Abstract:
A method for coupling a linear impedance control (LIC) type output driver to IEEE 1149.1 boundary scan circuitry includes entering a boundary scan load mode to load a test pattern into a chain of boundary scan registers (BSRs). The test pattern includes values corresponding to output enable and data signals according to the IEEE 1149.1 standard. Then these data and output enable signals from the BSRs are converted into test &#34;q --  up&#34; and &#34;q --  dn&#34; signals meeting the requirements of the LIC driver. These test &#34;q --  up&#34; and &#34;q --  dn&#34; signals are selectively provided to the LIC driver during boundary scan testing of the LIC driver. In a further refinement, the method enters a boundary scan capture mode to capture the response (i.e., the functional q --  up and q --  dn signals) of the circuit under test to input test patterns shifted into the BSRs. The functional q --  up and q --  dn signals are converted into response data and oe signals complying with the IEEE 1149.1 specification, which are then captured in the BSRs. Thus, this method allows the widely used IEEE 1149.1 boundary scan standard to be used with LIC drivers. The resulting compatibility simplifies the testing and use of the LIC drivers, and provides a boundary scan standard for use with LIC drivers that is compliant with the IEEE 1149.1 standard.

Description:
FIELD OF THE INVENTION 
     The present invention relates to digital circuits and, more particularly, to test circuits. Still more particularly, the present invention relates to boundary-scan circuits for use with linearized impedance control type output buffers. 
     BACKGROUND 
     Many complex circuits use boundary scan testing techniques to test the output buffers of the circuit. For circuits using conventional two-state or three-state CMOS output buffers, designers commonly use the boundary scan implementation defined in the IEEE Standard Test Access Port and Boundary-Scan Architecture IEEE Std 1149.1-1990 and IEEE Std 1149.1a-1993 (referred to herein as the IEEE 1149.1 Specification or Standard), which is incorporated herein by reference. As is well known, a boundary scan implementation allows for testing of interconnects in a board environment by loading or &#34;scanning in&#34; test patterns into a series of interconnected boundary scan registers (BSRs). Each test pattern loaded in the BSRs provides a different set of control and data signals to the output drivers. The responses of the output drivers to the test patterns can be captured by an adjacent circuit on the board and scanned out. To run a functional test vector, an input test pattern is scanned in through the BSRs. After one or more clock cycles, the response of the circuit can then be captured in the BSRs and either scanned out or monitored at the output pads. 
     FIG. 1 is a circuit diagram of a portion of a circuit 100 using a conventional boundary scan implementation for I/O drivers that have three-state drivers (TSDs). The circuit 100 includes a conventional TSD 102 serving as an output driver, having an output lead connected to an I/O pad 104. The I/O pad 104 is also connected to an input lead of an input driver 105, which drives any signal received from the I/O pad 104 to other portions (not shown) of the circuit 100 in the conventional manner. The circuit 100 also includes conventional BSRs 106 and 107, which are interconnected to form part of a &#34;scan chain&#34; for loading test patterns and scanning out capture data. The BSR 106 includes a capture, shift and update stage (CSUS) 108 that has an output lead connected to an input lead 111 of a two-input multiplexer 112. The other input lead 113 of the multiplexer 112 is connected to receive a fcn --  oe signal provided by another portion (not shown) of the circuit 100. The multiplexer 112 has an output lead 114 connected to the output enable lead of the TSD 102. 
     Similarly, the CSUS 110 has an output lead connected to an input lead 115 of another two-input multiplexer 116. The other input lead 117 of the multiplexer 116 is connected to receive a fcn --  data signal provided by another portion (not shown) of the circuit 100. The multiplexer 116 has an output lead 118 connected to an input lead of the TSD 102. 
     In operation during the boundary scan mode, the CSUSs 108 and 110 are loaded with a test pattern in the conventional manner (see the aforementioned 1149.1 specification). The test pattern is predetermined so that the CSUS 108 is loaded with a value for enabling or disabling the TSD 102, as desired. Thus, the CSUS 108 provides a bsr --  oe signal to the multiplexer 112. Similarly, the CSUS 110 is loaded with a desired value for the data signal to be provided to the input lead of the TSD 102. Thus, the CSUS 110 provides a bsr --  data signal to the multiplexer 116. The multiplexers 112 and 116 receive a mode signal via a line 120 that causes the multiplexers 112 and 116 to select the bsr --  oe and bsr --  data signals. A test access port (TAP) controller according to the 1149.1 specification typically provides this mode signal. Accordingly, the TSD 102 is controlled as desired by the test pattern loaded into the BSRs to test one of the various functions of the I/O driver. The output signal provided by the TSD 102 could then be monitored at the I/O pad 104 and compared to the expected result. Other test patterns may then be loaded to test other functions of the I/O drivers. 
     To test the input portion of the I/O driver, a test signal can be externally provided to the I/O pad 104. The driver 106 then drives the test signal to the rest of the circuit 100 (not shown). The response of the circuit 100 can then be captured in the BSRs. The capture data can then be scanned out from the BSRs and compared to the expected response. In this example, the CSUS 108 and 110 receive capture data through input leads 122 and 124, respectively. 
     During the functional mode, the mode signal is configured to cause the multiplexers 112 and 116 to select the fcn --  oe signal and the fcn --  data signal instead of the bsr --  oe and bsr --  data signals. Of course, the fcn --  oe and fcn --  data signals are generated by the circuit 100 during normal functional operation. Consequently, the multiplexers 112 and 116 provide the fcn --  oe and fcn --  data signals to serve as the data and oe signals received by the TSD 102. 
     However, some high performance circuits such as, for example, microprocessors, have to use other types of drivers for improved performance. One type that can be used is a linearized impedance control type (LIC) driver. FIG. 2 is a circuit diagram of an example of a portion of a circuit 200 including a LIC driver 202. Note, like reference numbers are used throughout the drawings for elements that has substantially similar structure and function. The LIC driver 202 includes a pull-up unit 204 and a pull-down unit 206. The pull-up unit 204 is connected to receive a q --  up signal via an input lead 208. Similarly, the pull-down unit 206 is connected to receive a q --  dn signal via an input lead 210. The LIC 200 can provide the functionality (i.e., a logic zero, logic one and high impedance state) of a conventional CMOS TSD through appropriate control of the logic levels of the q --  up and q --  dn signals, as summarized in Table 1 below. 
     
