Abstract:
A test pattern generation and comparison circuit creates test pattern stimulus signals for and evaluates response signals from logic or memory such as random access memory (RAM). It utilizes both parallel and serial interfaces to the logic/memory under test. The test pattern generation and comparison circuit further provides a method for testing logic and memory utilizing built-in self test (BIST) techniques. The method uses a programmable logic/memory commands which are translated into physical logic signals and timings for the logic or memory under test. The results of the test pattern generated and applied to the logic or memory are compared to expected results. The result of the comparison is a pass/fail designation. In addition, the comparison of the expected test results with the actual test results provides information on the exact location of the failure. Also, since the test pattern generation and comparison circuit architecture is compatible with hardware description languages such as Verilog HDL or VHDL, the test pattern generation and comparison circuit can be automatically generated with a silicon compiler.

Description:
BACKGROUND OF THIS INVENTION 
     1. Field of this Invention 
     This invention relates to circuits and methods for generating a stimulus signal and evaluating a response signal for testing of logic and memory located on an integrated circuit. More particularly, this invention relates to circuits and methods for generating test pattern signals and evaluating test response signals to verify operation and function of random access memory (RAM) integrated circuits. 
     2. Description of Related Art 
       FIG. 1  shows a typical random access memory (RAM) block diagram. The RAM  150  has address input terminals  141 , data input terminals  142  and timing and control input terminals  143 . The input decode logic  140  consists of address decoders which convert the address input terminals to array selection lines. These array selection lines can select a single memory bit within the RAM memory array  150  of memory cells or bits. The input decode logic also uses the timing and control input terminals  143  to produce electrical signals which facilitate the selection, reading and writing of the required memory bits. This selection of the memory bits is synchronized to timing clocks  143  so as to synchronize the RAM reading or output and the RAM writing or input with an access clock. This access clock synchronization allows capture of data at input terminals at a specified time with respect to the access clock waveform. It also allows presentation the RAM data at an output terminal  160  or memory read results at a specified time with respect to the access clock waveform. 
     The most common technique used currently in automatic test pattern generators is the D-algorithm, which is based on path sensitization. The main idea of path sensitization is to select a path through the combinatorial logic from the site of a potential fault to a primary output. Next, a path is followed through the logic circuit from the site of the potential fault to a primary output of the combinatorial logic, specifying the values along this logic path that are required to propagate the signal value on the faulty line to a primary output. The process of propagating a signal through a circuit is called forward drive. Similarly, the process of determining the primary inputs necessary to produce all of the signals required during the forward drive is called the backward trace. 
     The unique problem of testing sequential logic, which has both combinatorial logic and registers or flip-flops, is solved using scan testing. The idea is to scan in a predefined set of ones and zeros into a set of registers These ones and zeros become the applied inputs to a section or island of combinatorial logic. The results of combination of these inputs through the specified combinatorial logic are captured in output registers. These output registers are connected in a serial chain and can be shifted out serially (scanned out) to allow the testing of the ones and zeros with the expected outputs of the combinatorial section of logic under test. In summary, the D-algorithm is used on the combinatorial islands of logic, which the scan in of the input registers and the scan out of the output registers is used to test the sequential logic designs. 
     The specific example of memory testing, including dynamic random access memory (DRAM) and static random access memory (SRAM) is understood by reviewing the standard march memory test patterns. A march algorithm has several sets of up/down address settings, read/write operations, read/write data values, and different lengths of read/write data values. The objective of march test patterns is to store and read out alternating ones and zeros in the memory array to check for various known types of memory faults. Some of the memory faults that can be tested and located are stuck-at-one or stuck-at-zero faults, address decoder faults, transition from 1 to 0 and from 0 to 1 faults, stuck open faults, coupling faults, neighborhood pattern sensitive faults, and data retention faults. The required memory test patterns can be presented on parallel inputs, can be scanned in from an external tester via shift registers or can be internally generated via on-chip self test logic. 
