Abstract:
Disclosed are methods for utilizing programmable logic devices that contain at least one localized defect. Such devices are tested to determine their suitability for implementing selected designs that may not require the resources impacted by the defect. If the FPGA is found to be unsuitable for one design, additional designs may be tested. The test methods in some embodiments employ test circuits derived from a user&#39;s design to verify PLD resources required for the design. The test circuits allow PLD vendors to verify the suitability of a PLD for a given user&#39;s design without requiring the PLD vendor to understand the user&#39;s design.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/924,365 entitled “A METHOD OF USING PARTIALLY DEFECTIVE PROGRAMMABLE LOGIC DEVICES,” by Zhi-Min Ling et al., filed on Aug. 7, 2001, now U.S. Pat. No. 6,664,808 which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to programmable logic devices, and more particularly to methods for testing and using programmable logic devices that contain minor defects. 
     BACKGROUND OF THE INVENTION 
     Programmable logic devices (PLDs), such as field-programmable gate arrays (FPGAs), are user-programmable integrated circuits that can be programmed to implement user-defined logic circuits. In a typical architecture, an FPGA includes an array of configurable logic blocks (CLBs) surrounded by programmable input/output blocks (IOBs). A hierarchy of programmable routing resources interconnects the CLBs and IOBs. Loading a configuration bitstream into configuration memory cells of the FPGA customizes these CLBs, IOBs, and programmable routing resources. Additional resources, such as multipliers, memory, and application-specific circuits may also be included. 
     PLDs are growing ever larger as vendors attempt to satisfy customer demand for PLDs capable of performing ever more complex tasks. Unfortunately, as chip size increases, so too does the probability of finding a defect on a given chip. The process yield therefore decreases with PLD complexity, making already expensive PLDs still more expensive. 
     PLDS are not design specific, but instead afford users (e.g., circuit designers) the ability to instantiate an almost unlimited number of circuit variations. Not knowing in advance the purpose to which a given PLD will be dedicated places a heavy burden on the quality and reliability of the PLD because PLD vendors must verify the functionality of any feature that might be used. To avoid disappointing customers, PLD manufacturers discard PLDs that include even relatively minor defects. 
     PLD defects can be categorized in two general areas: gross defects that render the entire PLD useless or unreliable, and localized defects that damage a relatively small percentage of the PLD. It has been found that, for some large chips, close to two thirds of the chips on a given wafer may be discarded because of localized defects. Considering the costs associated with manufacturing large integrated circuits, discarding a large percentage of PLD chips has very significant adverse economic impact on PLD manufacturers. 
     SUMMARY 
     The present invention enables PLD manufactures to identify PLDs that, despite some defects, can flawlessly implement selected customer designs. 
     Subsequent to fabrication, the various chips on a given semiconductor wafer are tested for “gross” defects, such as power-supply shorts, that have a high probability of rendering a PLD unfit for any customer purpose. In a test methodology applicable to SRAM-based FGPAs, chips that survive gross testing are subjected to a “readback test” to verify the function of the configuration memory cells. Defect-free chips are subjected to further testing to ensure flawless performance, while chips that exhibit a large number or dense concentration of readback defects are rejected. Chips with relatively few defects are set-aside. as “ASIC candidates” and are subjected to further testing. Unlike the general tests normally performed to verify PLD functionality, in one embodiment the ASIC candidates are subjected to application-specific tests that verify the suitability of each candidate to function with one or more specific customer designs. 
     Some test methods in accordance with embodiments of the invention employ test circuitry derived from a user design to verify PLD resources required for the design. These methods verify the suitability of an FPGA for a given design without requiring an understanding of the design, and therefore greatly reduce the expense and complexity of developing design-specific tests for a user design. Also advantageous, narrowing test scope to those resources required for a given design reduces the time required for testing and increases the number of saleable PLDs. Using test circuits other than the user design to test the resources required for the user design facilitates comprehensive testing without requiring an understanding of the user design. 
     The foregoing test methods will not forestall PLD vendors from selling fully tested, defect-free PLDs. Customers will still require defect-free PLDs to develop customer-specific designs and to bring these designs to market quickly. However, once a customer has a specific design, the aforementioned test procedures can provide reduced-cost PLDs that are physically and functionally identical to the fully functional PLD or PLDs first used to develop the customer-specific design. 
     In accordance with one embodiment of the invention, a customer interested in the potential cost savings associated with recovered PLDs will send an expression of the customer-specific design (e.g., a data file) to the PLD vendor. The vendor will then use the expression to test ASIC candidates in the manner described above. ASIC candidates that are physically and functionally identical to the defect-free PLD first used to instantiate the customer-specific design can then be sold to the customer at reduced cost. 
     This summary does not limit the scope of the invention, as the scope of the invention is defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a flow chart  100  showing one embodiment of the present invention as applied to FPGAs. Subsequent to fabrication, the various chips on a given semiconductor wafer are tested for “gross” defects (step  105 ). 
     FIG. 2 is a block diagram of a conventional FPGA  200  in which is instantiated an illustrative user design. 
     FIG. 3 is a flowchart detailing the design-specific test step  121  of FIG.  1 . 
     FIG. 4 depicts an exemplary net under test  400  and associated signal source  405  and destination circuits  410  and  415 . 
     FIG. 5 depicts an FPGA  500  in which the CLBs, IOBs, and RAM block of the user design of FIG. 2 are shaded. 
     FIGS. 6A through 6D depict portions of circuit designs that can be instantiated on an FPGA to verify RAM-block functionality. 
     FIG. 7 schematically depicts a test circuit  700  adapted to verify the speed performance of a critical path  705  of a hypothetical user design. 
     FIG. 8 depicts a test circuit  800  that can be used in accordance with the invention to perform at-speed functional tests of FPGA resources, including internal memory and routing resources. 
     FIG. 9 depicts a test circuit  900  in accordance with another embodiment of the invention. 
     FIG. 10 depicts a test circuit  1000  in accordance with yet another embodiment of the invention. 
     FIG. 11 schematically depicts counter  1020 A of FIG. 10 (counter  1020 B is identical). 
     FIG. 12 schematically depicts an embodiment of LFSR  1030 A of FIG. 10 (LFSR  1030 B is identical to LFSR  1030 A, except that line /FE-CNT connects instead to line /LD-CNT). 
     FIG. 13 schematically depicts an embodiment of MSB comparator  1050 A of FIG.  10 . 
     FIG. 14 depicts an example of clock generator  1010  of FIG.  10 . 
     FIG. 15 depicts another type of clock generator  1500  for use in the present invention. 
     FIG. 16 depicts an embodiment of signature analyzer  1040  of FIG.  10 . 
     FIG. 17 shows how test circuit  1000  of FIG. 10 can be scaled to test additional resources on a given FPGA. 
