# Patent

## Title
Method and apparatus for diagnosing an integrated circuit

## Abstract

System and method for diagnosing failures within an integrated circuit is provided. In an embodiment, the apparatus includes a diagnostic cell coupled in series with a buffer chain. The diagnostic cell includes a plurality of logic operators that when activated invert a signal received from the buffer chain. The inversion of the signal from the buffer chain allows the diagnostic cell to determine the location of a failure within an integrated circuit previously determined by a scan chain design for test methodology to contain a failure.

## Background
This application claims the benefit of U.S. Provisional Application Ser. No. 61/163,543, filed on Mar. 26, 2009, entitled Method and Apparatus for Diagnosing an Integrated Circuit, which application is hereby incorporated herein by reference.


TECHNICAL FIELD
The present invention relates generally to a system and method for integrated circuit failure analysis and, more particularly, to a system and method for locating a failure in an integrated circuit that has been identified by a scan chain test.
BACKGROUND
Modern circuit designs incorporate methods and hardware that enable testing of an integrated circuit (IC) upon completion of production. Circuit designers refer to this as design for test (DFT) or design for testability. Upon completion of the IC product, IC testers apply manufacturing tests utilizing the methods and hardware included in the DFT design process. In this manner, IC testers validate that the IC hardware contains no defects that could prevent the IC from functioning as intended.
One DFT technique utilizes scan chains. Scan chains provide a simple way to set and observe every flip-flop in an IC. In order to utilize a scan chain, designers add a special signal called scan enable to an IC design. When the testing process asserts the scan enable signal, every flip-flop in the design connects into a long shift register or scan chain. One input pin provides data to this scan chain, and one output pin connects to the output of the scan chain. Using the chip's clock signal, a pre-determined pattern is entered into the chain of flip-flops, and upon completion of the test, a testing module reads out the state of every flip-flop.
The patterns entered into a scan chain are called test patterns. The state of every flip-flop upon completion of the test is generally referred to as the results or resultant pattern of the test. The testing system compares the results shifted out from the output pins against the expected “good machine” results. Good machine results are the bit patterns that are expected when the IC performed properly. In the event that the results pattern matches or compares to the good machine pattern, the IC performed in the designed manner. In the event that the good machine pattern does not match or miscompares, the IC did not perform in the designed manner, i.e., failed. When an IC test has failed, the unit conducting the test reports that a problem has been found with the IC.
Utilization of scan chains within an IC increases testability and the ability to observe the IC. However, while the scan chain diagnostic method determines when an IC has failed, scan chain diagnostics suffer particular problems when the failure occurs within a delay/buffer chain between flip-flops. Therefore, there is a need for a system and/or method for diagnosing failures within an IC that generally locates the failure and more particularly locates the failures that occur within a delay/buffer chain.
SUMMARY OF THE INVENTION
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by illustrative embodiments of the present invention which provide an apparatus for locating a failure within an integrated circuit, and a method for using the same.
In accordance with one embodiment of the present invention, an apparatus for diagnosing an integrated circuit is provided. The apparatus comprises a plurality of flip-flops, configured for scan chain design for test and a buffer chain having a plurality of buffers, the buffer chain communicatively coupled to a first flip-flop of the plurality of flip flops. The apparatus further comprises a first diagnosis cell, communicatively coupled in series with the buffer chain, configured to receive a buffer chain output signal from a first intermediate buffer in the buffer chain, a first input signal and a second input signal, and produce a diagnostic signal indicating the operational status of an integrated circuit, and a second intermediate buffer configured to receive the diagnostic signal and transmit the diagnostic signal to a second flip-flop of the plurality of flip-flops.
In accordance with another embodiment of the present invention, an apparatus for diagnosing integrated circuit is provided. The apparatus comprises a diagnostic cell comprising a first logic operator coupled to a second logic operator. The first logic operator is configured to receive a first input signal and a second input signal, and in response, produce a first logic operator output signal. The second logic operator is configured to receive a third input signal, and the first logic operator output signal, and in response produce a diagnostic signal indicating the operational status at a given location in the integrated circuit.