       FIG. 1  also shows other blocks, which serve as testing circuitry for the RAM. A built-in self-test (BIST) circuit  110  represents on-chip self-testing circuit. Typically, this self-testing circuitry provides testing of an entire chip, which includes RAM, logic, and even potentially analog circuitry. The outputs of the BIST go to the RAM test pattern generator  120  and to other test pattern generators  170 . This BIST output  180  includes command and background data lines. The command lines instruct the TPG  120 , which RAM tests to perform. The background data lines tell the TPG  120  what the expected RAM testing output results should be. Using this command and expected result information, the TPG  120  outputs a serial chain of stimulus or input values  124  to be applied to the RAM under test via the RAM data and control input block  140 . The RAM outputs go into the RAM output data and control block  160 . These RAM outputs are serially shifted through the test data output  164  into the comparator  130  shown in FIG.  1 . In addition, the TPG  120  delivers the expected test pattern results to the comparator  130 . The comparator compares the expected results to the actual RAM test results  164  and activates a Pass/Fail output  190  to indicate the results of the compare. The RAM  150  can be replaced by any logic function, and the same on-chip self-test methodology applies. This methodology is typical of the self-test techniques presently in use. 
     The input decode circuit  140  and the output buffer circuit  160  generally will each include a scan register. The scan register is effectively transparent during normal operation, but allows the transfer of test stimulus signals TS from the test pattern generator TPG  120  to the test access port TAP  144  of the input decode circuit. It is well known in the art that the test stimulus signals are transferred by way of a single connection to the test access port  144  and to the input of the scan registers in the input decode circuit. The normal operational signals, Address  141 , Data  142 , and timing and control  143  are disabled or alternately controlled by testing circuitry. 
     The test stimulus signals  124  are “scanned” in the scan register until the test stimulus signals  124  are aligned with the signal path for the normal operational signals. The appropriate timing signals are activated and the input decode circuit performs the operation indicated by the test stimulus signal TS  124 . A selected memory cell or cells of the RAM array  180  are written to or read from and the resultant output signals are transferred to the Output Buffer  160   
     The scan register Output Buffer  160  is connected to the Test Data Output port TDO  164 . At the completion of the transfer of the test stimulus TS to the test access port TAP  144 , the resultant output signals are “scanned” from the scan registers of the Output Buffer  160  through the Test Data Output port TDO  164  to the Q input of the comparator  130 . 
     The test expected results signal  125  is transferred from the Test Pattern Generator  120  to the comparator  130 . The comparator  130  compares the resultant output signals from the test data output port  164  with the test expected result signals  125 . The pass/fail signal  135  provides an indication of the success of the comparison. If the test is successful, the pass/fail signal  135  indicates a first logic level (1), and if the test is unsuccessful, the pass/fail signal  135  indicates a second logic level (0). 
     U.S. Pat. No. 5,377,148 (Rajsuman) describes hardware and methods to test variable size RAMs in a constant period of time. This is accomplished by partitioning the memory array into a plurality of individually accessible equivalently sized memory blocks. 
     U.S. Pat. No. 5,764,657 (Jones) presents a method for generating an optimal serial test pattern for sequence detection. The serial test pattern comprises a first plurality of bits and is generated by a pattern generator. 
     U.S. Pat. No. 6,061,817 (Jones et al) presents a method and apparatus for generating a serial test pattern for sequence detection. The serial test pattern has a first plurality of bits and is generated by pattern generator. 
     U.S. Pat. No. 6,094,738 (Yamada et al.) presents a test pattern generation apparatus and method for an SDRAM by adding a wrap address conversion circuit. Yamada et al. also describes a method of testing SDRAMs by converting address data from the pattern generator to the burst address of predetermined modes. 
     Kim et al., “On Comparing Functional Fault Coverage and Defect Coverage for Memory Testing,”  IEEE Transactions on Computer - Aided Design of Integrated Circuits and Systems . Vol. 18, No. 11, November 1999, IEEE, describes the evaluation of the effectiveness of the memory testing algorithms based on the defect coverage by comparing the defect coverage of known memory testing algorithms using the same defect statistics. 
     BRIEF SUMMARY OF THIS INVENTION 
     An object of this invention is to provide a circuit for testing to determine if the logic or memory meets the design specifications. 
     Another object of this invention is to provide methods for testing to isolate the errors found during any logic or memory tests, which fail the pass criteria. 
     Further, another object of this invention is to provide a test pattern generator circuit that is added to an integrated circuit during silicon compilation to automatically generate integrated photo masks for fabrication. 
     To accomplish these and other objects, an integrated test pattern generation and comparison apparatus is in communication with a built-in-self-test controller and functional integrated circuits formed on a semiconductor substrate. The integrated test pattern generation and comparison apparatus has a background and command decoder that is connected to receive test background and command codes from the test controller, to translate the test background and command codes to test stimulus signals that, when applied to the functional integrated circuits, create test response signals from the functional integrated circuits. The test stimulus signal is formed of a digital word having a number of bits. 