     FIG. 18 details MSB comparator  1050 B first introduced in FIG.  10 . 
     FIG. 19 schematically depicts a test circuit  1900  that includes N minor test circuits  1905 ( 1 )- 1905 (N), each comprised of a circuit portion  1000  and an associated MSB comparator (see FIG.  10 ). 
     FIG. 20 schematically depicts a test circuit  2000  that employs M column instances  1910 ( 1 )- 1910 (M) to populate every row and column of a Virtex™ FPGA. 
     FIG. 21 is a schematic diagram of a “slice”  2100 , one of two identical slices that make up an exemplary CLB in the Virtex™ family of devices available from Xilinx, Inc. 
     FIGS. 22A-22D depict four FPGA configurations for instantiating test circuit  2000  of FIG. 20 on an exemplary FPGA  2200 . 
     FIG. 23 (prior art) depicts a pair of sequential storage elements  2305  and  2310  interconnected via an exemplary register transfer path  2315 . 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to programmable logic devices. In the following description, numerous specific details are set forth to provide a more thorough understanding of embodiments of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known features have not been described in detail in order to avoid obscuring the present invention. 
     FIG. 1 is a flow chart  100  showing one embodiment of the present invention as applied to FPGAs. Subsequent to fabrication, the various chips on a given semiconductor wafer are tested for “gross” defects (step  105 ). For purposes of the present disclosure, “gross” defects are defects that have a high probability of rendering a PLD unfit for any customer purpose. Examples of gross defects include power-supply shorts, opens, excessive leakage, defective clock-management circuitry, and significant numbers of defective programmable memory cells. Chips with gross defects are discarded (decision  107 ). Various tests for gross defects are described in chapter 14 of “Application-Specific Integrated Circuits,” by Michael John Sebastian Smith (1997), which is incorporated herein by reference. 
     Chips that survive decision  107  are subjected to a “readback test” to verify the function of the configuration memory cells (decision  109 ). In this step, configuration memory is programmed to include various patterns of configuration data and then read back to verify the correct program states of those cells. In one embodiment, chips are rejected if they have a large number or concentration of defects. The number considered “large” will depend upon the size of the PLD in question and the distribution of the defects, as these parameters determine the likelihood of such defects rendering a PLD useless for instantiating customer designs (also referred to as “user designs”). 
     At decision  109 , defective chips with greater than the maximum allowable number of defects are discarded, chips with no defects are sent on to step  111  for comprehensive testing, and defective chips with a number of defects less than the maximum allowed number are identified as “ASIC candidates” (step  113 ). ASIC candidates are those chips that, though imperfect, may have adequate resources to instantiate some user designs. Other embodiments might separate ASIC candidates based on their likelihood of success at implementing a user design. For example, a device with only one defect might be considered more valuable than a device with five defects. 
     Those PLDs having no identified defects through decision  109  are thoroughly tested to ensure conformance to strict performance criteria. Circuit vendors must verify both speed performance and functionality of each device. Fully functional chips are identified as good devices (step  115 ). These chips are then packaged (step  117 ) and the resulting PLDs are subjected to the same series of tests as were the unpackaged chips, beginning once again at step  105 . The tests are run again to ensure no defects were introduced by or during the packaging process. If a packaged chip is defect free, the process will eventually return to step  115  and the packaged PLD will be binned accordingly and sold to a customer (step  118 ). Although not shown, the conventional test process for PLDs additionally includes speed binning. 
     Chips that are less than fully functional, but that nevertheless survived decisions  107  and  109 , are identified as “ASIC candidates” (step  113 ). Unpackaged ASIC candidates are packaged (step  117 ) and the resulting packaged PLDs are subjected to the same series of tests as were the unpackaged chips, beginning again at step  105 . Each packaged chip may be discarded by decisions  107  or  109 , or may be identified once again as an ASIC candidate (step  113 ). This time, however, the packaged device is binned as a packaged ASIC candidate (step  119 ) to be tested to determine whether, despite imperfections, it will reliably implement a user design. 
     At some time prior to the next step in the illustrated test method, the PLD manufacturer asks for and receives one or more user designs expressed using the appropriate PLD design software (step  120 ). These designs are adapted for use in the type of PLD under test (e.g., a PLD of the same size and pin configuration). A first of these customer designs is then analyzed to create design-specific test circuits to be instantiated on one of the ASIC candidates of step  119 . These test circuits are used to determine whether the ASIC candidate functions with the customer design. Such functionality is not unlikely, as customer designs typically leave a substantial portion of their programmable resources unused, and the defects in the PLD may be limited to these unused portions. Test  121 , a design-specific test, is detailed below. 
     In decision  123 , if the ASIC candidate under test is fully functional with the design of interest, the device is identified as acceptable for use with the particular design (step  125 ). The device is eventually sent to the customer (step  127 ), who then programs the ASIC candidate with the user design used in step  121  (step  129 ). Alternatively, if step  123  shows the design of interest is not fully functional in the selected device, one or more additional user designs may be tried (decision  131 ). The process is finished when for a given ASIC candidate the device is allocated for use with a customer design or the user designs are exhausted. If no suitable design is found, the ASIC candidate may be discarded or saved for testing on later received user designs. An ASIC candidate might be discarded after, for example, ten failed attempts to instantiate different user designs. 
     ASIC candidates allocated to a selected customer design are labeled accordingly to ensure they are not used in applications that may require defective resources. ASIC candidates may also be adapted to reject any but the verified user design. For example, a unique signature, such as a cyclic-redundancy-check (CRC) value of the bitstream for the verified design, may be stored in non-volatile memory on the PLD and used to verify the design. 
     In conventional testing, many PLDs are rejected for having a small number of random defects. Identifying ones of these that may nevertheless function perfectly with specific user designs allows PLD manufacturers and their customers to benefit from the use of PLDs that would otherwise be wasted. PLD manufactures benefit from significantly improved yield, and PLD customers benefit because PLDs suitable for their particular purpose are available at a lower price. Also advantageous from the user perspective, the recovered PLDs are physically and functionally identical to the fully functional PLDs first used to bring their products to market, so there are no engineering resources otherwise required to adapt their product to a new ASIC. The time normally required to adapt a product to a new ASIC is also reduced, allowing customers to move more quickly to a less expensive alternative to fully functional PLDS. 
     Flowchart  100  is illustrative; in practice, the flow may be quite different, with different steps accomplished in different orders and/or at different times. For example, step  121  may be performed using different test equipment than that used to verify “defect-free” PLDS. Moreover, flowchart  100  illustrates the case in which each wafer may provide PLDs and ASIC candidates. In other embodiments, wafers may be dedicated entirely to PLDs or entirely to ASIC candidates. PLD yield and the customer demand for ASIC candidates will be considered in determining the proportion of wafers or chips allocated to ASIC candidates. 