In accordance with yet another embodiment of the present invention, a method for diagnosing an integrated circuit is provided. The method comprises testing an integrated circuit using a scan chain test pattern as part of a scan chain design for test methodology, and in the event the test indicates that the integrated circuit does not perform as designed, enabling one or more diagnostic cells at given locations within the integrated circuit. The method continues by re-testing an integrated circuit using the scan chain test pattern, the one or more diagnostic cells selectively inverting an operational signal within the integrated circuit, and identifying the operational status at the given location within the integrated circuit based on the re-test of the integrated circuit.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a diagnostic cell in accordance with an embodiment of the present invention;
FIG. 2 is a truth table corresponding to the diagnostic cell embodiment of the present invention illustrated in FIG. 1;
FIG. 3 is a schematic diagram of a diagnostic cell within a circuit in accordance with another embodiment of the present invention;
FIG. 4 is a schematic diagram of a diagnostic cell within a circuit in accordance with yet another embodiment of the present invention;
FIG. 5 is a schematic diagram of a diagnostic cell within a circuit in accordance with still another embodiment of the present invention; and
FIGS. 6 and 7 are high level flowcharts illustrating operative steps of an embodiment of the present invention.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the presently illustrated embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
With reference now to FIG. 1, there is shown a schematic diagram illustrating a system 100 for diagnosing an integrated circuit (IC). System 100 comprises a buffer 111 and a diagnosis cell 120. The buffer 111 comprises a logic operator of a buffer type normally situated between flip-flops in a circuit in order to drive the signal to the next element in the circuit. The buffer 111 may also delay a signal while other operations are conducted in order to keep elements within a circuit synchronous. The buffer 111 receives a buffer chain signal 101 and transmits the buffer chain signal 101 to the next element in the system.
The diagnosis cell 120 receives the buffer chain signal 101, a control signal 102 and a diagnostic enable signal 103, and in response, the diagnosis cell 120 transmits a diagnostic signal 106. The control signal 102 comprises a signal instructing the diagnostic cell to invert or not invert the buffer chain signal 101. The control signal 102 may be received from a flip-flop within the system or from a primary input. Tying the control signal 102 to a flip-flop allows a tester to select inversion as part of the test pattern entered into the scan chain. A person of ordinary skill in the art will recognize that this value may be set before the test is run or selected in the event the system fails the initial scan chain test.
The diagnostic enable signal 103 comprises an enable value or a disable value set by the user such that a user may enable or disable the diagnosis cell 120 as necessary to diagnose failures within the system. Alternatively, the diagnostic enable signal 103 could be set automatically by test equipment. The diagnosis cell 120 comprises logic operators such that when the diagnostic cell 120 is enabled, the diagnostic signal 106 will comprise a value that is the inverse of the buffer chain signal 101, produced according to the diagnostic cell truth table of FIG. 2. For example, where the control signal 102 comprises an invert value, and the diagnostic enable signal 103 comprises an enable value, the diagnostic signal 106 will be the inverse of the buffer chain signal 101.
In the illustrated embodiment, the diagnosis cell 120 further comprises a first logic operator 121 and a second logic operator 122. The first logic operator 121 receives the control signal 102 and the diagnostic enable signal 103 and transmits the first logic operator signal 105. The first logic operator 121 comprises one or more logic operators such that the first logic operator 121 will produce a first logic operator signal 105 according to the diagnostic cell truth table of FIG. 2. For example, where the control signal 102 comprises an invert value, and the diagnostic enable signal 103 comprises an enable value, the first logic operator signal 105 produced by the first logic operator 121 will comprises an invert value.
The second logic operator 122 receives the buffer chain signal 101 and the first logic operator signal 105 and produces the diagnostic signal 106 in response. The second logic operator 122 comprises one or more logic operators such that the second logic operator 122 will produce the diagnostic signal 106 according to the diagnostic cell truth table of FIG. 2. A person of ordinary skill in the art will understand that the first logic operator 121 and the second logic operator 122 may comprise any logic elements that, with equivalent inputs, produce a diagnostic signal 106 equivalent to that illustrated in the diagnostic cell truth table of FIG. 2. In addition, the first logic operator 121 and the second logic operator 122 may comprise a single logic element or a plurality of logic elements, so long as the result of the logic operations produces an equivalent result.
In an exemplary operative embodiment, when the diagnostic enable signal 103 comprises a disable value, the diagnosis cell 120 allows the buffer chain signal 101 to pass through, such that the diagnostic signal 106 is equivalent to the buffer chain signal 101. When the diagnostic enable signal 103 comprises an enable value, the diagnosis cell 120 allows a user to invert the value of the buffer chain signal 106 based on the control signal 102. In this manner, a user may determine where a failure has occurred within an IC. For example, in the event that a scan chain test has identified a failure in the circuit, inverting the buffer chain signal 101 identifies whether the failure occurred before or after the diagnostic cell 120. In the event that an inversion of the buffer chain signal 101 corrects the resultant pattern in a subsequent scan chain test then the failure is located before the diagnostic cell 120. In the event that an inversion does not correct the resultant pattern in a subsequent scan chain test, the failure is located after the diagnostic cell 120.