     The test pattern generation and comparison apparatus further has a number of latency buffers connected to the background and command decoder receive the test stimulus signals and to adjust in time the relationship of the test stimulus signals as required by the functional integrated circuits. There will be one set of latency buffers for each test access port of the functional integrated circuit. Each latency buffer is a plurality of serially connected flip-flop circuits. A first flip-flop circuit of the plurality of serially connected flip-flop circuits has a data input connected to the background and command decoder to receive one bit of the test stimulus signal and an output connected to a subsequent flip-flop circuit of the serially connected flip-flop circuits, whereby each subsequent flip circuit of the serially connected flip-flop circuits has an output connected to the input of a following flip-flop circuit of the plurality of serially connected flip-flop circuits, and whereby a last flip-flop circuit has an input connected to an output of a previous flip-flop circuit and an output containing a delayed bit of the test stimulus signal. The number of flip-flop circuits of each latency buffer is the number of bits in one test stimulus signal. The test stimulus signals are adjusted in time as a function of the number of flip-flop circuits in the plurality of serially connected flip-flop circuits. 
     The test pattern generation and comparison apparatus has a plurality of parallel-to-serial converters. Each parallel-to-serial converter is connected to one group of the plurality of latency buffers, to convert the test stimulus signals to a serialized test stimulus signals to be scanned to a scan register of the functional integrated circuit. 
     The parallel-to-serial circuit has a first plurality of flip-flops. Each flip-flop has a data input to receive one of a first portion of bits of the test stimulus signal and a clock input to receive a first clocking signal to latch the first portion of the bits of the test stimulus signal. The parallel-to-serial circuit further has a first plurality of multiplexor circuits. Each multiplexor circuit has a first input to receive one of a remaining portion of bits of the test stimulus signal, second input to receive an output of one of the first plurality of flip-flops, and a select input to receive a second docking signal to selectively transfer the remaining bit of the test stimulus signal and the output of one of the first plurality of flip-flops to an output of the multiplexor circuit. The parallel-to-serial circuit additionally has a second plurality of flip-flops. Each flip-flop of the first plurality of flip-flops has a data input connected to an output of one of the first plurality of multiplexor circuits, and a clock input connected to receive the first clocking signal to latch the output of one of the first plurality of multiplexor circuits to the output of the flip-flop of the plurality of flip-flops. Finally, the serial-to-parallel circuit has a second plurality of multiplexor circuits. Each multiplexor circuit has a first input connected to a first flip-flop of the plurality of flip-flops, second input connected to a second flip-flop of the second plurality of flip-flops, and a select input connected to the first clocking signal to alternately transfer the first input to an output of the multiplexor circuits and the second input to the output, as the first clocking signal changes from a first level to a second level and from the second level to the first level. 
     The test pattern generation and comparison apparatus has a test response comparison circuit. The test response comparison circuit is connected to the background and control decoder to receive an expected test response signal providing a correct response expected from the integrated circuits in response to the test stimulus signals, and connected to the integrated circuit to receive a test response signal that is the response of the integrated circuit to the test stimulus signal. The test response comparison circuit has a comparator circuit to receive the test response signal and the expected test response signal, compare the test response signal to the expected test response signal and produce a test results signal indicating functioning of the integrated circuits. The comparator circuit is comprised of comparator logic of exclusive-ORs and ORs which compare the data out read from the RAM under test and the expected value from the Background logic section. The output of the comparator circuit is the Pass/Fail signal where a high level indicates Pass or equality or a low level indicates a Fail or inequality. 
     The test response comparison circuit further has a error-handling module to receive the test response signal and the expected test response signal and creates a diagnostic signal indicating a location of any fault determined to exist within the integrated circuits. The error handling module includes a parallel-loadable shift register. The input of this shift register are the data outputs from the RAMs. The load signal for the shift register comes from the Pass/Fail signal of the comparator. The diagnostic output is the serial output of the shift register. 
     The test pattern generation and comparison apparatus is structured such that a hardware description of the test pattern generation and comparison apparatus requires the number of bits of the test stimulus signal and the adjusting in time of the test stimulus signal as parameters to automatically create a physical description of the test pattern generation and comparison apparatus during an automatic physical design of the integrated circuit for placement on the semiconductor substrate. 
     The test pattern generation and comparison apparatus is applicable to testing logic circuits and memory array circuits. However, the preferred embodiment of this invention is applicable for the testing of random access memories (RAM) such as dynamic RAM, static RAM, and other known RAM arrays. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram of on-chip self testing of the prior art. 
         FIG. 2  is a high level diagram of an embodiment of an on-self testing circuit of this invention. 