                       TABLE 1______________________________________q.sub.-- up    q.sub.-- dn                 LIC out______________________________________0              0      00              1      Z1              0      Illegal1              1      1______________________________________ 
    
     The &#34;Z&#34; in Table 1 indicates a high impedance state. As is well known in the art of LIC drivers, the q --  up and q --  dn signals must be generated so that the q --  up signal is not at a logic one level at the same time the q --  dn signal is at a logic zero level. 
     It is appreciated that the q --  up and q --  dn signals of the LIC driver are not equivalent to the data and oe signals of a conventional CMOS TSD. That is, the oe and data signals cannot simply be replaced by the q --  up (or q --  dn) signals in an I/O driver using a LIC driver. Thus, circuits using boundary scan implementations according to the IEEE 1149.1 specification cannot be used with circuits having LIC drivers. However, because the IEEE 1149.1 standard is widely used in the industry, there is a need for a system that allows LIC drivers to be used with boundary scan implementations according to the IEEE 1149.1 specification. 
     SUMMARY 
     In accordance with the present invention, a circuit is provided for coupling a LIC driver to a boundary scan implementation. In one embodiment adapted for the IEEE 1149.1 boundary scan standard, the circuit includes a logic circuit that converts the data and oe signals of the IEEE 1149.1 specification to q --  up and q --  dn signals meeting the requirements of the LIC driver. In a further refinement, the logic circuit also converts functional q --  up and q --  dn signals provided by the circuit under test to the data and oe signals of the IEEE 1149.1 specification. This feature is advantageously used to capture data into the BSRs of the IEEE 1149.1 boundary scan implementation. As a result, the logic circuit allows the widely used IEEE 1149.1 boundary scan standard to be used with LIC drivers. The resulting compatibility simplifies the testing and use of the LIC drivers, and develops a new boundary scan standard for use only with LIC drivers that is compliant with the IEEE 1149.1 standard. 
     In a particular implementation of the above embodiment, the logic circuit includes a first logic circuit for converting the data and oe signals from the CSUSs to q --  up and q --  dn signals. The logic circuit also includes a second logic circuit for converting the functional q --  up and q --  dn signals (i.e., generated by the circuit under test) into &#34;response&#34; oe and data signals to be captured in the BSRs. The first and second logic circuits of the logic circuit thereby allow the IEEE 1149.1 boundary scan standard to be used with LIC drivers in a manner that is transparent to boundary scan tester. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram of a portion of a circuit with a conventional boundary-scan implementation; 
     FIG. 2 is a circuit diagram of a portion of a circuit with a conventional linearized impedance control type output driver; 
     FIG. 3 is a block diagram of an electronic system having a logic circuit for use with a linearized impedance control type output driver, in accordance with one embodiment of the present invention; 
     FIG. 4 is a circuit diagram of a portion of a circuit with a logic circuit for use with a linearized impedance control type output driver, in accordance with one embodiment of the present invention; 
     FIGS. 5A and 5B are flow diagrams illustrative of the operation of the logic circuit of FIG. 4; 
     FIG. 6 is a circuit diagram of an LIC-to-boundary scan logic circuit, in accordance with one embodiment of the present invention; 
     FIG. 7 is a circuit diagram of the LIC-to-boundary scan logic circuit of FIG. 6, according to one embodiment of the present invention; 
     FIG. 8 is a circuit diagram of a boundary scan-to-LIC logic circuit, in accordance with one embodiment of the present invention; 
     FIG. 9 is a circuit diagram of one embodiment of the boundary scan-to-LIC logic circuit of FIG. 8, in accordance with the present invention; and 
     FIG. 10 is a circuit diagram of another embodiment of the boundary scan-to-LIC logic circuit of FIG. 8, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 is a block diagram of an electronic system 300 according to one embodiment of the present invention. The electronic system 300 includes an integrated circuit 301 with a logic circuit for interfacing boundary scan circuitry with a linearized impedance control type output driver, a memory 303, interfaces 305 and peripherals 307 1  -307 N . 
     The electronic system 300 can be any type of electronic system. In this embodiment, the electronic system 300 is a computer system in which the integrated circuit 301 is a processor connected to the memory 303 and to interfaces 305. The processor can be any type of processor such as, for example, Pentium®, X86, Sparc®, Alpha®, MIPS®, HP®, and PowerPC® processors. The interfaces 205 are connected to peripherals 307 1  -307 N , thereby allowing the processor to interact with these peripherals. The memory 303 and the interfaces 305 can be any type of memory or interface for use in computer systems. Likewise, the peripherals can be any type of peripheral such as, for example, displays, mass storage devices, keyboards or any other type of input or input-output device. In accordance with the present invention, the logic circuit used in the integrated circuit 301 allows IEEE 1149.1 boundary scan circuitry to be used with a linearized impedance control type of output driver. 
     FIG. 4 is a circuit diagram of a portion of a logic circuit 400 that includes a logic circuit 402 for coupling LIC drivers to IEEE 1149.1 standard BSRs, in accordance with one embodiment of the present invention. The logic circuit 402 is connected to receive the bsr --  oe and bsr --  data signals from the CSUSs 108 and 110 of the BSRs 106 and 107. The logic circuit 402 is also connected to receive the &#34;functional&#34; (i.e., generated by the circuit 400 during normal functional operation) q --  up and q --  dn signals. The logic circuit 402 outputs a signal bsr --  q --  up, a signal bsr --  q --  dn, a signal rsp --  oe and a signal rsp --  data. 
     FIGS. 5A and 5B are flow diagrams illustrative of the operation of the logic circuit 402 (FIG. 4). Referring to FIGS. 4-5B, the logic circuit 402 operates as follows. In a step 501 (FIG. 5A), the logic circuit 402 enters a boundary scan &#34;shift&#34; mode in which a test pattern is shifted into the BSRs in the conventional manner. As previously described, the test pattern includes values stored in the CSUSs 108 and 110 for &#34;setting&#34; the output enable and data signals to predetermined logic levels to test the functionality of the LIC driver 202 (FIG. 2). 
     In a step 503 (FIG. 5A), the logic circuit 402 receives the output enable (i.e., bsr --  oe) signal and the data (i.e., bsr data) signal from the CSUSs 108 and 110. In a next step 505 (FIG. 5A), the logic circuit 402 converts the signals bsr --  oe and bsr --  data from the CSUSs 108 and 110 into q --  up and q --  dn signals (i.e., the bsr --  q --  up and bsr --  q --  dn signals), respectively. More specifically, for this &#34;boundary scan-to-LIC&#34; feature, the logic circuit 402 implements the truth table shown below in Table 2. 
     