Referring now to FIG. 2, there is shown a truth table for the diagnostic cell 120 of FIG. 1. When the diagnostic enable signal 103 comprises a disable value, the diagnostic cell 120 bypasses the buffer chain signal 101, i.e., passes the buffer chain signal through unchanged. Similarly, when the control signal 102 comprises a do not invert value, the diagnostic cell 120 bypasses the buffer chain signal 101. As illustrated in FIG. 2, when the diagnostic cell 120 bypasses the buffer chain signal 101, the diagnostic signal 106 comprises a value equivalent to the buffer chain signal 101. When the diagnostic enable signal 103 comprises an enable value, the control signal 102 comprises an invert value, the diagnostic cell 120 inverts the buffer chain signal 101, thus the diagnostic signal 106 comprises the inverse value of the buffer chain signal 101.
The schematic diagram of FIG. 3 illustrates system 300, an embodiment of the present invention, wherein the diagnosis cell 120 of FIG. 1 is implemented in a larger circuit utilizing scan chain design for test (DFT). A person of ordinary skill in the art will recognize that the elements of system 300 shown and described herein are only a portion of the circuit elements contained within a larger integrated circuit (IC) of which system 300 is a part. Many additional features may be incorporated as part of the invention as the elements shown are for exemplary purposes and are not otherwise meant to be limiting. System 300 comprises a first flip-flop 301, a second flip-flop 302, a third flip-flop 303, a first buffer 310, a second buffer 311, a third buffer 312, a first MUX 331, a second MUX 332, a third MUX 333, and a diagnosis cell 120.
In the illustrated embodiment, the diagnosis cell 120 comprises a first logic operator 121, such as the AND gate shown in FIG. 3, and a second logic operator 122, such as the XOR gate shown in FIG. 3. A person of ordinary skill in the art will understand that the first logic operator 121 and the second logic operator 122 may comprise a combination of one or more logic operators such that the selected logic operators produce a diagnostic signal 325 equivalent to that described in the diagnostic cell truth table of FIG. 2. Additionally, the above examples are provided for illustrative purposes, to further explain applications of the present invention and are not meant to limit the present invention in any manner. Other circuit elements may be used as desired for a given application.
The first logic operator 121 receives a diagnostic enable signal 327 that comprises an enable or disable value, allowing the diagnosis cell 120 to operate in diagnosis mode or function as a pass through element, depending on the status of the diagnostic enable signal 327. The first logic operator 121 is communicatively coupled to the second logic operator 122 as indicated by a first logic operator signal 328. In the illustrated embodiment, the first logic operator signal 328 comprises an invert or do not invert value instructing the second logic operator 122 to invert the buffer chain signal 324. The first logic operator signal 328 is produced in response to the receipt by the first logic operator 121 of the control signal 326, and the diagnostic enable signal 327.
In the illustrated embodiment, the second flip-flop 302 stores the control signal 326. Setting the control signal 326 to a flip-flop in the circuit allows a tester to design the test pattern to include further diagnostics based on the results of the initial test. An invert value clocked into the node represented by the second flip-flop 302 allows use of the initial test pattern to more particularly locate the failure within the IC. In the event that the test pattern returns a failing resultant pattern, the test pattern may be clocked into the IC again, and where the bit value in the initial test pattern clocked into the second flip-flop 302 comprises an invert value, the user or testing equipment may more particularly locate a failure by enabling the diagnostic cell 120 without the requirement of additional inputs to control inversion.
In the illustrated embodiment, the flip-flops in the circuit of system 300 are designed to incorporate scan chain DFT and may operate in parallel load or serial shift mode. When the circuit is in operational mode, the flip-flops are configured to operate in parallel, and the values stored in the flip-flops are latched into the next logic element in the system. When the circuit is to undergo testing, the flip-flops are configured to serially shift values into the flip-flops or serially shift values out of the flips-flops of the circuit, thus allowing the circuit to be observed at each flip-flop, i.e. during a scan chain test process it is possible to set each input value preceding a logic element and know how each logic element responded to that input.
