Abstract:
Architecture, circuitry, and methods are provided for testing one or more integrated circuits which may be arranged upon a printed circuit board. The integrated circuits include sequential and combinatorial logic used by the integrated circuit during normal functioning thereof. Testing of that logic can occur by sending test vectors in parallel or serial form to input pins of the integrated circuit. The test results can either be read as a serial data stream or as a parallel-delivered data stream. If the test information and results occur in parallel fashion, than automated test equipment can be used which do not require compliance with having a single serial fed test vector input and test result output, normally found in a TAP application. A parallel/serial multiplexer is used to select whether the integrated circuit receives parallel or serial test vectors, and another parallel/serial multiplexer is used to select whether the test results are to be delivered in parallel or serial fashion. A test access port controller is linked to various boundary scan cells, and control signal generators within the controller orchestrate movement of test data or normal functional data into and from the integrated circuit core logic. Parallel fed test vectors can be shifted from the input cells as serial fed test vectors and then back to the input cells, where the test vectors can then be sent to the core logic among numerous conductors in parallel fashion. Likewise, the test results can be converted from a parallel to serial form and fed back to the output cells, whereupon the test results can be sent to the test equipment as parallel-delivered test results.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an architecture, circuitry and method for testing one or more integrated circuits, each of which implements a test access port (“TAP”) such as the JTAG access port described in IEEE Std 1149.1. The test circuitry can place test signals in parallel upon an integrated circuit, shift the test signals through boundary scan cells while using the serialized boundary scan architecture, and thereafter dispatch the test results in parallel from the integrated circuit. 
     2. Description of the Related Art 
     Testing an integrated circuit can be performed in various ways. For example, the integrated circuit can be tested while in wafer form using test probe operation. Additionally, or alternatively, the integrated circuit can be tested after it is scribed and packaged. In either instance, sequential and/or combinatorial logic of the integrated circuit must be tested using input test data, generally referred to as “test vectors.” Test vectors are supplied from a commercial test machine or automated test equipment (“ATE”). Alternatively, the test vectors can be provided from circuitry upon the integrated circuit. Such circuitry is often referred to as built in self test (“BIST”) circuitry. BIST circuitry may use a pseudo-random sequence generator to produce test vectors forwarded to the functional core logic of the integrated circuit. 
     BIST has been used successfully on a number of integrated circuits. However, BIST is difficult to use when testing arbitrary random logic, and requires adding significant test circuitry to the integrated circuit area. If BIST is not used and the test vectors are applied from an external ATE, it is desirable that the test vectors not only verify the integrated circuit operation but also the integrated circuit as it exists on a printed circuit board. 
     In the early 1990&#39;s, a standard was developed and approved as IEEE Std. 1149.1 and 1149.1a, henceforth referred to as the JTAG standard. The JTAG standard was envisioned to allow testing of the integrated circuit after it had been assembled onto the printed circuit board. Moreover, the JTAG standard provided for testing numerous integrated circuits on the board as well as the interconnect of those circuits to the printed conductors of the board. In-system testing was therefore provided for testing the entire, assembled printed circuit board using pins associated with a test access port (“TAP”). 
     An integrated circuit that is JTAG compliant will have reserved a four or five signal TAP. If testing of the integrated circuit involves a boundary scan mechanism, then each integrated circuit also contains one or more boundary scan cells and a TAP controller for orchestrating signal flow within and through each of those cells. 
     FIG. 1 illustrates a printed circuit board  10  having multiple integrated circuits  12 ,  14 , and  16  mounted thereon. For sake of brevity, only three integrated circuits are shown. However, it is recognized that a printed circuit board may embody certainly more than three integrated circuits. FIG. 1 also illustrates three input conductors  18  and three output conductors  20  associated with integrated circuit  14 . Almost all integrated circuits are known to include more than three input conductors (or pins), and more than three output conductors (or pins). Therefore, FIG. 1 is used only as an abbreviated example so as not to unduly complicate the drawing. 