         FIG. 3  is a detailed block diagram of a test pattern generation and comparison circuit of this invention. 
         FIG. 4  is a logic diagram of the latency buffer of this invention. 
         FIG. 5  is a logic diagram of the parallel to serial converter of this invention. 
         FIG. 6  is a timing diagram of a command decode to form test stimulus signals as output of the test pattern generator of this invention. 
         FIG. 7  is a timing diagram that illustrates the latency and serial-to-parallel signals of the test pattern generator of this invention. 
         FIG. 8  is a block diagram of the serial-to-parallel circuit of this invention. 
         FIG. 9  is a timing diagram that illustrates the operation of the serial-to-parallel circuit of FIG.  8 . 
         FIG. 10  is a block diagram of the background and command decoder of this invention. 
         FIG. 11  is a block diagram of the comparator circuit of the test pattern generation and comparison circuit of this invention. 
         FIG. 12  is a block diagram of the error handling module of the test pattern generation and comparison circuit of this invention. 
         FIG. 13  is a flowchart of the method for generation of test stimulus signals and the analysis of test response signals to verify function of integrated circuits of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Refer now to  FIG. 2  for a discussion of a test pattern generation and comparison circuit of this invention built into or embedded within an integrated circuit. The test circuit of this invention is used to verify the function and operation of an integrated circuit. The built-in self test (BIST) logic  200  is a logic circuit, which controls the testing of the various logic and memory sections of the integrated circuit chip. It consists of a BIST controller  230 , which triggers the beginning and ending of the various chip self tests. The BIST logic also consists of the Sequencer  240  which contains the individual programmable memory and logic test algorithms and individual test pattern generator interface TPG signals. The sequencer  240  drives the individual test pattern generators, TPG&#39;s, for logic  270  and memory  250 . The Sequencer communicates with the TPG blocks via a Command bus and a Background bus  245 . The RAM TPG  250  presents test pattern input signals  210  and receives test pattern output results  220  from the RAM  260 . The serial output from the RAM  220  is sent to a Comparator  255  where it is compared with the expected RAM test results that came from the Command/Background bus. The results of the RAM results comparison are communicated via the DIAG bus  259  and the PASS/FAIL line  257 . The DIAG bus  259  contains information on the exact location of the error found. Similarly, the Logic TPG  270  presents test pattern input signals  285  and receives test pattern output results  295  from the Logic  280 . The serial output from the Logic  295  is sent to a Comparator  275  where it is compared with the expected Logic test results  290  that came from the logic test pattern generator  270 . The results of the Logic results comparison are communicated via the DIAG bus  279  and the PASS/FAIL line  277 . The DIAG bus  279  contains information on the exact location of the error found. The PASS/FAIL line  277  indicates whether and error has occurred with no indication of the type of error or its location. 
       FIG. 3  illustrates the test pattern generation and comparison circuit  250  of this invention. The serial test data outputs  310 , . . . ,  315 ,  320  provide the appropriate data, control and timing signals to the RAM such that the RAM may be tested for correct operation. The serial test data output  310 , . . . ,  315 , and  320  collectively form the test stimulus signals  210  of FIG.  2 . The test response signals Q A    326 , Q X    327 , . . . , Q Y    328  of  FIG. 3  represent the serial test data output TDO  220  of FIG.  2 . The background and command decoder  330  accepts input from the high level command bus  331  and the encoded background bus  332 . The number of commands acceptable from the command bus  331  is  2   n  commands, where n is number of terminals or bits of the command bus  331 . The number of connections or bits of the background bus  332  depends on the word length in memory. The access clock is used to synchronize the test pattern and generation circuit with the remaining integrated circuits to be placed on the chip. The access clock loads the flip-flops of the latency buffers  340 , . . . ,  345 ,  350 ,  355 ,  375 , and is, in the preferred embodiment, the master clock of the remaining integrated circuits to be placed on the chip. The test stimulus signals  334 ,  335 ,  336 , and  337  are structured to form the memory data, address and control signals to be applied to the RAM array  260  of FIG.  2 . The test stimulus signals  334 ,  335 ,  336 , and  337  are each connected to latency buffers  340 ,  345 ,  350 , and  355 . In addition, the output enable signal  374  and the parity signal  376  from the background and command decoder  330  is applied to latency buffers  356 . 