The first MUX 331 is communicatively coupled to the first flip-flop 301, the second MUX 332 is communicatively coupled to the second flip-flop 302, and the third MUX 333 is communicatively coupled to the third flip-flop 303. The first MUX 331 receives a scan status signal 341, a first scan input signal 342a, and a first data input signal 343. The scan status signal 341 comprises a scan enable or a scan disable value instructing the first MUX 331 to include or not include the first flip-flop 301 in a scan chain within the IC, i.e., to communicatively connect the first flip-flop 301 as part of a series so that a pattern may be clocked in or out of the IC, or to communicatively connect the flip-flop 301 so that the IC may operate normally. Based on the scan status signal 341, each respective MUX controls the inclusion of a respective flip-flop in a scan chain.
In the illustrated embodiment, the first scan input signal 342a comprises a primary input, set to an input/output (I/O) pin, allowing system 300 to clock a test pattern into the flip-flops of system 300. The data input signal 343 comprises a logic signal that may be set to other elements (not shown) within the larger IC of which system 300 is a part. The second MUX 332 receives the scan status signal 341, a second scan input signal 342b, and a second data input signal 344. The second scan input signal 342b comprises the output of the first flip-flop 301. The second data input signal 344 comprises a logic signal that may be set to other elements (not shown) within the larger IC of which system 300 is a part.
The third MUX 333 receives the scan status signal 341, a third scan input signal 342c, and a third buffer signal 329. The third scan input signal 342c comprises the output of the second flip-flop 302. As described in more detail below, the first, second, and third MUXs, 331, 332, and 333, respectively, allow for implementation of a scan chain test. A person of ordinary skill in the art will recognize that the scan chain DFT system described herein is illustrated in simplified form for illustrative purposes only and is not intended to otherwise limit the scope of the invention. Other methods and devices may be used to implement a scan chain DFT system and are contemplated by the present invention.
As illustrated in FIG. 3, the first flip-flop 301 is communicatively coupled to the first buffer 310, the first buffer 310 is communicatively coupled to the second buffer 311, and the second buffer 311 is communicatively coupled to the second logic operator 122, shown in FIG. 3 as an XOR gate. The second logic operator 122 is communicatively coupled to the third buffer 312, and the third buffer 312 is communicatively coupled to the third MUX 333 such that the signal 329 produced by the third buffer 312 may be stored in the third flip-flop 303. The first buffer 310, the second buffer 311, and the third buffer 312 delay a signal between the first flip-flop 301 and the third flip-flop 303. Delaying the signal allows the portion of the IC embodied in system 300 to be synchronous with other elements of the larger IC of which system 300 is a part.
In the operative embodiment of system 300, the scan status signal 341 initially comprises a scan enable value. The first MUX 331 receives the scan status signal 341, and selects the first scan input signal 342a to pass through to the first flip-flop 301 in response. Similarly, the second and third MUXs 332 and 333, respectively, receive the scan status signal 341 and, in response, select the second and third scan input signals 342b and 342c, respectively, to pass to the second and third flip-flops 302 and 303, respectively. Thus, when the scan status signal 341 comprises a scan enable value, the first, second, and third MUXs, 331, 332, and 333, respectively, communicatively connect each flip-flop into series as part of a scan chain. When the scan input signal 342a is set to a test pattern, each clock cycle will insert one bit of the test pattern into the scan chain and advance the bit in each flip-flop into the next flip-flop in the scan chain. For example, the bit value clocked into the first flip-flop 301 will advance to the second flip-flop 302, and the bit value in the second flip-flop 302 will advance to the third flip-flop 303. Similarly, the value stored in the third flip-flop 303 will advance to the next flip-flop in the scan chain, or to a primary output or I/O pin as illustrated by signal 346. In this manner a test pattern may be clocked into system 300.
Once a scan chain test pattern has been clocked into the flip-flops of the IC, the scan status signal 341 is set to a disable value. The MUXs respectively select the data input signal to pass to the each respective flip-flop. System 300 is operated for a predetermined number of clock cycles, based on the testing parameters, allowing the system to function in operational mode with each logic element operating based on the value stored in the flip-flop preceding the particular element. A resultant bit value produced by a logic element is stored in the flip-flop following the particular element. For example, in normal operation of system 300, when the scan status signal 341 is set to a disable value, operation of the clock latches the value stored in the first flip-flop 301 into the first buffer 310. At each successive clock cycle the logic element functions and latches a value to the next element, first to the second buffer 311, then the second logic operator 122, then the third buffer 312 and finally to the third flip-flop 303. Similarly, the value stored in the second flip-flop 302, and the value stored in the third flip-flop 303 latch into the subsequent element in the IC (not shown).