     Coupled between the input pins and the to-be-tested core logic  22  are respective boundary scan cells  24 . Likewise boundary scan cells  26  are shown between core logic  22  and the output pins. Depending on the state of TAP controller  28 , each boundary scan cell can either allow core logic  22  to be connected to external integrated circuits  12  and/or  16 , or can enable boundary scan testing from a test data input conductor, TDI. TDI along with the test data output conductor TDO, the test mode select conductor TMS, and the test clock conductor TCK form four of possibly five pins attributed to TAP. TDI and TDO are daisy-chained through each boundary scan cell  24  and  26 , and from integrated circuit to integrated circuit, whereas TCK and TMS are broadcast. Prior to entering the first of the daisy-chained integrated circuits, signals upon TDI, TDO, TMS, and TCK of the TAP are derived from an ATE. 
     In addition to boundary scan cells  24  and  26  attributed to each input/output pin, integrated circuit  14  also may include scan elements (SEs)  25  arranged internal to the circuit  14 . Scan elements  25  are chained together and used for internal manufacturing testing of the core logic, for example. Scan elements  25  communicate with corresponding portions of the core logic  22 . Each scan element may receive a bit, and shift that bit to the next scan element in the chain such that when the scan is complete, a multiple number of scan elements contain a rather large test vector comprising a plurality of bits. Thus, the scan elements may be rather numerous and may include more than one chain to facilitate design debug and manufacturing testing. Scan elements  25  are serially accessible from the TDI, TDO, TMS and TCK signals. The IEEE Std 1149.1 specification provides for multiple scan chains, some of which may be known to the integrated circuit manufacturer only. An example by which the scan elements  25  are used arise in programmable logic devices, and are expressed in various data sheets of manufacturers such as Cypress Semiconductor Corp. 
     Depending on what is placed into a scan chain, its length can be quite large. Shifting test information into or out of a rather long scan register chain can demand a lot of input and output cycles and associated vector memory on the ATE channels assigned to access the TAP pins of the integrated circuit under test. The IEEE Std 1149.1 makes known the use of bypass registers, or identification code (IDCODE) registers on dedicated chains within possibly numerous scan element chains that extend throughout an integrated circuit. While the boundary scan cells  24  and  26  can be parallel accessible through input/output pins, the internal scan elements  25  can only be accessed through the serial chain. Thus, the boundary scan cells may be deemed a specific implementation of the scan technique to test any core logic whatsoever, and scan elements are specific to the core logic across which they are chained together in serial fashion. 
     FIG. 2 illustrates in more detail the daisy-chaining of multiple integrated circuits  30  that are solder connected to surface-mount pads or vias of a printed circuit board  10 . Depending on the number of integrated circuits having a TAP access, a single serialized test signal stream can be input throughout boundary scan cells  32   a  and  32   b  of each integrated circuit and thereafter placed in parallel within the core logic  34  of those integrated circuits. The test vectors are thereby said to be serially fed and parallel placed (i.e., scanned) into the functional core logic. Conversely, the test vectors can only be sent in series into the scan elements via TDI, and received from the scan elements in series via TDO. The ATE connected to TDI and TDO thereby not only produces the test vectors for testing the assembled printed circuit board, but also reads the test vector results. Details of the JTAG and TAP architecture are provided in the 1149.1 and 1149.1a specification. Further details of boundary scan systems and test vectors forward from an ATE to and from boundary scan cells are provided in U.S. Pat. Nos. 5,751,163 and 5,805,607 (both of which are herein incorporated by reference). It is noted that when describing the serial placement of test vectors into the boundary scan cells, serial placement of test vectors occurs within the internal scan elements—the only difference being that the scan chain of the scan elements can in some instances be much longer than, and have more scan elements than, the boundary scan cells. 
     While the provisions of JTAG and generally the concept of serially feeding test data into a single input pin and reading those results from a single output pin has many advantages, it also presents numerous disadvantages. For example, applying serially fed test vectors from an ATE to a TDI pin requires a significant amount of test time. In order to apply all the necessary test vectors to a relatively large logic portion, the serially fed test vectors must be quite lengthy. Each test vector must be serially scanned from one boundary scan cell to the next and, of greater criticality, from one scan element to the next within a rather large scan chain. The serially fed test vector proceeds from one scan cell or scan element until it reaches the appropriate “input” scan cell (or scan element) in the chain where it can then be applied to the core logic. Thereafter, an “output” scan cell (or scan element) will receive the test result of that test vector where output shifting is further necessary to present the result as a serially fed output signal on the TDO conductor. Serially feeding test vectors and serially reading those test results is not only time consumptive, but also is limiting as to the type of ATE that can be used. Specifically, the ATE is limited to one which can serially feed rather large numbers of test vectors and serially read those test results on a small number of pins. 