     FIG. 2 is a block diagram of a conventional FPGA  200  in which is instantiated an illustrative user design. FPGA  200  includes a collection of programmable logic, including a plurality of input/output blocks (IOBs)  205 , an array of configurable logic blocks (CLBs)  210 , and a plurality of RAM blocks  215 , all of which may be selectively interconnected via programmable routing resources. CLBs  210  are the primary building blocks and contain elements for implementing customizable gates, flip-flops, and wiring; IOBs  205  provide circuitry for communicating signals with external devices; and RAM blocks  215  allow for synchronous or asynchronous data storage, though each CLB can also implement synchronous or asynchronous RAMS. For a detailed treatment of one FPGA, see the Xilinx advance product specification entitled “Virtex-II 1.5V Field-Programmable Gate Arrays,” DS031-2 (vl.9), Nov. 29, 2001, which is incorporated herein by reference. 
     FIG. 3 is a flowchart detailing the design-specific test step  121  of FIG.  1 . This test advantageously facilitates comprehensive testing without the expense and complexity of developing design-specific tests for a user design. 
     To begin with, software analyzes the user design to identify the resources required for the design (step  300 ). Such resources are depicted in the fictional example of FIG. 2 as the shaded IOBs  205 , CLBs  22 , RAM block  215 , and the interconnect resources  220  used to interconnect them (the remaining interconnect resources are omitted here for clarity). Also included but not shown are the programmable memory cells required to define the user configuration. Step  303  is a readback test to verify the functionality of the configuration bits required for the user design. Software that carries out steps of test methods in accordance with embodiment of the present invention can be stored on any computer-readable medium. Examples of computer-readable media include magnetic and optical storage media and semiconductor memory. 
     Next, the software divides the interconnect specified in the user design into a set of nets (step  305 ), wherein each net is a portion of the interconnect defined between source and destination nodes. A number of sample nets are depicted in FIG. 2 using bold lines. In general, each net includes a single signal source and one or more signal destinations. The collection of nets together includes all signal paths defined by the user design, and the nets used for testing are selected to minimize overlap (that is, minimize the number of wires used to relay signals but not actually used in the design). A FOR loop defined between steps  310 A and  310 B (FIG. 3) defines a test for verifying the integrity of each net. 
     Dividing the interconnect into a collection of nets is only one way to test the interconnect resources associated with a given design. For other methods of testing interconnect resources, see e.g. pending U.S. patent application Ser. No. 09/837,380 filed Apr. 17, 2001 entitled “Providing Fault Coverage of Interconnect in an FPGA,” by Robert W. Wells, et al., which is incorporated herein by reference. 
     Beginning at step  315 , the source element of a given net is configured as a signal generator, and the destination element or elements are configured as signal observers. The signal generator then provides signals to the destination circuits over the net to confirm the functionality of the net. 
     FIG. 4 depicts an exemplary net under test  400  and associated signal source  405  and destination circuits  410  and  415 . Source  405  and circuits  410  and  415  are CLBs in the example, but each element might also be e.g. an IOB or a RAM block. In the example, source  405  is configured as a test-signal generator that produces a sequence of ones and zeros in response to a test clock TCLK. Source  405  includes a flip-flop  420  connected through a look-up table (LUT)  425  to net  400 . Source  405  need not include LUT  425 , but this example assumes the portion of net  400  within the CLB used to instantiate source  405  is a portion of the user design. If the interconnect in the user design instead extended directly from flip-flop  420 , then the preferred simulated net would similarly extend directly from flip-flop  420 . 
     Destination circuits  410  and  415  are configured as test-signal observers. Each includes a respective LUT  425  and flip-flop  420 . Flip-flops  420  are adapted to store signals presented to destinations  410  and  415  over net  400 . To test net  400  (step  320  of FIG.  3 ), signals from source  405  are clocked across net  400  into destination circuits  410  and  415  using a common test clock TCLK. The resulting contents of the flip-flops  420  in destination circuits  410  and  415  are then read back to ensure net  400  passed the correct data. The portions of net  400  that extend within destinations  410  and  415  are preferably the same portions employed by the user design. In the example, the user design includes local routing within destination  410  that conveys a signal to the respective LUT  425  and local routing within destination  415  that conveys a signal to the respective flip-flop  420 . 
     If a net fails to convey the appropriate data at speed, the ASIC candidate fails for the design of interest (FIG. 3, decision  345 ). The process then moves on to the next design, if any (step  131  of FIG.  1 ). The foregoing process continues for each net of the user design until the PLD fails or the entire collection of nets is validated. 
     The programming process used to generate the configuration data defining the various test circuits, including the test-signal generators and observers, typically utilizes design entry software (e.g., synthesis or schematic tools), place-and-route software, and bitstream generation software executed on a personal computer or workstation. The software includes a library of pre-defined circuit “macros” that define the test-signal generator and observer logic functions for each type of programmable block for a given PLD type. The use of “macros” in PLD programming processes is well known. 
     Programmable blocks (e.g., IOBs, CLBs, and RAM) typically include memory elements and local routing (FIG. 21, discussed below, depicts one type of conventional CLB). In verifying the routing path between programmable blocks in a given customer design, it is preferred that the local routing within the blocks be the same local routing used in the customer design. Consequently, the cores used to instantiate test-signal generators and receivers include, where possible, internal routing identical to their counterparts in the customer design. 
     In one embodiment, a library of software macros includes, for each type of logic block, a set of signal generators and observers that includes every possible configuration of local interconnect resources. Providing a test circuit for a net in a customer design then includes selecting test-signal generator and observer library elements whose local interconnect configurations best match the corresponding logic-block configurations in the customer design. 
     Some logic-block functionality may be difficult to test using the types of signal generators and observers described above. It may be difficult, for example, to create a toggle flip-flop that.includes the carry chain resources available in some CLBs. In such cases, the logic blocks that cannot be modeled as a signal generator and/or signal observer are instead instantiated between two other logic blocks, one of which is configured as a test-signal generator, the other of which is configured as a test-signal observer. In this instance, the logic blocks that cannot be modeled as a signal generator or observer become a portion of the net connecting two other logic blocks. 
     Some embodiments of the invention perform further testing of the nets to locate shorts between interconnect resources that might impact a customer design. In one embodiment, for example, each net is tested with neighboring interconnect resources held high (e.g., to a logic one). The test can be repeated with neighboring resources held to a logic zero. A short between the net under test and a neighboring interconnect line will corrupt the data transmitted over the net. 
