a test pattern by shifting a respective bit from the test stimuli source into each of a first group of scan chains of a circuit;
applying the updated test pattern to the circuit;
shifting out a respective bit of previously compacted test response data from each of a second group of scan chains of the circuit, into the test response collector;
compacting an updated test response corresponding to the updated test pattern with the previously compacted test response data to produce updated compacted test response data in the second group of scan chains; and
repeating the above operations for a predetermined number of times.

16. The test-per-clock circuit test system of claim 15, wherein the method further comprises:
applying a control signal to configure the first group of scan chains to operate in a shifting-launching mode, to configure the second group of scan chains to operate in a capturing-compacting-shifting mode, and to configure a third group of scan chains to operate in a mission mode, wherein the third group of scan chains comprises all scan chains of the circuit excluding the first and second groups of scan chains.

17. The test-per-clock circuit test system of claim 16, further comprising:
a configuration source;
wherein the method further comprises writing the control signal from the configuration source to a register in the circuit.

18. The test-per-clock circuit test system of claim 16, wherein the control signal is a first control signal, wherein application of the first control signal is performed before any of the updating and shifting operations, and wherein the method further comprises:
reconfiguring at least some of the scan chains of the circuit, after completion of the repeating operations, by application of a second control signal different from the first control signal.

19. The test-per-clock circuit test system of claim 16, wherein parallel outputs of scan chains in the shifting-launching mode are blocked from driving the circuit.

20. The test-per-clock circuit test system of claim 16, wherein scan chains in the capturing-compacting-shifting mode are blocked from capturing test responses.