     It would desirable to utilize any conventional ATE, and not simply those designed to forward serialized test vectors and read serialized test results. Conventional testers which forward test vectors in parallel and receive test results in parallel would advantageously speed up the overall test process while maximizing the available vector memory applied to the integrated circuit via multiple tester pins. The desired parallel-delivered and parallel-received test mechanism should beneficially operate within the constraints of the JTAG compliant integrated circuit and JTAG printed circuit board architecture by only slightly modifying the boundary scan circuitry of that architecture. The improved test architecture, circuitry, and method could therefore have broader applications to highly complex integrated circuit testing while using a relatively small parallel-delivered test vector table employed in many conventional, multiple-pin testers. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part solved by an architecture which can apply test vectors in parallel to one or more integrated circuits arranged on and connected to a printed circuit board. The integrated circuit can therefore be tested by accessing at least some, if not all, boundary scan cells and internal scan elements arranged within one or more scan chains. The scan chains of the integrated circuit are tested either before or after assembly on or apart from a printed circuit board by any tester which can deliver test vectors and/or receive test results across numerous pins or conductors. Parallel delivery and receipt of test vectors and test results to and from the boundary scan cells provides for ease by which the test vectors can be placed upon the scan elements arranged in one or more chains internal to the core logic. Parallel delivery and receipt greatly enhances the overall test time and minimizes the test vector memory access times of the ATE. Additionally, depending on the test mode used, test vectors can be placed from a parallel-fed ATE or a serial-fed ATE. 
     Parallel delivery and receipt of test information preferably occurs to any and all scanable storage elements configured upon the integrated circuit. The delivery mechanism may, if desired, advantageously use the JTAG and/or boundary scan architecture of the integrated circuit and/or board. Preferably, the parallel delivered test vectors are forwarded onto the input pins of the integrated circuit, captured by the boundary scan cells, and shifted between boundary scan cells to present a test signal having serialized test vectors. The serial test vectors can then be sent to the scan elements on the appropriate scan chain inside the core logic and/or integrated circuit 
     When the proper number of vectors have been shifted to the appropriate elements, an update latch within each scan element may be used to present its corresponding vector to a conductor extending from the scan element to the core logic circuitry. After the core logic has been tested, the test result is output and selected for transmission into a series of capture latches arranged near the output conductors of the core logic. The capture latches of the output scan elements perform similar to the capture latches of the input scan elements in that the capture latches will serialize the test results through a series of shift operations. 
     Included within the integrated circuit is a parallel/serial multiplexer that is responsive to whether the test vectors were sent from the ATE in parallel or serial fashion. In other words, the multiplexers determine whether the scan elements will receive parallel-driven test vectors via the boundary scan cells or will receive serial-driven test vectors. If presented serially, then the ATE would be connected to the TDI pin of the TAP. Otherwise, the ATE is connected to multiple pins of the integrated circuit that may or may not be connected to the printed circuit board. Application of the parallel-driven ATE to the multiple pins can occur through a bed-of-nails type test apparatus, for example. Another parallel/serial multiplexer may be used at or near the output pins of the integrated circuit and is configured based on whether the test results will be forwarded from the integrated circuit in parallel or in serial fashion. Thus, each integrated circuit preferably includes a pair of parallel/serial multiplexers. Depending on how those multiplexers are configured, either a serial test mechanism or a parallel test mechanism can be used, with selectivity therebetween. 
     According to one embodiment, test architecture is provided. The architecture includes an integrated circuit having a logic portion (i.e., core logic), and a plurality of input pins adapted to receive a parallel delivered test signal. A plurality of capture latches may be configured to receive the test signal in serial fashion and to deliver the test signal to a plurality of update latches. Conductors within the core logic portion are coupled to receive the test signal in parallel fashion from each of the plurality of update latches. The capture latches and update latches are thereby associated with any scanable device linked together in serial fashion, including the boundary scan cells or the scan elements. 
     The logic portion can also deliver test results to a plurality of capture latches, whereby those latches are clocked so as to serialize the test results for serial read out through the TDO pin. Alternatively, the test results can be delivered, either directly or from the TDO through serial-to-parallel conversion, upon a plurality of output pins of the integrated circuit. 