After the predetermined number of clock cycles, the scan status signal 341 is again set to an enable value communicatively connecting the flips-flops of system 300 into series as described above. At each clock cycle, the bit values stored in the respective flip-flops are advanced to the next flip-flop in the series chain. In the illustrated embodiment, as each bit value is clocked into the third flip-flop 303, it is then clocked out to a primary output as shown by signal line 346, producing a resultant pattern.
The resultant pattern is compared to a good machine pattern, i.e., the resultant pattern that the IC would produce in the event that the IC functioned correctly. Where the resultant pattern compares to the good machine pattern, the IC passed the scan chain test. Where the resultant pattern does not match the good machine pattern, the IC is identified as having a failure. Because each bit value corresponds to a particular flip-flop and logic element or series of logic elements the location of the failure can be traced to a particular logic block by identifying the failed bit value, the flip-flop to which it corresponds, and the logic block that preceded the flip-flop. However, scan chain testing is unable to identify which logic element within a logic block failed. For example, where a failure is diagnosed to exist in a buffer chain such as that embodied by the first buffer 310, the second buffer 311, and the third buffer 312, the scan chain method is unable to particularly identify which particular buffer of the buffer chain failed.
In the illustrated embodiment, in the event that the resultant pattern miscompares to the good machine pattern, thus identifying a failure in the buffer chain, the initial test pattern is clocked into the scan chain again as described above, and the diagnostic enable signal is set to an enable value. At each clock cycle, the values stored in the respective flip-flops sequence through into the next element in the circuit. For example, at a rising clock edge, the value in flip-flop 301 is latched into the first buffer 310, and the value in flip-flop 302 is latched into the diagnosis cell 120 as control signal 326. As described above, control signal 326 will comprise an invert value.
The system 300 functions in operational mode for a predetermined number of clock cycles, and the diagnostic cell 120 inverts the buffer chain signal 324, in the process described above with respect to FIG. 1. The resultant pattern is clocked out and compared to the good machine pattern. Were the failure in the buffer chain to occur in the first buffer 310, or the second buffer 311, the inversion by the diagnostic cell 120 will correct the resultant pattern. Thus, in the event that the resultant pattern matches the good machine pattern, then the failure occurred prior to the diagnostic cell 120 in the first buffer 310 or the second buffer 311.
Were the failure in the buffer chain to occur in the third buffer 312, the inversion by the diagnostic cell 120 will not correct the resultant pattern. Thus, in the event that the resultant pattern does not match the good machine pattern, then the failure occurred after the diagnostic cell 120 in the third buffer 312. A person of ordinary skill in the art will recognize that additional diagnostic cells placed between each buffer or series of buffers will allow more particular identification of a failing buffer. Additionally, a person of ordinary skill in the art will recognize that the use of the diagnostic cell may be expanded to include use along any desired signal path within an integrated circuit.
Referring now to FIG. 4, there is shown system 400, another embodiment of the present invention. System 400 comprises a first flip-flop 401, a second flip-flop 402, a first buffer 410, a second buffer 411, a third buffer 412, a first MUX 431, a second MUX 432 and a diagnosis cell 120. In the illustrated embodiment, the diagnosis cell 120 comprises a first logic operator 121, such as the AND gate shown in FIG. 4, and a second logic operator 122, such as the XOR gate shown in FIG. 4. A person of ordinary skill in the art will understand that the first logic operator 121 and the second logic operator 122 may comprise a combination of one or more logic operators such that the selected logic operators produce a diagnostic signal 429 equivalent to that illustrated in the diagnostic cell truth table of FIG. 2.
The first flip-flop 401, the second flip-flop 402, the first MUX 431, and the second MUX 432 represent portions of a scan chain used in a design for test process similar to that described in FIG. 3.
In the illustrated embodiment, the diagnosis cell 120 comprises a first logic operator 121, such as the AND gate shown in FIG. 4, and a second logic operator 122, such as the XOR gate shown in FIG. 4. A person of ordinary skill in the art will understand that the first logic operator 121 and the second logic operator 122 may comprise a combination of one or more logic operators such that the selected logic operators produce a diagnostic signal 425 equivalent to that described in the diagnostic cell truth table of FIG. 2. Additionally, the above examples are provided for illustrative purposes, to further explain applications of the present invention and are not meant to limit the present invention in any manner. Other circuit elements may be used as desired for a given application.
The first logic operator 121 receives a diagnostic enable signal 427 that comprises an enable or disable value, allowing the diagnosis cell 120 to operate in diagnosis mode or function as a pass through element, depending on the status of the diagnostic enable signal 427. The first logic operator 121 is communicatively coupled to the second logic operator 122 as indicated by a first logic operator signal 428. In the illustrated embodiment, the first logic operator signal 428 comprises an invert or do not invert value instructing the second logic operator 122 to invert the buffer chain signal 424. The first logic operator signal 428 is produced in response to the receipt by the first logic operator 121 of the control signal 426, and the diagnostic enable signal 427.