     As is known, the structure of the integrated circuit may be such that the test stimulus signal  210  generated by the test pattern generator  250  of  FIG. 2  may be multiple test stimulus lines fed to multiple test access ports for the input data and decode circuitry for other RAM arrays  260  placed in the integrated circuit. Further, each RAM array  260  may require its own unique set of test stimulus signals. Thus to accomplish this, the background and command decoder  330  provides multiple test stimulus signals  334 ,  335 ,  336 , and  337  to the latency buffers A, . . . , Z  340 ,  345 ,  350 , and  355 . The latency buffers  340 ,  345 ,  350 , and  355  adjust or delay the test stimulus signals  334 ,  335 ,  336 , and  337  such that are delayed in time by a predetermined amount relative to the Access Clock. The delayed test stimulus signals  342 , . . . ,  347  are transferred to the parallel-to-serial converter circuits  380 , . . . ,  385 . The parallel-to-serial converter circuits  380 , . . . ,  385  converts the parallel delayed test stimulus signals  342 , . . . ,  347  to the serial test stimulus signals  310 , . . . ,  315 . 
     The delayed test stimulus signals  352  are transferred to the parallel-to-serial converter  390 . The serialized test stimulus signal is then transferred to the tri-state buffer  395 . The output of the tri-state buffer  395  is the serial test data  320 . The delayed test stimulus signal  367  acts as the tri-state control for the tri-state buffer  395 . The tri-state buffer  395  is employed in test structures including input/output pads where the output of the RAM test pattern generator  210  of  FIG. 2  must be brought to a high impedance or disabled to prevent interference with normal operation. 
     Refer now to  FIG. 10  for a discussion of the structure and operation of the background and command decoder  330 . The high level command bus  331  and the encoded background bus  332  are connected to the combinatorial logic  1030 . This block of logic produces an output enable signal OE, which when equal to zero tells the Comparator  360  in  FIG. 3  to compare the background pattern  1050  in  FIG. 10  to the parallel data from the serial-to-parallel block  325  in FIG.  3 . The parity output  1040  tells the Comparator  360  that the background pattern has been inversed. The high level commands are decoded in the combinatorial logic and the RAM signals X, Y, W[ 0 ], and W[ 1 ] are generated and funneled through parallel-to-serial converters. These serial signals are then presented to the RAM inputs. 
     Refer now to  FIG. 4  for a discussion of the structure of the latency buffers  340 ,  345 ,  350 ,  355  and  356 . Each set of latency buffers  400  includes multiple register sets  405   a , . . . ,  405   z . Each register set  405   a , . . . ,  405   z  includes a group of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n . One of the test stimulus signals  420   a , . . . ,  420   z  from the background and command decoder  330  of  FIG. 3  provides the data input to the first flip-flop  410   a ,  415   a  of the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n . The outputs of each flip-flop of the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n  are connected to the input of each subsequent flip-flop. The output of the last flip-flop  410   n ,  415   n  of the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n  form the delayed test stimulus signals  425   a , . . . ,  425   z . The access clock provides the timing signal to cause the test stimulus signals  420   a , . . . ,  420   z  to be transferred through each of the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n.    
     Refer now to  FIG. 7  for a discussion of the operation of the latency buffers  400 . At a time t 0  the background and command decoder  330  of  FIG. 3  receives a command CMD such as test write or test read from the sequencer  240  of FIG.  2 . The command is decoded to create the test stimulus signals A[ 0 ], A[ 1 ], A[ 2 ], and A[ 3 ]. The test stimulus signals A[ 0 ], A[ 1 ], A[ 2 ], and A[ 3 ] are, in this example, the inputs  334  to the latency buffer  340 . The latency buffer  340  delay the test stimulus signals A[ 0 ], A[ 1 ], A[ 2 ], and A[ 3 ] by the time λ during the time period t 1 . The time delay λ is a fixed number of cycles or period of the access clock and determines the number of flip-flops in the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n . The number of flip-flops in the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n  is determined by the formula: 
       N   =     λ   ϕ         
 
where:
         N is the number flip-flops in each of the groups of serially connected flip-flops  410   a ,  410   b , . . . ,  410   n ,  415   a ,  415   b , . . . ,  415   n.      λ is the required delay time.   φ is the period of the access clock.       

     The test stimulus signals A[ 0 ], A[ 1 ], A[ 2 ], and A[ 3 ] that have been delayed by the delay time λ form the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d that are active at the time period t 2.    