     A test access port (TAP) controller is also provided. The controller includes a first control signal generator, a first clock signal generator, and a second control signal generator. Each of the generators are operably coupled to boundary scan cells and scan elements of an associated integrated circuit. The first control signal generator is adapted to capture a parallel delivered test signal and the first clock signal generator is adapted to shift the captured test signal onto a single conductor as a serial test signal. The second control signal generator can then disburse in parallel format the test information from the serial test signal onto separate conductors. The separate conductors are those which extend into the core logic (if the controller is controlling input boundary scan cells), or the separate conductors are output pins if the controller is used to control output boundary scan cells. 
     A method is also provided for testing an integrated circuit or circuit board containing multiple integrated circuits. The method includes applying a test signal to a plurality of input pins of an integrated circuit. The test signal may then be shifted upon a single conductor, whereupon a plurality of test signals are drawn from the single conductor and forwarded to combinatorial and sequential logic preferably within the core logic portion of the integrated circuit. The test signal applied to the plurality of input pins can be selected rather than a test signal applied to a TAP serial input conductor of the integrated circuit. Test results of the test signal upon the core logic may then be forwarded to a plurality of output pins, or to a single TAP serial output conductor of the integrated circuit. Parallel-driven test vectors from the ATE can be written into any scanable device (i.e., scan cell or scan element), and test results can be drawn from the scanable devices in parallel via the output pins of the integrated circuit. The scanable device may or may not have direct access to the integrated circuit input/output pins depending on whether the scanable devices are scan cells external to the core logic or scan elements internal to the core logic. Parallel-placement of the test vectors occur using primarily the already available boundary scan elements on a boundary scan portion of the integrated circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 is a block diagram of boundary scan cells arranged upon an integrated circuit between core logic and input/output pins, scan elements arranged within the core logic, and a TAP controller which orchestrates serialized test vectors placed into the boundary scan cells and the scan elements; 
     FIG. 2 is a block diagram of a printed circuit board having interconnected integrated circuits, and serialized test signal flow between boundary scan cells on separate integrated circuits; 
     FIG. 3 is a circuit schematic of a boundary scan cell or a scan element, and the signals which control data and test signal flow therein according to one embodiment; 
     FIG. 4 is a block diagram of the test signals selectively applied either in parallel to the input/output pins or in series to the boundary scan cells and scan elements through the TAP according to separate embodiments; and 
     FIG. 5 is a partial state diagram for the TAP controller in FIG. 4 for carrying out a parallel or serial test access of the core logic. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 3 illustrates a boundary scan cell or scan element, hereinafter scan device  40 , that is configured to operate as an intermediary between the core logic of an integrated circuit and an output pin (or input pins) of the integrated circuit. Depending on how output multiplexer  42  is configured, scan device  40  can be arranged either as an input scan device or as an output scan device. The example shows the device as an output scan device, but it is recognized that it could equally perform as an input scan device using the same configuration. In the example shown, device  40  receives data from the core logic, shown as DATA OUT. Also, the device receives serialized test data from the previous device, shown as SERIAL TEST IN. Depending on whether the integrated circuit is in the test mode or in normal operation mode, either the SERIAL TEST IN signal or the DATA OUT signal will be selected by a tap control signal (TAP CNTL  1 ) forwarded to input multiplexer  44 . Output multiplexer  42  will determine whether the DATA OUT signal will be transferred to the output pin, or whether the captured and updated test vector will be forwarded to the output pin of the integrated circuit if the scan device is a boundary scan cell. 
     The output of input multiplexer  44  forms the input of a capture latch  46  and the output of capture latch  46  forms the input of an update latch  48 . Latches  46  and  48  are respectively clocked by clocking signals (TAP CLK  1  and TAP CLK  2 ) sent from the TAP controller. In a normal functional operation, where test vectors are not applied to nor read from the integrated circuit, the TAP controller is inactive and the output multiplexer  42  will be controlled by TAP CNTL  2  signal such that the signal DATA OUT is connected directly to the output pin of the integrated circuit to transfer the inner core logic test results to the output pin. During normal functional operation, control of the input multiplexer  44  is unimportant and the clock signals TAP CLK  1  and TAP CLK  2  are preferably inhibited. 