In the operative embodiment, system 400 of FIG. 4 operates in a manner similar to that of system 300 of FIG. 3, however, the control signal 426 is set to a primary input or I/O pin allowing the control signal to be switched independently of the test pattern, thus giving the tester greater flexibility while conducting the test of the larger IC of which system 400 is a part.
FIG. 5 illustrates system 500, another embodiment of the present invention. System 500 comprises a first flip-flop 501, a second flip-flop 502, a first buffer 510, a second buffer 511, a third buffer 512, a first MUX 531, a second MUX 532, a first diagnosis cell 120a, and a second diagnosis cell 120b. A person of ordinary skill in the art will recognize that the first flip-flop 501, the second flip-flop 502, the first MUX 531, and the second MUX 532 represent portions of a scan chain used in a design for test process similar to that described in FIG. 3. A person of ordinary skill in the art will also recognize that the elements shown are only a portion of the circuit elements contained within an integrated circuit, and that many additional features may be incorporated as part of the invention; the elements shown are for exemplary purposes and are not otherwise meant to be limiting.
In the illustrated embodiment, the first diagnosis cell 120a comprises a first logic operator 121a, such as the AND gate shown in FIG. 5, and a second logic operator 122a, such as the XOR gate shown in FIG. 5. A person of ordinary skill in the art will understand that the first logic operator 121a and the second logic operator 122a may comprise a combination of one or more logic operators such that the selected logic operators produce a first diagnostic signal 524 equivalent to that described in the diagnostic cell truth table of FIG. 2.
The second diagnosis cell 120b comprises a third logic operator 121b, such as the AND gate shown in FIG. 5, and a fourth logic operator 122b, such as the XOR gate shown in FIG. 5. A person of ordinary skill in the art will understand that the third logic operator 121b and the fourth logic operator 122b may comprise a combination of one or more logic operators such that the selected logic operators produce a second diagnostic signal 529 equivalent to that described in the diagnostic cell truth table of FIG. 2.
The first flip-flop 501 is communicatively coupled to the first buffer 510, as shown by signal 522. The first buffer 510 is communicatively coupled to the second logic operator 122a, as shown by signal 523, and the second logic operator 122a is communicatively coupled to the second buffer 511 as shown by a first diagnostic signal 524, such that the first diagnostic signal 524 of the first diagnosis cell 120a latches into the second buffer 511. The second buffer 511 is communicatively coupled, as shown by signal 525, to the fourth logic operator 122b, shown in FIG. 5 as an XOR gate. The fourth logic operator 122b is communicatively coupled to the third buffer 512, and the third buffer 512 is communicatively coupled to the second flip-flop 502.
The first logic operator 121a receives a diagnostic enable signal 527 that comprises an enable or disable value, allowing the first diagnosis cell 120a to operate in diagnosis mode or function as a pass through element, depending on the status of the signal 527. The first logic operator 121a also receives a first control signal 528 that comprises an invert or do not invert value that may be communicatively connected to any primary input.
The third logic operator 121b receives the diagnostic enable signal 527 that comprises an enable or disable value, allowing the second diagnosis cell 120b to operate in diagnosis mode or function as a pass through element, depending on the status of the signal 527. As illustrated in FIG. 5, the first diagnostic cell 120a and the second diagnostic cell 120b may be enabled simultaneously by the diagnostic enable signal 527. In an alternative embodiment, the first diagnostic cell 120a and the second diagnostic cell 120b may be enabled independently by separate diagnostic enable signals. The third logic operator 121b also receives a second control signal 526b that comprises an invert or do not invert value that may be communicatively connected to any primary input.
In the operative embodiment, system 500 of FIG. 5 operates in a manner similar to that of system 300 of FIG. 3, modified as described below, however, the control signals 526a and 526b are set to a primary input or I/O pin allowing the control signals to be switched independently of the test pattern, thus giving the tester greater flexibility while conducting the test of the larger IC of which system 500 is a part.
In the illustrative operative embodiment, in the event that the buffer chain fails the scan chain test, the first diagnosis cell 120a and the second diagnosis cell 120b may be enabled. When the first and second diagnosis cells 120a and 120b, respectively, are enabled, the output of the first and second diagnosis cells 120a and 120b, respectively, may be controlled so that a failure in the IC may be located. In an exemplary operation, the first control signal 526a is set to an invert value and the second control signal 526b is set to a do not invert value. The test pattern is clocked into the flip-flops of the system as described above with respect to FIG. 3, and the system operates for a predetermined number of clock cycles, the first diagnostic cell 120a inverting the buffer chain signal 523, and the second diagnostic cell 120b bypassing the buffer chain signal 525. Following the predetermined number of clock cycles, the resultant pattern is clocked out of the flip-flops as described above with respect to FIG. 3. In the event that the resultant pattern does not miscompare with the good machine pattern, then the failure is located prior to the first diagnostic cell 120a in the first buffer 510.