     Assuming all nets pass, the test procedure of FIG. 3 moves on to test the various IOBs  205  and CLBs  210  employed in the user design (e.g., the shaded IOBs  205  and CLBs  210  of FIG.  2 ). Many methods for testing programmable blocks are known to those skilled in testing programmable logic devices, and all of these may be used. Another method for testing IOBs, CLBs, and other resources involves programming the PLD to include special test circuits that exercise programmable logic. In one embodiment, the resources to be tested are configured to create a counter circuit connected to the address terminals of a linear-feedback shift register (LFSR). The interconnect employed to instantiate the shift registers for test are preferably the same interconnect verified during the foregoing test sequence. 
     LFSRs are cyclic, in the sense that when clocked repeatedly they go through a fixed sequence of states. Consequently, an LFSR that starts with a known set of data will contain a predictable set of data after a given number of clock periods. The fixed states of an LFSR are pseudo-random, with repetition rates that can be of virtually any length. The pseudo-random nature of LFSRs ensures that the internal memory and routing resources used to instantiate them are treated to a large number of permutations, provided that each LFSR is allowed to shift for a significant number of clock periods. 
     In accordance with one embodiment, an LFSR is preset to a known count (e.g., zero) and clocked a known number of times. The resulting count is then compared with a reference. If the resulting count matches the reference, then all of the resources used to implement the test circuit, including the memory and routing resources used to implement the LFSR, are deemed fully functional at the selected clock speed. If, however, the LFSR count does not match the reference, then the test fails. The test can be run at a number of different speeds to determine the maximum clock speed for the device under test. 
     FIG. 5 depicts an FPGA  500  in which the CLBs, IOBs, and RAM block of the user design of FIG. 2 are shaded. Some of these resources are interconnected to form a pair of LFSRs using, in large part, routing resources (nets or portions of nets) verified as good in preceding test steps. The LFSRs can then be run to test their constituent logic resources. If the test fails (decision  333  of FIG.  3 ), then the test rejects the ASIC candidate for use with the present user design and attempts another user design, if any. If the test passes, then the test procedure moves to step  335 , in which any RAM blocks are tested. RAM blocks can be tested using many test procedures. Specific examples employing LFSRs to test FPGA resources are described below in detail. 
     Should the RAM block test of step  335  fail, then the test rejects the PLD for use with the present user design and attempts another user design, if any. If the RAM block passes, the PLD may be subjected to a parametric test that verifies appropriate speed performance for signals traversing and critical paths in the user circuit of interest. (One method for verifying speed performance is discussed below.) If the PLD fails the speed test (decision  345 ), then the test rejects the PLD for use with the present user design and attempts another user design, if any. Otherwise, the ASIC candidate is deemed fit for use with the user&#39;s design, and is consequently allocated for sale to the appropriate customer (step  125 ). 
     FIGS. 6A through 6D (prior art) depict portions of circuit designs that can be instantiated on an FPGA to verify RAM-block functionality. Many other suitable embedded-memory tests are well known to those of skill in the art. 
     FIG. 6A schematically depicts an LFSR circuit  600  adapted to generate a pseudo-random sequence of addresses and data for testing a dual-port RAM block (i.e., a RAM block with ports A and B). For an N-bit LFSR counter, the maximum number of states the counter cycles through before repeating is 2 N −1. The missing state is the all zero state. Because LFSR  600  is to test all RAM addresses, including address zero, LFSR  600  is an N+1 bit LFSR, where N is the width of the RAM address port. 
     If a typical N+1 LFSR counter were used to test a dual-port RAM in this manner, there would be an address collision between ports A and B when all the bits are one (i.e., both ports are addressing the same memory cell). An AND gate  610  and associated circuitry eliminates the “all ones” state to avoid such a collision. LFSRs are discussed in more detail below in connection with FIGS. 8-22. 
     One property of the LFSR counter  600  is that the data in storage element K+1 (i.e., Q [K+1] ) equals the data in storage element K (i.e., Q [K] ) after each clock. Using this property and assigning the RAM address of port A as Q [N:1]  and RAM address of port B as Q [N+1:2] , the address of port A will be the address of port B on the next clock cycle. we can exploit this property with, the following scheme: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Port A 
                 Port B 
                 Port A 
                 Port B 
                 Port A 
                 Port B 
               
               
                 Address 
                 Address 
                 R/W 
                 R/W 
                 Data 
                 Data 
               
               
                   
               
             
             
               
                 Addr-1 
                 Addr-0 
                 Read 
                 Read 
                  Data-1 
                 ˜Data-0 
               
               
                 Addr-1 
                 Addr-0 
                 Write 
                 Write 
                 ˜Data-1 
                  Data-0 
               
               
                 Addr-2 
                 Addr-1 
                 Read 
                 Read 
                  Data-2 
                 ˜Data-1 
               
               
                 Addr-2 
                 Addr-1 
                 Write 
                 Write 
                 ˜Data-2 
                  Data-1 
               
               
                   
               
             
          
         
       
     
     With this scheme of exercising the Block RAM, the contents of each RAM cell is read from and written to from both ports and the data is returned to its initial state. This allows the entire test to be repeated and looped as long as the test circuit remains functional. If any of the contents of the RAM is corrupted, the test circuit is reconfigured. 
     FIG. 6B depicts a data generator  615  for generating RAM test data from the thirteen outputs LOUT [1:13] from LFSR  600  of FIG.  6 A. incidentally, signal designations terminating in an alphanumeric designation between parentheses identify the figure to or from which the signal line extends. In FIG. 6B, for example, signal line LOUT[1] ( 6 A) comes from FIG.  6 A. 
     The data DATA_A for address port A is based on the XNOR of LFSR output bits LOUT[1:12], whereas the data DATA_B for address port B is based upon the OR of LFSR output bits LOUT[2:13]. This method allows the expected data to be deterministic: DATA_A and DATA_B are known for any given address. 
     Data generator  615  includes a pair of flip-flops  617  and  619  for storing the complements COMP_A and COMP_B of DATA_A and DATA_B, respectively. Complements COMP_A and COMP_B are used to detect read and write errors. 
     FIG. 6C depicts a RAM block  620  connected to an error detector  625 . On every clock cycle, error detector  625  compares outputs DOA and DOB from Block RAM  620  with respective complementary signals COMP_A and COMP_B from FIG.  6 B. This comparison occurs even if block RAM  620  is not enabled, and thus verifies whether block RAM  620  is enabled. During a read cycle, the data presented to block RAM  620  and the expected output signal are opposite, which ensures that a write cycle did not occur during a read cycle. A latch  630  captures any error condition and provides the resulting error signal on an output line ERR_OUT. The FPGA under test is rejected if RAM block  620 , or any other RAM block associated with the user design of interest, produces an error signal. 