       FIG. 5  illustrates an embodiment of the parallel-to-serial converters  380 ,  385 ,  390  of FIG.  3 . In this implementation of the embodiment of this invention the background and command decoder  330  produce one test stimulus signal having a width of 4 bits, represented by the test stimulus signals A[ 0 ], A[ 1 ], A[ 2 ], and A[ 3 ]. These signals are then delayed as described above through the latency buffer  340  to form the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d. The delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d are the inputs to the parallel-to-serial converter  500 . The low order bit A[ 0 ]_d of the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]d is one input to the two bit multiplexor  510 . The next higher even bit A[ 2 ]d of the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d is the data input to the flip-flop  530 . The output of the flip-flop  530  is a second input to the multiplexor  510 . The output A_even of the multiplexor  510  is the data input to the flip-flop  540 . The lowest order odd bit A[ 1 ]_d of the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d is the first input of the two bit multiplexor  520  and the highest order bit A[ 3 ]_d of the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d is the data input of the flip-flop  560 . The output of the flip-flop  560  is the second input to the two bit multiplexor  520 . The output A_odd of the multiplexor  520  is the data input to the flip-flop  550 . The outputs of the flip-flops  540  and  550  are the input to the two bit multiplexor  570 . The output of the two bit multiplexor  570  is the serial test data out  580 . 
     The load signal  515  provides the select signal to determine which of the two signals applied to the inputs of the two bit multiplexors  510  and  520  is transferred to the outputs A_even and A_odd. The memory clock  535  provides the clock signal for the flip-flops  530 ,  540 ,  550 ,  560 , that “latches” the input signals present at the inputs of the input of the flip-flops  530 ,  540 ,  550 ,  560  to their respective outputs. Further, the memory clock  535  provides the select signal for the two bit multiplexor  570 . 
     Refer again to  FIG. 7  for a discussion of the function of the parallel-to-serial converter  500 . At the time ti, the delayed test stimulus signals A[ 0 ]_d, A[ 1 ]_d, A[ 2 ]_d, and A[ 3 ]_d are applied to the input terminals as above-described. During the beginning of the time segment t 1 , the load signal remains at a high logic level (1) and the two bit multiplexor  510  transfers the low logic level (0) of the lowest order bit A[ 0 ]_d to the flip-flop  540 . Simultaneously the two bit multiplexor  520  transfers the high logic level (1) of the second lowest order bit A[ 1 ]_d to the flip-flop  550 . The bits A[ 2 ]_d, and A[ 3 ]_d of the delayed test stimulus signals A[ 0 ]_d, A[ 1 ] d, A[ 2 ] d, and A[ 3 ] d are respectively the data inputs of the flip-flops  530  and  560 . At the change of the memory clock from the low logic level (0) to a high logic level (1) the data inputs of the flip-flops  530 ,  540 ,  550 , and  560  are “latched” to the outputs of the flip-flops  530 ,  540 ,  550 , and  560 . The multiplexor  570  is activated with the high level (1) of the memory clock during the time t 3  to transfer the low logic level (0) of the test stimulus signal A[ 0 ]d from the first input of the multiplexor  570  to the serial data output  580 . At the beginning of the time t 4  the memory clock changes from the high logic level (1) to the low logic level (0) and the output of the multiplexor  570  now receives the contents A[ 1 ]_d of its second input which is the output of the flip-flop  550  The test stimulus signal A[ 1 ]_d is now the serial data output  580 . During the time period t 3  and prior to the change of the memory clock from the high logic level (1) to the low logic level (0) at the beginning of the time period t 4 , the load signal changes from the high logic level (1) to the low logic level (0). This causes the multiplexors  510  and  520  to be activated to respectively transfer the contents A[ 2 ]_d, and A[ 3 ]_d of the output of the flip-flops  530  and  560  respectively to outputs A_even and A_odd of the multiplexors  510  and  520 . At the beginning of time t 5  , the memory clock changes from the low logic level (0) to the high logic level (1) and the test stimulus data A[ 2 ]_d, and A[ 3 ]_d is “latched” to the outputs of the flip-flops  540  and  550 . During the time t 5  the test stimulus data A[ 2 ]_d is transferred to the serial data output. When the memory clock changes from the high logic level (1) to the low logic level (1), the multiplexor transfers the second input which is the contents A[ 3 ]_d of the output of the flip-flop  550  to the serial data output. 