     When vector testing is to be performed, the TAP controller upon the integrated circuit controls TAP CNTL  2  such that output multiplexer  42  connects the output of output latch  48  to its output. TAP CNTL  1  signal is controlled such that the SERIAL TEST IN signal is forwarded to the output of input multiplexer  44 , so that when the TAP controller serially outputs a sequence of test vectors, those vectors will be presented to capture latch  46  in serial fashion. After a plurality of clock cycles, dependent on the number and type of pin connections used, the TAP CLK  1  signal will forward the series of test vectors upon the output of capture latch  46 . Each vector may be a single bit or, depending on the number of cycles used, can encompass a field of bits. 
     After the requisite number of vectors have been captured and serially fed to the next and succeeding scan device capture latch, a test vector is therefore said to be present at the output of each cell&#39;s capture latch. Thereafter, the TAP controller will then transition TAP CLK  2  signal such that the test bit on each capture latch output will appear on the output of each respective update latch  48 . If the scan device is a boundary scan cell coupled to an output pin, the update latch output will thereafter appear on the output pin of that integrated circuit. If the scan device is a boundary scan cell coupled to an input pin, then the updated test vectors will be presented to corresponding input conductors of the core logic. If the integrated circuit output pads are connected to similar boundary scan cells configured as input boundary scan cells, then the TAP controllers controlling those input boundary scan cells can serially link their respective boundary scan cells so that a check can be carried out to ensure that the value written to the boundary scan cell by a particular TAP controller was successfully communicated to another boundary scan cell to which it is connected. In this manner, the interconnections between various integrated circuits can be tested. 
     FIG. 4 illustrates test architecture, circuitry, and methodology for selecting between parallel delivered and received test information or serial delivered and received test information. More specifically, FIG. 4 illustrates, in block diagram form, an integrated circuit  50  mechanically and electrically connected to printed circuit board  51 . Integrated circuit  50  includes many input, output, and input/output pins. For sake of simplicity and brevity in the drawings, only three input pins and three output pins are shown. Each input pin is electrically connected to an input boundary scan cell  52  and each output pin is electrically connected to an output boundary scan cell  54 . An ATE  56  can deliver and receive either parallel test information or serial test information. If used to deliver and receive serial test information, then the test vectors will be delivered serially to the TAP and the serial test results are received by TDO, as shown. Alternatively, however, the same ATE or a different ATE  56  can deliver and receive test information across a plurality of input and output pins. ATE can deliver and receive across N number of pins depending upon the amount of test vectors needed and the available size of ATE memory. In the example shown, ATE  56  can deliver to and receive from three respective pins. However, it is to be understood that more than three pins can be used to deliver and receive test information. Regardless of the number used, ATE  56  can deliver and receive the test information at the corresponding pins while the pins are soldered and otherwise electrically connected to various other integrated circuits  58  and  60 . Thus, the test architecture is an in-system test architecture used to not only test core logic  62  within integrated circuit  50 , but also the interconnections between integrated circuited and the printed conductors extending across PCB  51 . 
     Each output boundary scan cell  54  has input and output multiplexers as wells as capture and update latches, similar to that shown in FIG. 3 with possibly a slight modification thereto. More specifically, the output multiplexer  42  may be reversed with an input pad of the integrated circuit and the update latch output connected to the input of multiplexer  42 , and the output of multiplexer  42  connected as DATA IN to an input conductor of the core logic portion. Thus, the input boundary scan cells are configured similar to the output boundary scan cells, in that each have an input multiplexer, an output multiplexer, a capture latch, and an update latch. 
     Referring again to FIG. 4, a test (TST) signal may be forwarded from the ATE  56  to determine whether the test information will be provided as parallel information or as serial information. For example, the test signal can default to a serial test information selection unless activated. When activated, however, the TDI of the TAP will be disregarded and preferably inhibited. Instead, all test information will be derived from and sent to ATE  56  through a set of pins coupled to respective input/output pins on the integrated circuit. In the input example, the pins are input pins which receive parallel delivered test vectors across multiple pins (three in the example shown). The test vectors may then be placed into corresponding input boundary scan cells which can shift each bit or field of bits of the corresponding cell into the subsequent cell and so forth to present a serial output of cells. The serialized test vectors can then be fed to the parallel/serial multiplexer  66 . If the test signal remains active to multiplexer  66 , the serial information from the boundary scan cells  52  can be shifted into the scan elements  53 . Scan elements  53  are eventually populated with respective test vectors and upon activation of each update latch, the corresponding test vectors are forwarded into conductors of core logic  62 . 