     FIG. 6D depicts token circuitry  640  that can be used to sequentially enable RAM blocks like block  620  for test. With large devices, particularly those with substantial amounts of block RAM (e.g., the Virtex™ V3200E or the V812EM available from Xilinx, Inc.), care should be taken to limit the amount of circuitry active during test, and thus reduce the amount of noise and dynamic current within the device. To this end, token circuitry  640  enables only one RAM block at a time. A low-to-high transition on an input terminal Token_In activates token circuitry  640 . When the Token_In signal is detected, token circuitry  640  enables block LFSR  600 , data generator  615 , and RAM  620 . Token circuitry  640  also generates a RESET pulse that resets the flip-flops of FIGS. 6A and 6B and RAM  620  of FIG.  6 C. The flip-flops of FIGS. 6A-6D for which no reset line is shown are all reset before the test begins. 
     When LFSR  600  completes its count, an end-count signal END_CNT from AND gate  610  disables LFSR  600  and causes token circuit  640  to generate a “Token_Out” signal on a line TOKEN_OUT. Line TOKEN_OUT connects to the TOKEN_IN line of the next block RAM under test, if any. The token is thus passed from one RAM block to the next until all the RAM blocks in the user design are tested. 
     FIG. 7 schematically depicts a test circuit  700  adapted to verify the speed performance of a critical path  705  of a hypothetical user design. Test circuit  705  includes a CLB  710  configured as a toggle flip-flop to generate a test signal for transmission through critical path  705 . CLB  710  conventionally includes a flip-flop  715  and a LUT  720 . Test circuit  700  additionally includes a CLB  725  adapted to receive and store data signals from CLB  710 . CLB  725  includes a flip-flop  730  and a LUT  735 . 
     In this illustrative example, critical path  705  is the path between flip-flop  715  and flip-flop  730 , as indicated using relatively bold lines. The example assumes the critical path in the user design under test includes LUTs  720  and  735 , a pair of CLBs  740  and  745 , and three interconnect subnets  750 . These elements are included in test circuit  705  because critical path  705  is preferably as similar as practical to the critical path in the user design. CLBs  740  and  745  are not necessarily programmed as in the user design, but can instead be programmed to merely pass the test signal (e.g., as buffers). 
     Critical path  705 , and other critical paths, are identified by a conventional static timing analyzer and extracted from the user design of interest. For example, a user design intended to operate at 50 KHz will have a maximum register-to-register delay of about 20 us. The timing analyzer can be used to identify register-to-register paths with delays closest to  20 us and then test these critical paths to ensure they pass both rising and falling edges in under 20 us. In practice, each critical path should be somewhat faster than required, so an appropriate guard band is subtracted from the minimum propagation delay. 
     Having identified the critical paths, test circuit  700  is created in the same manner described above in connection with FIG.  4 . Whereas the test of FIG. 4 determined whether each net was functional, the test associated with test circuit  700  determines whether critical path  705  operates at speed. The speed test may be performed in a number of ways. In one embodiment, for example, flip-flops  715  and  730  are preset to known states and then clocked at the required frequency for a number of periods using a test clock TCLK on a like-named signal line. The state of flip-flop  730  is then read back to verify that the data from flip-flop  715  was received and stored properly. The test can be performed different ways, using different numbers of clocks, toggling the flip-flops on different edges, beginning with different stored data, etc., to ensure proper operation in a variety of circumstances. 
     For a discussion of alternative methods of performance testing PLDs, see U.S. Pat. Nos. 6,075,418 and 6,232,845, both to Kingsley, et al., and the above-incorporated Smith reference. Both Kingsley et al. patents are incorporated herein by reference. Using the test procedures outlined in the Kingsley et al. patents, collections of configurable resources are configured in a loop so that they together form a free-running ring oscillator. The oscillator produces an oscillating test signal in which the period is proportional to the speed of the components in the loop. Many such oscillators can be instantiated on a given PLD to measure speed performance. 
     In some embodiments, ASIC candidates can be tested using the methods and circuits described in Kingsley et al. The resources used in the customer design can be tested for speed. Alternatively, more exhaustive speed testing can be done before or after design-specific testing. In one embodiment, oscillators of the type described by Kingsley et al. are distributed across the PLD to test for speed. Some oscillators may not function at all due to the defects present in ASIC candidates. These oscillators are, in some embodiments, simply disregarded: the other tests outline above ensure the defects do not impact customer designs. 
     In addition to the test described above, ASIC candidates can be subjected to the same types of physical and reliability testing as the equivalent standard PLDs. Holding defective parts to high standards for specific customer designs may be important for encouraging customers to use the type of ASIC candidates identified using the above methods. 
     Once an ASIC candidate is verified for a particular customer design, the PLD manufacturer may want to prevent customers from using the PLD for other designs. One embodiment prevents such use by authenticating the customer design each time the PLD is configured by performing a hash function on the configuration bitstream. The result of the hash function is then compared to a proprietary hash key provided in non-volatile memory by the PLD manufacturer. The PLD will not function if the bitstream does not produce the required hash result. 
     FIGS. 8-22 and the associated text describe methods and test circuits that can be employed to test every programmable block on an FPGA. These methods and circuits are useful for performing the comprehensive testing of step  111  (FIG.  1 ); however, such comprehensive testing is not preferred for the design-specific testing of step  121  (FIGS.  1  and  3 ). The methods and test circuits are therefore limited to include those resources of the user design identified in step  300  (FIG. 3) and as little overhead as possible. Limiting the test-circuit overhead minimizes the possibility of rejecting a defective chip for design-specific use due to defects that have no impact on the specific design. 
     FIG. 8 depicts a test circuit  800  that can be used in accordance with the invention to perform at-speed functional tests of FPGA resources, including internal memory and routing resources. In accordance with an embodiment of the invention, test circuit  800  is instantiated using resources employed by a user design and a minimal amount of additional resources. Test circuit  800  includes a counter  810  having output terminals  820  connected to the address terminals of a RAM array  830  configured as a linear-feedback shift register (LFSR). Test circuit  800  additionally includes a clock generator  835  configured to provide a clock signal on line CLK and a reference register  840  having an output terminal connected to an input terminal of a comparator  850 . LFSR  830  connects to another input terminal of comparator  850  via routing resources  860 . 
     The pseudo-random nature of LFSR  830  ensures that the memory locations within LFSR  830  will be treated to a large number of permutations if LFSR  830  is allowed to shift for a significant number of clock periods. Referring to decision block  870 , if, after some number of clock transitions, the count provided to comparator  850  matches the correct reference number in register  840 , then all of the resources used to implement test circuit  800  are deemed fully functional at the selected clock speed. If, however, the LFSR count does not match the count in register  840 , then the test fails. The test can be run at a number of different speeds to determine the maximum clock speed for a device under test. 
     FIG. 9 depicts a test circuit  900  in accordance with another embodiment of the invention. Test circuit  900  is in some ways similar to test circuit  800  of FIG. 8, like-numbered elements being the same. Test circuit  900  additionally includes a second LFSR  910  also connected to output terminals  820  of counter  810 . LFSR  910  is configured to produce the same output pattern as LFSR  830 , so that the final results and each intermediate value from LFSR  830  and LFSR  910  should match after each clock cycle. As indicated in flowchart  915 , test circuit  900  indicates an error if the output of a synchronous comparator  920  indicates a mismatch after any clock cycle. 