                       TABLE 2______________________________________bsr.sub.-- data     bsr.sub.-- oe             bsr.sub.-- q.sub.-- up                         bsr.sub.-- q.sub.-- dn                                LIC out______________________________________0         1       0           0      0X         0       0           1      Z1         1       1           1      1______________________________________ 
    
     The X and Z in Table 2 respectively indicate a &#34;don&#39;t care&#34; and a high impedance condition. The logic circuit 402 provides the bsr --  q --  up and bsr --  q --  dn signals on output leads that are respectively connected to the input leads 111 and 115 of the multiplexers 112 and 116 (FIG. 1). As summarized in Table 2, the mapping of the bsr --  data and bsr --  oe signals into the bsr --  q --  up and bsr --  q --  dn signals omits the illegal condition of the bsr --  q --  up signal being at a logic one level at the same time that the bsr --  q --  dn signal is at a logic zero level. Particular embodiments of circuitry implementing the functionality of Table 2 are described below in conjunction with FIGS. 8-10. Of course, in light of the present disclosure, those skilled in the art of digital circuits can design many other circuits that implement the functionality of Table 2 without undue experimentation. 
     Conversely, during boundary scan testing of the function of the circuit 400, the logic circuit 402 enters a boundary scan capture mode during a step 507 (FIG. 5B). In a next step 509 FIG. 5B), the logic circuit 402 receives the functional LIC control signals (i.e., fcn --  q --  up and fcn --  q --  dn) resulting from the test pattern. Then in a step 511 (FIG. 5B), the logic circuit 402 converts the fcn --  q --  up and fcn --  q --  dn signals into &#34;response&#34; oe and data signals (i.e., the rsp --  oe and rsp --  data signals) to be captured in the CSUS 108 and 110 in the conventional manner. More specifically, for this &#34;LIC-to-boundary scan&#34; feature, the logic circuit 402 implements the truth table shown below in Table 3. 
     
                       TABLE 3______________________________________fcn.sub.-- q.sub.-- up    fcn.sub.-- q.sub.-- dn                  rsp.sub.-- data                           rsp.sub.-- oe______________________________________0        0             0        10        1             X        01        1             1        1______________________________________ 
    
     The X in Table 3 indicates a &#34;don&#39;t care&#34; condition. The logic circuit 402 provides the signals rsp --  oe and rsp --  data on output leads that are respectively connected to the input leads 122 and 124 of the CSUS 108 and 110. As summarized in Table 3, the mapping of the fcn --  q --  up and fcn --  q --  dn signals into the rsp --  data and rsp --  oe signals does not include the illegal condition of the fcn --  q --  up signal being at a logic one level at the same time that the fcn --  q --  dn signal is at a logic zero level. Particular embodiments of circuitry implementing the functionality of Table 3 are described below in conjunction with FIGS. 6 and 7. Of course, in light of the present disclosure, those skilled in the art of digital circuits can design many other circuits that implement the functionality of Table 3 without undue experimentation. Then in a next step 513, the response signals rsp --  oe and rsp --  data are shifted out of the BSRs in the conventional manner. Alternatively, the step 505 may be performed so that the captured values are used to generate LIC control signals, which causes LIC drivers to output the response. 
     FIG. 6 is a circuit diagram of an LIC-to-boundary scan (LIC-BSR) logic circuit 602, in accordance with one embodiment of the present invention. In this embodiment, the LIC-BSR logic circuit 602 forms part of the logic circuit 402 (FIG. 4) to implement the aforementioned &#34;LIC-to-boundary scan&#34; feature. The LIC-BSR logic circuit 602 has input leads 604 and 606 and output leads 608 and 610. The input leads 604 and 606 are respectively connected to receive the signals fcn --  q --  up and fcn --  q --  dn generated by the circuit 800 during normal functional operation. The output leads 608 and 610 are respectively connected to the input leads 122 and 124 of the CSUS 108 and 110. The LIC-BSR logic circuit 602 operates during the boundary scan mode to convert the signals fcn --  q --  up and fcn --  q --  dn into the signals rsp --  oe and rsp --  data (conforming to the IEEE 1149.1 specification) to be captured in the CSUS 108 and 110. More specifically, the LIC-BSR logic circuit 602 implements the truth table shown below in Table 5. In this way, the LIC-BSR logic circuit 602 serves to make the IEEE 1149.1 boundary scan standard interoperable with the LIC driver 202. 
     