     Multiplexers  66  and  68  exists to provide selectibility. If they did not exist in a conventional sense, then input from the TAP controller  70  would feed directly to scan elements  53 . Likewise, the output from the last daisy-chained scan element  53  would be fed to the TAP controller  70 , absent the selectibility of multiplexer  68 . Also, absent multiplexers  66  and  68 , output from input boundary scan cells  52  feed directly to output boundary scan cells  54 . However, by using multiplexers  66  and  68 , the test vectors can be placed in parallel on the boundary scan cells  52  and, through multiplexer  66 , read the parallel-fed input test vectors into scan elements  53 . Similarly, the serial output from scan elements  53  are made available to at least a subset of the boundary scan cells (e.g., output boundary scan cells)  54  via multiplexer  68  when test signal (TST) is a logic one value. Thus, when TST is a logic one, the output from boundary scan cells  52  is made available to the scan elements  53  (via multiplexer  66 ), and the output from scan elements  53  are made available to boundary scan cells  54 . 
     The scan elements  53  are arranged similar to the boundary scan cells  52  and  54 , in that each scan element contains corresponding input/output multiplexers, capture latches and update latches such as those shown in FIG.  3 . Data from the output multiplexer  42  is either sent to the core logic from the boundary scan cells or from the scan elements, depending on whether the scan device  40  is a boundary scan cell or a scan element. Alternatively, data from the output multiplexer  42  can be sent to the output pins if the scan device  40  is a boundary scan cell  54 . Thus, the scan device  40  can be thought of as either an input boundary scan cell  52 , an output boundary scan cell  54 , or a scan element  53 . Each bit or field of bits captured in parallel by the input boundary scan cell or output boundary scan cell is maintained within corresponding capture latches, and the bit or field of bits serially fed into the scan element  53  is captured by corresponding capture latches before being fed in parallel by the update latches. 
     Cells  52  receive in parallel, and elements  53  as well as cells  54  deliver in parallel. The test vectors are shifted into the scan elements  53  and delivered onto corresponding conductors of the core logic in parallel. Test results are either shifted in series upon a single conductor to TDO, or are forwarded to the output pins directly or, alternatively, converted from serial form to parallel form before being placed on the output pins. The status of test signal TST determines whether output multiplexer  68  will forward the serialized information from the input boundary scan cells  52  or the scan elements  53 . If the test signal is active, then test results fed into the scan elements  53  are forwarded through the boundary scan cells  54 , where they can be delivered in parallel from the output pins of the integrated circuit  50 . It is noted that FIG. 4 illustrates only a single, serial chain of scan elements  53  only for sake of brevity and clarity in the drawings. Numerous scan elements  53  can be linked upon dissimilar chains, each of which is selectibly routed to output multiplexer  68 . 
     It will be appreciated upon reviewing FIG. 4, in combination with FIG. 5, that TAP controller  70  comprises a state machine which produces various signals at select times. Those signals are control and clocking signals that are broadcast delivered to either input cells  52 , elements  53  and/or output cells  54 , depending on whether test vectors are being delivered or test results are being received (either from an external cell or an internal element). Accordingly, control lines are present between TAP controller  70  and the various cells. The control lines are used to orchestrate the control and clocking signals sent to the input and output multiplexers (shown in FIG.  4 ), as well as the capture and update latches (shown in FIG.  3 ). 
     A partial state diagram  80  is shown in FIG.  5 . The state diagram illustrates six of possibly numerous control states of TAP controller  70 , shown in FIG.  4 . Referring to FIGS. 4 and 5 in combination, TAP controller  70  forwards various control signals to the respective scan devices  52 ,  53  or  54  depending on the state of its state machine. 