     LFSR  830  is implemented using on-chip memory to test the functionality of that memory. The remaining components, including the various interconnections between LFSR  910 , counter  810 , clock generator  835 , and comparator  850 , can also be implemented using FPGA resources. Thus configured, a passing test sequence on either of test circuits  800  or  900  indicates correct function, at speed, for all of the FPGA resources used in the test. 
     FIG. 10 depicts a test circuit  1000  specifically adapted for testing FPGAs in the Virtex™ family available from Xilinx, Inc., of San Jose, Calif. Test circuit  1000  includes a clock generator  1010  connected to a pair of counters  1020 A and  1020 B and a respective pair of RAM LFSRs  1030 A and  1030 B. Counters  1020 A and  1020 B are connected to respective RAM LFSRs  1030 A and  1030 B via buses  1035 A and  1035 B. Test circuit  1000  additionally includes a signature analyzer  1040  and a most-significant-bit (MSB) comparator  1050 A. These circuit components are externally accessible via a number of input and output pins depicted as flag-shaped circuit nodes. 
     A clock line CLK from clock generator  1010  connects to each of counters  1020 A and  1020 B, LFSRs  1030 A and  1030 B, and MSB comparator  1050 A. A load/count pin  1060  can be externally set to logic zero (e.g., zero volts) to transfer data on a data line D_IN into each of LFSRs  1030 A and  1030 B. As discussed below, the force-error/count signal /FE-CNT on pin  1063  allows an external tester to verify that the test circuit  1000  correctly flags errors. 
     In the depicted embodiment, line D_IN is tied to a logic zero (e.g., zero volts); in other embodiments, line D_IN is externally accessible, allowing an external tester to load desired data patterns into LFSRs  1030 A and  1030 B. Once each of LFSRs  1030 A and  1030 B are loaded, load/count pin  1060  and force-error pin  1063  are set to logic ones, thereby allowing LFSRs  1030 A and  1030 B to count through a pseudo-random sequence. 
     The most-significant bit of LFSR  1030 A connects to each of signature analyzer  1040  and MSB comparator  1050 A via a line MSB_A; the most-significant bit of LFSR  1030 B connects to MSB comparator  1050 A via a line MSB_B. Both LFSRs are configured the same, and should therefore generate the same pseudo-random sequence of most-significant bits. MSB comparator  1050 A compares the most-significant bit of each LFSR after each clock to ensure that both LFSRs step through the same sequence. MSB comparator  1050 A flags any mismatch between MSB_A and MSB_B by providing a logic zero on an external pin  1070 . Such an error indicates that the FPGA resources used to instantiate test circuit  1000  do not function properly at the selected clock frequency. 
     Signature analyzer  1040  is, in one embodiment, a seventeen-bit LFSR counter with a clock terminal connected to line MSB_A. Because LFSR  1030 A runs through a deterministic sequence, line MSB_A should transition between one and zero a certain number of times for a given number of transitions on clock line CLK. Signature analyzer  1040  therefore indicates an error if the count stored in signature analyzer  1040  does not indicate the correct number of signal transitions. (The discussion of FIG. 16 below details one embodiment of signature analyzer  1040 .) 
     Force-error pin  1063  enables an external tester to determine whether test circuit  1000  will flag an error if the output of RAM LFSR  1030 A differs from that of RAM LFSR  1030 B. Such a test is performed by leaving input pin  1063  low when pin  1060  is brought high. LFSR  1030 B will therefore increment, while LFSR  1030 A merely repetitively loads zeros. The respective most-significant bits MSB_A and MSB_B will eventually fail to match, a condition that should result in an error flag (i.e., a logic zero on pin  1070 ). Further, LFSR  1030 A&#39;s failure to count should cause signature analyzer  1040  to contain the wrong count. Pin  1063  can thus be employed to ensure that test circuit  1000  is capable of noting errors. 
     FIG. 11 schematically depicts counter  1020 A of FIG. 10 (counter  1020 B is identical). Counter  1020 A can be any type of synchronous counter, but is configured as an LFSR in the depicted embodiment because LFSRs are very fast and require fewer resources than other counters of comparable size. Counter  1020 A includes four D flip-flops  1100 - 1103  and a 16×1 RAM  1105 . Each address line of RAM  1105  is tied to a logic zero. Thus configured, RAM  1105  acts as a one-bit storage element similar to flip-flops  1100 - 1103 . This configuration is used to ensure that counter  1020 A packs efficiently into Virtex™ FPGAs. 
     Counter  1020 A includes a feedback circuit  1110  with the requisite logic to preset counter  1020 A and to provide feedback signals to flip-flop  1100  so that counter  1020 A operates as an LFSR. Line /LD-CNT can be the same or different from the like-named line of FIG.  10 . Clocking counter  1020 A while line /LD-CNT is a logic zero loads counter  1020 A with zeros; clocking counter  1020 A while line /LD-CNT is a logic one causes counter  1020 A to count through each of the 32 possible combinations of five binary digits. Bus  1035 A conveys these states to LFSR  1030 A. 
     For a detailed description of LFSRs and how to implement them using FPGAs, see the Xilinx application note entitled “Efficient Shift Registers, LFSR Counters, and Long Pseudo-Random Sequence Generators,” by Peter Alfke, XAPP 052 Jul. 7, 1996 (Version 1.1), which is incorporated herein by reference. 
     FIG. 12 schematically depicts an embodiment of LFSR  1030 A of FIG. 10 (LFSR  1030 B is identical to LFSR  1030 A, except that line /FE-CNT connects instead to line /LD-CNT). LFSR  1030 A includes three D flip-flops  1200 - 1202  and two 16×1 RAM arrays  1205  and  1210 . Each address line of RAM arrays  1205  and  1210  is tied to a respective line from counter  1020 A on bus  1035 A (it does not matter which address line connects to which line of bus  1035 A). The remaining line from bus  1035 A connects to respective write-enable and clock-enable terminals of the various flip-flops and RAM arrays via a line CE-WE. The sense of each clock terminal of flip-flops  1200 - 1202  and RAM arrays  1205  and  1210  is inverted to allow the signals from bus  1035 A time to settle before LFSR  1030 A is clocked. 