     Refer now to  FIGS. 3 and 6  for a discussion of the operation of the test pattern generator  250  of this invention. The memory clock, the access clock, and the load signal provide the timing and control signals for the test pattern generator  250 . A command signal CMD  331  is applied to the background and command decoder  330 . The background and command decoder  330  decodes the command signal CMD to form the test stimulus signals  334 ,  335 , and  336 . In this example, the command signal CMD forms four serial test data signals A, B, C, and D that would be illustrative of the signal contents of the serial test data ports  310 ,  315 , and  320 . The parameters that determine the structure of the decoded test stimulus signals are the latency and the packet length. The latency determines the relative timing of the serial test data signals A, B, C, and D for each of the serial test data ports  310 ,  315 , and  320  in relation to the application of the command CMD signal. The packet length is the number of serial test data bits to be provided by a particular command signal CMD. 
     The command signal CMD is decoded to form the signals A[ 0 ], B[ 0 ], B[ 1 ], C[ 0 ], C[ 1 ], C[ 2 ], C[ 3 ], D[ 0 ], D[ 1 ], D[ 2 ], and D[ 3 ] that are the test stimulus signal  334 ,  335 ,  336 , and  337 . In the case of the test stimulus signal for port A the number of bits is one A[ 0 ], the number of bits for port B is two B[ 0 ], B[ 1 ], for ports C and D the number of bits is four C[ 0 ], C[ 1 ], C[ 2 ], C[ 3 ], D[ 0 ], D[ 1 ], D[ 2 ], and D[ 3 ]. The serial test data signals for ports A, B, C, and D of  FIG. 6  illustrate by example the timing relationships of the test data signals for ports A, B, C, and D. Since the test stimulus signal for the port A has one bit, the serial test data signal of port A has packet length of one during one access clock. The latency of the serial port A is set to zero or, in other words, the serial test data for port A coincides with the command signal CMD. Since the test stimulus signal for the port B has two bits, the serial test data signal of port B has packet length of two during one access clock. The latency of the serial port B is set to one or the serial test data for port B is delayed one access clock cycle with respect to the command signal CMD. Since the test stimulus signal for the port C has four bits, the serial test data signal of port C has packet length of four during one access clock. The latency of the serial port C is set to zero or the serial test data for port A coincides with the command signal CMD. The test stimulus signal for the port D has four bits, the serial test data signal of port D has packet length of four during one access clock. The latency of the serial port D is set to four or the serial test data for port D is delayed four access clock cycles with respect to the command signal CMD. 
     If the access clock frequency equals the memory clock frequency, the maximum packet length would be two. If the memory clock frequency equals to twice the access clock frequency, the maximum packet length would be four. In general, the maximum packet length equals two times the memory clock frequency divided by the access clock frequency. 
     The serial test data  310 ,  315 ,  320  is scanned to the respective test access ports for the testing the RAM array  260  of FIG.  2 . The appropriate controls are activated to test the function of the RAM array  260 . The test data output TDO  220  contains the serial test results data that is transferred to one serial data input Q A , . . . , Q X , Q y  of the test pattern comparison circuit  255 . Each serial test results data input Q A    326 , . . . , Q X    327 , Q Y    328  is received by the serial-to-parallel converter  325 . The serial test results data inputs Q A    326 , . . . , Q X    327 , Q Y    328  are converted to a parallel test result data word  362 ,  364 , and  366 . 
     Refer to  FIG. 8  for discussion of the structure and function of the serial-to-parallel converter  325 .  FIG. 8  shows two serial outputs  860  from the RAM  870 . These two serial signals are converted to four parallel signals via the connection of several flip-flops (FF) such as  810 . The memory clock  820  captures RAM Data Out  0   860 . The Access clock shifts the data from the input FF to the output FF to produce Data Out  0   850 . 
       FIG. 9  is a timing diagram of the operation of the serial-to-parallel converter  325 . As explained above, the memory clock, the access clock and the load signal provide the timing and control signals for the serial-to-parallel converter. The serial data output for test result data port D is by example, illustrative of two successive data packets Wd[ 0 ] and Wd[ 1 ]. The bits of the packet word Wd[ 0 ] are transferred serially to the data input Q of port D during the times t 0 , t 1 , t 2 , and t 3 . The bits of the packet word Wd[ 1 ] are transferred serially to the data input Q of port D during the times t 4 , t 5 , t 6 , and t 7 . The test results data word Wd[ 0 ] is contained in the parallel test response word D[ 0 ], D[ 1 ], D[ 2 ], and D[ 3 ] during the times t 4 , t 5 , t 6 , and t 7 . The bit D[ 0 ] contains the test result data of the time t 0 , the bit D[ 1 ] contains the test result data of the time t 1 , the bit D[ 2 ] contains the test result data of the time t 2 , and the bit D[ 3 ] contains the test result data of the time t 3 . The test results data word Wd[ 1 ] is contained in the parallel test response word D[ 0 ], D[ 1 ], D[ 2 ], and D[ 3 ] during the time t 8 . The bit D[ 0 ] contains the test result data of the time t 4 , the bit D[ 1 ] contains the test result data of the time t 5 , the bit D[ 0 ] contains the test result data of the time t 6 , and the bit D[ 3 ] contains the test result data of the time t 7 . 