In the event that the resultant pattern continues to miscompare with the good machine pattern, then the failure is located after the diagnostic cell 120a. The first control signal 526a is set to a do not invert value, and the second control signal 526b is set to an invert value. Again, the test pattern is clocked into the flip-flops of the system as described above with respect to FIG. 3, and the system operates for a predetermined number of clock cycles, the first diagnostic cell 120a bypassing the buffer chain signal 523, and the second diagnostic cell 120b inverting the buffer chain signal 525. In the event that the resultant pattern miscompares with the good machine pattern, the failure is identified to have occurred after the second diagnostic cell 120b in the third buffer 512. In the event that the resultant pattern compares with the good machine pattern, the failure is located prior to the second diagnostic cell 120b in the second buffer 511. Thus, using two diagnostic cells as illustrated in FIG. 5 allows the user to more particularly identify the location of a failure. A person of ordinary skill in the art will recognize that a plurality of diagnostic cells may be used according to the specific needs required by the tester.
The previously described systems operate generally as described with respect to FIG. 6 and FIG. 7. FIG. 6 and FIG. 7 illustrate a high-level flow chart that depicts logical operational steps performed by, for example, system 400 of FIG. 4, which may be implemented in accordance with an embodiment. As indicated at block 605, the process begins when a scan enable signal is applied to the system, communicatively connecting all of the flip-flops of the system into a long scan chain. Next as indicated at block 610, a precreated test pattern is clocked into the scan chain.
The process continues when the scan enable signal is disabled and the system is operated for a predetermined number of clock signals, as indicated at block 615. Normal operation of the system proceeds, as indicated at block 620, by re-enabling the scan enable signal, and clocking out the resultant pattern. As indicated at block 625, the resultant pattern is then compared to the good machine pattern. In the event that the resultant pattern compares to the good machine pattern, then, as indicated at decisional block 630, the process continues on the YES path, where the process ends.
As indicated at decisional block 630, in the event that the resultant pattern miscompares with the good machine pattern, the process continues on the NO path to block 635, where the scan enable signal is applied to the system and the test pattern is clocked into the scan chain again. The system then sets the scan enable signal to disable, and the diagnosis enable signal to enable, as indicated at block 640. The process continues as shown in block 645, where an invert control signal is applied to the diagnosis cell.
The system is operated, as described at block 650, and a resultant pattern is produced in the flip-flops of the system. Next, as indicated at block 655 (FIG. 7), the system re-enables the scan enable signal and clocks out the resultant pattern. The resultant pattern is again compared to the good machine pattern, as indicated at block 660. In the event that the resultant pattern does not compare to the good machine pattern, as shown at decisional block 665, the system continues on the NO path to block 670, where the failure is identified to have occurred after the diagnostic cell. In the event that the resultant pattern compares with the good machine pattern, as indicated at decisional block 665, the process continues on the YES path to block 675, where the failure is identified to have occurred before the diagnostic cell.
A person of ordinary skill in the art will recognize that the apparatus and process described by the present invention will provide a means by which to diagnosis and particularly locate failures within a delay/buffer chain that is compatible with present testing systems, most notably the common automatic test pattern generation (ATPG) systems without hindering the normal operation of the systems.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. An apparatus for diagnosing an integrated circuit comprising:
a plurality of flip-flops arranged to perform scan chain design for test procedures;
a buffer chain having a plurality of buffers separate from the plurality of flip-flops, the buffer chain communicatively coupled to an output of a first flip-flop of the plurality of flip flops;
a first diagnosis cell, communicatively coupled in series with the buffer chain, configured to receive a buffer chain output signal from a first intermediate buffer in the buffer chain, a first input signal, and a second input signal, and produce a diagnostic signal indicating the operational status of the integrated circuit based on at least the buffer chain output signal and the first input signal; and
a second intermediate buffer configured to receive the diagnostic signal and transmit the diagnostic signal to an input of a second flip-flop of the plurality of flip-flops.