                       TABLE 5______________________________________fcn.sub.-- q.sub.-- up    fcn.sub.-- q.sub.-- dn                  rsp.sub.-- data                           rsp.sub.-- oe______________________________________0        0             0        10        1             X        11        1             1        1______________________________________ 
    
     The X indicates a &#34;don&#39;t care&#34; condition. As summarized in Table 5, the mapping of the signals fcn --  q --  up and fcn --  q --  dn into the signals rsp --  data and rsp --  oe omits the illegal condition of the fcn --  q --  up signal being at a logic one level at the same time that the fcn --  q --  dn signal is at a logic zero level. A particular embodiment of a circuit implementing the functionality of Table 5 is described below in conjunction with FIG. 7. Of course, in light of the present disclosure, those skilled in the art of digital circuits can design many other circuits that implement the functionality of Table 5 without undue experimentation. 
     FIG. 7 is a circuit diagram of the LIC-BSR logic circuit 602 (FIG. 6), according to one embodiment of the present invention. In this embodiment, the LIC-BSR logic circuit 602 includes a non-inverting buffer 702 and a two-input OR gate 704. The buffer 702 has an input lead connected to receive the signal fcn --  q --  up and has an output that serves as the output lead 610 FIG. 6) of the LIC-BSR logic circuit 602. The OR gate 704 has one input lead connected to receive the signal fcn --  q --  up and has another input lead connected to receive the signal fcn --  q --  dn. The output lead of the OR gate 704 serves as the output lead 608 (FIG. 6) of the LIC-BSR logic circuit 602. 
     This embodiment of the LIC-BSR logic circuit 602 operates as follows. When the signals fcn --  q --  up and fcn --  q --  dn are both at logic zero levels, (i) the buffer 702 outputs the signal rsp --  data with a logic zero level to be captured in the CSUS 110 (FIG. 6), and (ii) the OR gate 704 outputs the signal rsp --  oe with a logic one level to be captured in the CSUSS 108 (FIG. 6). Thus, in this case, this embodiment of the LIC-BSR logic circuit 602 conforms to the first row of Table 5. 
     When the signal fcn --  q --  up is at a logic zero level and the signal fcn --  q --  dn is at a logic one level, the buffer 702 outputs the signal rsp --  data with a logic zero level while the OR gate 704 outputs the signal rsp --  oe with a logic one level. Thus, in this case, this embodiment of the LIC-BSR logic circuit 602 conforms to the second row of Table 5. 
     Then, when the signal fcn --  q --  up is at a logic one level and the signal fcn --  q --  dn is at a logic one level, the buffer 702 and the OR gate 704 output the signals rsp --  data and rsp --  oe with logic one levels. Thus, in this case, this embodiment of the LIC-BSR logic circuit 602 conforms to the third row of Table 5. 
     FIG. 8 is a circuit diagram of a portion of a circuit 800 having a boundary scan-to-LIC (also referred to herein as &#34;BSR-LIC&#34;) logic circuit 802, in accordance with one embodiment of the present invention. In this embodiment, the BSR-LIC logic circuit 802 forms part of the logic circuit 402 (FIG. 4) to implement the aforementioned &#34;boundary scan-to-LIC&#34; feature. The boundary scan implementation used in the circuit 800 is substantially similar to the boundary scan implementation used in the circuit 400 (FIG. 4), except that in the circuit 800, the boundary scan implementation also supports a high impedance signal hiz. More specifically, the boundary scan implementation in the circuit 800 also supports the optional high impedance signal of the IEEE 1149.1 standard. The signal hiz is a global signal that can be used to configure several LIC drivers into the high impedance state using a single signal (i.e., without having to set several pairs of q --  up and q --  dn signals). 