     RAM arrays  1205  and  1210  each act as sixteen one-bit storage elements. Flip-flops  1200 - 1202  are also one-bit storage elements. LFSR  1030 A is therefore a 105-bit LFSR. 35-bit LFSRs can have billions of states, so LFSR  1030  will not repeat for a very long time, even at relatively high clock frequencies. Consequently, each bit of LFSR  1030 A provides a non-repeating, pseudo-random sequence of ones and zeroes during a test period. The most-significant bit of LFSR  1030 A (or any other bit) is conveyed to MSB comparator  1050  (FIGS. 6 and 10) on line MSB_A for comparison with the same bit from LFSR  1030 B. LFSR  1030 A includes a feedback circuit  1215  with the requisite logic to preset LFSR  1030 A with data from terminal D_IN (e.g., all zeros) and to provide feedback signals to RAM array  1205  so that the RAM arrays and flip-flops operate as an LFSR. 
     FIG. 13 schematically depicts an embodiment of MSB comparator  1050 A of FIG.  10 . Comparator  1050 A includes an XNOR gate  1300  and a D flip-flop  1310 . The Q output of flip-flop  1310  is preset to logic one. Comparator  1050 A compares the logic levels on lines MSB_A and MSB_B. Any mismatch between MSB_A and MSB_M produces a logic zero that is stored in flip-flop  1310  and presented on error line /ERR. As with other gates depicted in the Figures, XNOR gate  1300  may be implemented using FPGA logic resources, as will be understood by those of skill in the art. 
     FIG. 14 depicts an example of clock generator  1010  of FIG.  10 . Clock generator  1010  includes an AND gate  1400 , a non-inverting delay element  1410 , and a buffer  1420 . AND gate  1400  acts as a simple inverter when the clock enable signal CE is a logic one, so that delay element  1410  and AND gate  1400  form a ring oscillator. Delay element  1410  can include any resource that produces a desired delay, a chain of look-up tables (LUTs) and interconnect resources, for example. Clock generator  1010  can be implemented entirely on-chip, and is therefore useful for field diagnostics. In one embodiment implemented on a Virtex™ FPGA, delay element  1410  is a chain of twelve buffers. 
     FIG. 15 depicts another type of clock generator  1500  for use in the present invention. Clock generator  1500  includes a clock-pulse generator  1510  and a buffer  1520 . Clock-pulse generator  1510  uses four external clocks on pins P 1 -P 4 , each with a different amount of delay with respect to one another, to produce an output clock of higher frequency than the external clocks. Thus, for example, four clocks from a 50 MHz tester can be combined to create a single 200 MHz clock on line CLK. A conventional tester typically provides the external clock signals. Some PLDS, such as those of the Virtex™ II family of FPGAs available from Xilinx, Inc., include clock management resources that can be used to develop appropriate test clock signals. 
     FIG. 16 depicts an embodiment of signature analyzer  1040  of FIG.  10 . Signature analyzer  1040  can be any suitable signature analyzer, a circuit for performing a conventional cyclic redundancy check (CRC), for example, but is an LFSR in the embodiment of FIG.  16 . Signature analyzer  1040  includes seventeen D flip-flips  1601 - 1617  (flip-flops  1602 - 1613  are omitted from FIG. 16 for simplicity). An XOR gate  1620  provides the requisite feedback to configure signature analyzer  1040  as an LFSR. Before running a test, signature analyzer  1040  is reset using a set/reset line SR (FIG. 14) connected to each flip-flop. Then, during a subsequent test, signature analyzer  1040  clocks once for each zero-to-one signal transition on line MSB_A from LFSR  1030 A. Because LFSR  1030 A runs through a deterministic sequence, line MSB_A should transition between one and zero a certain number of times for a selected number of transitions on clock line CLK. Signature analyzer  1040  therefore indicates an error if the count stored in signature analyzer  1040  does not correspond to the correct number of signal transitions. The correct number of transitions can be determined by simulating test circuit  1000  (FIG. 10) for the selected number of clock periods. 
     Using a seventeen-bit LFSR assures that the pattern generated by signature analyzer  1040  will not soon repeat. Also, as noted above, LFSRs are useful counters because they are relatively fast and require relatively low overhead. In one embodiment, flip-flops  1601 - 1617  are output flip-flops in the IOBs of an FPGA under test. If necessary, the flip-flops can be placed in unbonded sites so that switching signals do not affect external pins. 
     FIG. 17 shows how test circuit  1000  of FIG. 10 can be scaled to test additional resources on a given FPGA. This scaling is accomplished by duplicating a portion of test circuit  1000 —labeled  1700  in FIG.  17 —as many times as necessary. Portion  1700  includes counters  1020 A and  1020 B and LFSRs  1030 A and  1030 B. One instance of portion  1000  connects MSB comparator  1050 A; the remaining one or more portions  1000  use a slightly different MSB comparator  1050 B, which is described below in connection with FIG.  11 . 
     Each instance of portion  1700  is arranged in series, such that the MSB comparator associated with each downstream instance compares each line MSB_A and MSB_B of that instance with MSB_B of the preceding instance. In the depicted example, MSB comparator  1050 B compares the signals on lines MSB_A and MSB_B of the second instance—labeled MSB_A′ and MSB_B′, respectively—with each other and with MSB_B of the first instance. Any mismatch between or within instances results in an error flag (a logic zero) on error pin  1070 . Incidentally, there need be only one signature analyzer  1040 , because comparator circuits  1050 A and  1050 B ensure that the outputs of each RAM LFSR match. 
     FIG. 18 details MSB comparator  1050 B first introduced in FIG.  10 . MSB comparator  1050 B includes a collection of gates and a flip-flop  1800  preset to logic one. MSB comparator  1050 B compares each line MSB_A′ and MSB_B′ of that instance with MSB_B of the preceding instance. Any mismatch between or within instances results in an error flag (logic zero) on the error line /ERR′ extending from the MSB comparator. Each MSB comparator  1050 B also receives the error signal from a previous MSB comparator on line /ERR. Thus, errors reported by upstream comparators propagate through the test circuit, eventually to be reported on an external pin. 
     FIG. 19 schematically depicts a test circuit  1900  that includes N minor test circuits  1905 ( 1 )- 1905 (N), each comprised of a circuit portion  1000  and an associated MSB comparator (see FIG.  10 ). When test circuit  1900  is instantiated in Virtex™ FPGAS, each minor test circuit  1905 ( 1 -N) occupies four CLBs. Each member of the Virtex™ family includes an array of CLBs with an even number of rows R and an even number of columns C. In one embodiment, test circuit  1900  is extended to fill two columns of a Virtex™ FPGA using R/2 instances. Minor test circuits  1905 ( 1 )- 1905 (N) are collectively termed a “column” instance  1910 . 
     FIG. 20 schematically depicts a test circuit  2000  that employs M column instances  1910 ( 1 )- 1910 (M) to populate every row and column of a Virtex™ FPGA. As discussed above, each member of the Virtex™ family includes an even number C of columns, and each column instance  1910  occupies two columns. Thus, C/2 column instances can be used to populate an entire Virtex™ FPGA. 