     During normal functional operation of integrated circuit  50 , TAP controller  70  is in state  82 . In this state, the data on the input conductor of input scan device (DATA IN) is connected to DATA OUT sent into the core logic. More specifically, in a normal functional operation, whatever is input to the input pad will be forwarded into core logic, and whatever is output from the core logic will be forwarded to the output pad. Connection of DATA IN and DATA OUT is effectuated through control of TAP CNTL  2  signal. The TAP CNTL  2  signal as well as TAP CNTL  1 , TAP CLK  1 , and TAP CLK  2 , are derived from a master clock signal which is a buffered version of the TCK signal defined in the JTAG standard. Upon commencement of a test operation, TAP controller  70  will first transition to test-idle state  84 . Transitions  86  occur between states  82  and  84  depending on whether normal operation or test operation occurs. However, once in an appropriate state, TAP controller  70  will remain in that state unless a transition  86  occurs. If a scan test is to be carried out, the TAP controller  70  will, on the next cycle of the master clock, enter selective-scan state  88 . On the next clock cycle of the master clock, TAP controller  70  will enter a capture state  90 . When in state  90 , TAP CNTL  1  signal will be such that the input pin will be connected directly to the input of the capture latch  46 . On the next transition of the master clock, a shift state  92  occurs. In this state TAP controller  70  produces a plurality of clock cycles, such as TAP CLK  1  while maintaining TAP CNTL  1  at a level such that the SERIAL TEST IN signal appears at the input of capture latch  46 . Each transition of TAP CLK  1  will serially shift the captured ATE test data into the test next scan device. The number of shifts N is dependent upon the number of input (or output) boundary scan cells and/or the number of scan elements within the internal scan chain. Thus, the number of clock cycles necessary to shift the test vectors from cells  52  into the core logic (or into the scan elements  53 ) and from the core logic to scan elements  53  (or boundary cells  54 ) is represented as 2N+2M, where N represents the number of boundary scan cells and M represents the number of scan elements. Hence, for 2N+2M cycles of the master clock, shift state  92  is maintained. On the next cycle of the master clock or sometime thereafter, an update state  94  is entered. While in the update state  94 , TAP controller  70  will cause the output of update latch  48  to be updated by generating a clock cycle of TAP CLK  2 . The test vector data bit or field of bits, on the input of update latch  48  will therefore appear on the output of latch  48 , and consequently to the output pin as DATA OUT, or to the core logic as DATA IN. In the input boundary scan example, all of the DATA IN signals will have been updated and thereafter sent into integrated circuit  14 , as well as various integrated circuits serially connected thereto. 
     Test data for internal scan elements are captured on N inputs, then shifted out N cycles. This process is repeated until all of the M scan elements are loaded in M/N capture operations. In the absence of the advantages described herein, it would have taken M shifts through the TDI with unique data, or M vectors. When ATE data is captured in parallel and shifted serially, the extra clock cycles necessary to complete the shift usually does not introduce additional test vectors because after data is captured in parallel, it does not matter what data appears at the parallel inputs while the shift operation is in progress. 
     Once the test vectors are input into the core logic via the update state  94 , state machine will transition to state  84  and remain in state  84  until the output boundary scan cells receive the test results. Then the test results are captured by transitioning TAP CLK  1  while the input multiplexer  44  is controlled through TAP CNTL  1  such that the signal of the DATA OUT line appears on the output of input multiplexer  44 . Capture latch  46  will capture the test result during state  90 . On the next master clock cycle, shift state  92  is entered and, once again, the results captured on the output of capture latch  46  are serially clocked out of the TAP controller under the control of TAP CLK  1  as SERIAL TEST OUT signal. The capture result is clocked out of the capture latch and a new set of test bits can be clocked into corresponding latches. The update state  94  can then be used to forward the capture test result data bit or field to an update latch and thereafter to an output pin of the integrated circuit, whereupon ATE  56  receives the test result in parallel with other output pin test results. The above described sequence of states is repeated continuously for all scan devices that are serially linked between the parallel-fed input pins and the parallel-delivered output pins. 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that the embodiments described are believed applicable to testing either in parallel or serial fashion one or more integrated circuits which may be mechanically and electrically connected to a printed circuit board. The ATE can either be one which delivers and receives test information in parallel or serial fashion and, therefore, selectivity between the various test modes is achieved by forwarding a test mode signal to parallel/serial multiplexers configured upon one or more integrated circuits. The various architectures, circuitry, and methodologies used to produce these advantages are set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense as to possibly numerous architectures, circuitry, and methodologies which fall within the spirit and scope of the present invention.