     In particular, the BSR-LIC logic circuit 802 is connected to receive the signal bsr --  oe from the CSUS 108, the signal bsr --  data from the CSUS 110 and a signal bsr --  hiz --  n. The signal bsr --  hiz --  n is used during boundary scan testing and corresponds to the complement of a signal hiz typically provided by a TAP controller (not shown) compliant with the IEEE 1149.1 standard. The CSUSs 108 and 110 form part of a &#34;scan chain&#34; in the boundary scan implementation. Although in this embodiment the signals bsr --  oe and bsr --  data are provided from adjoining BSRs, it will be appreciated that these signals can be provided from non-adjoining BSRs as well. Further, in other embodiments, the bsr --  hiz --  n signal may be replaced with a bsr --  hiz signal, with the inversion performed in the BSR-LIC logic circuit 802. 
     The BSR-LIC logic circuit 802 has an output lead connected to the input lead 111 of the multiplexer 112, through which the BSR-LIC logic circuit 802 outputs the signal bsr --  q --  up. The other input lead 113 of the multiplexer 112 is connected to receive the &#34;functional&#34; q --  up signal (i.e., signal fcn --  q --  up) generated by the circuit 800 during normal functional operation. In addition, the BSR-LIC logic circuit 802 has another output lead connected to the input lead 115 of the multiplexer 116 through which the BSR-LIC logic circuit 802 outputs the signal bsr --  q --  dn. The other input lead 117 of the multiplexer 116 is connected to receive the &#34;functional&#34; q --  dn signal (i.e., signal fcn --  q --  dn) generated by the circuit 800 during normal functional operation. 
     This portion of the circuit 800 operates as follows. During the boundary scan mode, the BSR-LIC logic circuit 802 receives the signals bsr --  oe and bsr --  data from the CSUSs 108 and 110 and the signal bsr --  hiz --  n from the TAP controller (not shown). The BSR-LIC logic circuit then converts the signals bsr --  oe, bsr --  data and bsr --  hiz --  n into the signals bsr --  q --  up and bsr --  q --  dn that are used by the LIC driver 202. This conversion is performed so that the LIC driver 202 responds to the signals bsr --  oe, bsr --  data and bsr --  hiz --  n in the same way that a conventional CMOS TSD would respond. For example, when the signal bsr --  oe is deasserted during boundary scan testing, the BSR-LIC circuit 802 causes the signals bsr --  q --  up and bsr --  q --  dn to be at logic zero and logic one levels, respectively. Consequently, the LIC driver 202 enters a high impedance output state. Also, when the signal bsr --  hiz --  n is at a logic one level, the BSR-LIC logic circuit functions in response to the signals bsr --  oe and bsr --  data as described above in conjunction with Table 2. However, when the signal bsr --  hiz --  n is at a logic zero level during boundary scan testing, the BSR-LIC logic circuit 802 causes the signals bsr --  q --  up and bsr --  q --  dn to be at logic zero and logic one levels, respectively, which causes the LIC driver 202 to enter a high impedance output state. More specifically, the BSR-LIC logic circuit 802 implements the truth table shown below in Table 4. 
     