     Test circuit  2000  includes a number of circuit elements in common with test circuits  1000  and  1900  of FIGS. 10 and 19, respectively, like-numbered elements being the same. Test circuit  2000  additionally includes circuitry that sequences through each column instance and indicates an error if any column instance generates an error signal on respective error lines /ERR( 1 )-/ERR(M). 
     The additional common circuitry includes a counter  2010 , a column sequencer  2015 , an M-input OR gate  2020 , an M-input AND gate  2025  preceded by a collection of M ones catchers  2027 , an M-input AND gate  2030  followed by a ones catcher  2032 , and output pins  2035  and  2040 . Each ones catcher in collection  2027  is identical to ones catcher  2032 . Column sequencer  2015  can be a conventional one-hot register with a single memory location connected to each column instance  1910 ( 1 ) through  1910 (M) via respective ones of lines  2045 . Counter  2010  counts up to some number of clock cycles sufficient to test one column instance before clocking column sequencer  2015 . Sequencer  2015  cycles a single logic one from one register to the next, thus enabling each column instance in turn. Once each column instance has been tested, column sequencer  2015  stops counting and issues a logic one on line DONE to external pin  2035 . 
     In one embodiment instantiated on a Virtex™ FPGA, each element of the common circuitry is implemented using IOB resources. This advantageously allows the CLBs to be populated using sets of identical circuits. In another embodiment, the common circuitry is implemented using resources external to the FPGA. Yet another embodiment dispenses with counter  2010  and column sequencer  2015 , relying instead on external controls to provide the equivalent functionality. Ones catcher  2032  and identical ones catchers  2027  capture error signals and hold them on external pins  2040  and  1070 . 
     FIG. 21 is a schematic diagram of a “slice”  2100 , one of two identical slices that make up an exemplary CLB in the Virtex™ family of devices available from Xilinx, Inc. All of the terminals to and from slice  2100  are connected to horizontal or vertical interconnect lines (not shown) through which they can be programmably connected to various other components within the FPGA. 
     Slice  2100  includes two 4-input LUTs  2105 A and  2105 B. LUTs  2105 A and  2105 B are each capable of implementing any arbitrarily defined Boolean function of up to four inputs. In addition, each of LUTs  2105 A and  2105 B can provide a 16×1-bit synchronous RAM. Furthermore, the two LUTs can be combined to create a 16×2-bit or 32×1-bit synchronous RAM, or a 16×1-bit dual-port synchronous RAM. 
     Slice  2100  also includes a pair of sequential storage elements  2110 A and  2110 B that can be configured either as edge-triggered D-type flip-flops or as level-sensitive latches. The D inputs can be driven either by LUTs  2105 A and  2105 B or directly from input terminals, bypassing LUTs  2105 A and  2105 B. Each storage element includes an initialization terminal INIT, a reverse-initialization terminal R, an enable-clock terminal EC, and a clock terminal conventionally designated using the symbol “&gt;”. The INIT terminal forces the associated storage element into an initialization state specified during configuration; the reverse-initialization terminal R forces the storage element in the opposite state as the INIT terminal. Terminals INIT and R can be configured to be synchronous or asynchronous, and the sense of each control input can be independently inverted. 
     Configuration memory cells define the functions of the various configurable elements of slice  2100 . An exemplary two-input multiplexer  2125  includes a pair of MOS transistors having gate terminals connected to respective configuration memory cells  2130 . Other configuration memory cells used to define the functions of the remaining programmable elements of slice  2100  are omitted for brevity. The use of configuration memory cells to define the function of programmable logic devices is well understood in the art. 
     A detailed discussion of slice  2100  is not necessary for understanding the present invention, and is therefore omitted for brevity. For a more detailed treatment of the operation of many components within slice  2100 , see the co-pending U.S. patent application Ser. No. 08/786,818 entitled “Configurable Logic Block with AND Gate for Efficient Multiplication in FPGAS,” by Chapman et al., U.S. Pat. No. 5,889,413 entitled “Lookup Tables Which Double as Shift Registers,” by Bauer, and U.S. Pat. No. 5,914,616, entitled “FPGA Repeatable Interconnect Structure with Hierarchical Interconnect Lines,” by Steven P. Young, et al. Each of the foregoing documents is incorporated herein by reference. 
     FIGS. 22A-22D depict four FPGA configurations for instantiating test circuit  2000  of FIG. 20 on an exemplary FPGA  2200 . For simplicity, FPGA  2200  includes an eight by twelve array of CLBs  2110 , each CLB including two slices  2100 . Virtex™ FPGAs are larger than FPGA  2200 , however, the smallest Virtex™ FPGA having a sixteen by twenty-four array of CLBs. 
     When test circuit  2000  is instantiated on a Virtex™ FPGA, each minor test circuit  1905 ( 1 -N)—detailed in FIG.  19 —occupies four CLBs, leaving only a single unused LUT for every four CLBs. Columns of test circuits  1905 ( 1 -N) are connected together to form column instance  1910 ( 1 -M), also detailed in FIG.  19 . An exemplary test circuit  1905 ( 1 ) and an exemplary column instance  1910 ( 3 ) are labeled in FIG.  22 A. The unused LUTs from the test circuits can then be configured to implement other circuits, if desired, such as clock generator  1010 , clock-pulse generator  2210 , XOR gate  920 , OR gate  2020 , AND gate  2025 , and AND gate  2030 . 
     The four configurations of FIGS. 22A-22D together provide 100% test coverage of all CLB flip-flops and LUT RAM. However, populating an FPGA with these test circuits will not test the myriad potential routing paths between registers, conventionally referred to as “register transfer paths.” FIG. 23 (prior art) depicts a pair of sequential storage elements  2305  and  2310  interconnected via an exemplary register transfer path  2315 . Transfer path  2615  includes routing resources and possibly combinatorial and/or sequential logic. Fortunately, verifying the correct operation of the various memory elements also verifies the operation of the register transfer paths used to interconnect those elements. The present invention can therefore be extended to test routing resources. 
     Testing routing resources in accordance with the invention is an iterative process. The test circuits described above can be run a number of times, in each case employing a different set of interconnect resources. Those of skill in the art are familiar with the process of rerouting designs to use different routing resources in an effort to test those resources. 
     Virtex™ FPGAs are fully compliant with the IEEE Standard 1149.1 Test Access Port and Boundary-Scan Architecture, commonly referred to as the “JTAG standard,” or simply “JTAG.” Using JTAG, FPGA resources can be field tested by importing a serial stimulus vector to program the FPGA to include the above-described test circuitry. Then, as described above, the test circuitry can be run at speed to determine whether the resources occupied by the test circuitry function properly at speed. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, the foregoing test describes just a few ways to test programmable resources and circuits instantiated in programmable resources; many other test methods might also be used. Those of skill in testing PLDS can adapt many standard tests for use with the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.