     Referring back now to  FIG. 3 , the parallel test result data words  362 ,  364 , and  366  are the inputs to the comparator  360  and the error handling module  370 . The comparator  360  receives the expected test response data  372  decoded from the encoded background data  332  by the background and command decoder  330 . Further, the background and command decoder  330  provides the output enable signal  374 , and the parity signal  376 . The output enable signal  374 , and the parity signal  376  are appropriately delayed by the latency buffer  375  and applied to the comparator  360  and the error handling module  370 . The function of the latency buffers  375  is as described in  FIG. 4  to delay the output enable signal  374 , and the parity signal  376 . The output enable signal OE determines if the comparator needs to compare the expected data and the data output from the serial to parallel modules. The function of the parity signal is to select whether the expected output should equal the background data directly or the inverse of the background data. If parity is 1, the expected data equals the background data. If parity is 0, the expected data equals the inverse of the background data. 
     The comparator  360  compares the expected test result data pattern  332  to the parallel test result data words  362 ,  364 , and  366  and provides a pass/fail signal  373  indicating whether the tested integrated circuit is functioning properly. Refer now to  FIG. 11 , for a discussion of the comparator  360   
     The comparator in  FIG. 11  receives the Data Out  1160  from the Serial-to-Parallel module and compares it to the Background data pattern  1110 . The Parity signal  1140  indicates whether to negate the background data. The output enable signal  1150  indicates whether to perform the compare if OE=0. If the Background=the Data, Pass/Fail=Pass. If the Background does equal the Data, Pass/Fail=Fail. If OE=1, the comparator does not compare and the Pass/Fail  1170  equals Pass The comparison takes place via the XOR and OR logic tree  1130 . 
     An optional function of the test pattern comparison circuit  255  is the error handling module  370 . The error handling module compares the expected test result data pattern  332  to the parallel test result data words  362 ,  364 , and  366  and further compares them to identify and locate any faults present in the RAM array  260  of FIG.  2 . 
     Refer now to  FIG. 12  for a discussion of the structure and operation of the error handling module  370 . The parallel data (0-n)  1250  from the S2P module is captured into a shift register of length n, if there is a failure indicated by the Pass/Fail signal  1240  from the comparator. The shift register which is loaded with the incorrect data result is then shift out serially on the DIAG output  1230 . This diagnostic output can be used to analyze the location and type of logic faults. 
     Refer now to  FIG. 13  for a summary flowchart of the method for generating a test stimulus pattern to be applied to an integrated circuit such as a RAM array and for comparing a test result from the integrated circuit to verify function of the integrated circuit of this invention. The first step is to transmit the command and background codes  1310  from the BIST logic to the test pattern generation (TPG) logic. Next, the TPG decodes  1320  the Command and Background codes to determine which test to perform and to extract the expected test results for the requested test. Then, test signals  1330  are generated for the logic or memory under test. The test signals are delayed  1340  with respect to the access or memory clocks in order to be compatible with the timing requirements of the logic or memory under test. Next, the delayed test signals are serialized and transferred to the logic or memory under test  1350 . After the specified test is performed on the logic or memory, the test results are received by the test comparison circuit  1360 . The test comparison circuit analyzes  1370  the test results and reports a pass or fail. In addition, the test report can optionally include a diagnostic, which isolates the circuit location of any test failures. 
     One of the aspects of this invention is that this architecture of the TPG blocks for both logic and memory testing is compatible with silicon compilation systems. These systems generate integrated circuit designs and fabrication masks from a high level hardware design language, such as VHDL. The high level hardware design language provides a software description of the logic and memory. The latency parameter λ is used by the silicon compilers to determine which latency buffer circuit to use. Further, the packet length is determined as a function of the standardized tests chosen to test the integrated circuit. This silicon compiler decision is based on the amount of delay through the memory logic required to establish proper timing relationships of the test signals to properly exercise the operation of the integrated circuit. The high level hardware description language coding the latency parameter λ and the packet-length permits automatic specification of the test pattern generation and comparison circuit of this invention within and integrated circuit for inclusion on a semiconductor substrate. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.