2. The apparatus of claim 1, wherein the first diagnosis cell comprises:
a first logic operator communicatively coupled to a second logic operator, the first logic operator configured to receive the first input signal and the second input signal, and in response, produce a first logic operator output signal;
the second logic operator configured to receive the buffer chain output signal and the first logic operator output signal, and in response produce the diagnostic signal.

3. The apparatus of claim 2, wherein the first logic operator and the second logic operator have a first plurality of logic gates configured to pass-through the buffer chain output signal in response to the first input signal and the second input signal and have a second plurality of logic gates configured to invert the buffer chain output signal in response to the first input signal and the second input signal.

4. The apparatus of claim 1, wherein the first input signal comprises a diagnostic enable signal having a diagnosis cell enable state and a diagnosis cell disable state.

5. The apparatus of claim 1, wherein the second input signal comprises a control signal having an invert state and a do not invert state.

6. The apparatus of claim 1, further comprising a third flip-flop of the plurality of flip-flops outputting the second input signal to the first diagnosis cell.

7. The apparatus of claim 1, further comprising a second diagnosis cell communicatively coupled to receive an input signal from a third intermediate buffer in the buffer chain, and produce a second diagnostic signal that drives a fourth intermediate buffer in the buffer chain.

8. An apparatus for diagnosing an integrated circuit comprising:
a diagnostic cell comprising:
a first logic operator coupled to a second logic operator, the first logic operator configured to receive a first input signal and a second input signal, and in response, produce a first logic operator output signal; and
a second logic operator configured to receive a third input signal from an element external to the diagnostic cell and the first logic operator output signal, and in response produce a diagnostic signal indicating an operational status at a given location in the integrated circuit based on at least the second input signal and the third input signal.

9. The apparatus of claim 8, further comprising a first intermediate buffer in a buffer chain outputting the third input signal to an input of the second logic operator in series with the first intermediate buffer, the second logic operator outputting the diagnostic signal to an input of a second intermediate buffer in the buffer chain in series with the second logic operator.

10. The apparatus of claim 8, further comprising a first intermediate circuit element outputting the third input signal to the second logic operator, the second logic operator outputting the diagnostic signal to a second intermediate circuit element.

11. The apparatus of claim 8, further comprising a first flip-flop outputting the second input signal to the first logic operator.

12. The apparatus of claim 11, further comprising a second flip-flop configured to receive the diagnostic signal.

13. The apparatus of claim 8, wherein the first input signal comprises a diagnostic enable signal having a diagnostic cell enable state and a diagnostic cell disable state.

14. The apparatus of claim 8, wherein the second input signal comprises a control signal having an invert state and a do not invert state.

15. The apparatus of claim 8, wherein the first logic operator and the second logic operator comprise a first plurality of logic gates configured to pass-through the third input signal in response to the first input signal and the second input signal, the first logic operator and the second logic operator comprising a second plurality of logic gates configured to invert the third input signal in response to the first input signal and the second input signal.