                       TABLE 4______________________________________bsr.sub.-- data  bsr.sub.-- oe           bsr.sub.-- hiz.sub.-- n                     bsr.sub.-- q.sub.-- up                             bsr.sub.-- q.sub.-- dn                                    LIC out______________________________________0      1        1         0       0      0X      0        1         0       1      Z1      1        1         1       1      1X      X        0         0       1      Z______________________________________ 
    
     The Xs indicate &#34;don&#39;t care&#34; conditions whereas the Zs indicate a high impedance state. As summarized in Table 4, the mapping of the signals bsr --  hiz --  n, bsr --  data and bsr --  oe into the signals bsr --  q --  up and bsr --  q --  dn omits the illegal condition of the bsr --  q --  up signal being at a logic one level at the same time that the bsr --  q --  dn signal is at a logic zero level. Particular embodiments of circuitry implementing the functionality of Table 4 are described below in conjunction with FIGS. 9-10. Of course, in light of the present disclosure, those skilled in the art of digital circuits can design many other circuits that implement the functionality of Table 4 without undue experimentation. 
     FIG. 9 is a circuit diagram of one embodiment of the BSR-LIC logic circuit 802 (FIG. 8), in accordance with the present invention. In this implementation, the BSR-LIC logic circuit 802 includes two-input multiplexers 902 and 904 and a two-input AND gate 906. The AND gate 906 is connected to receive from the CSUS 108 and 804 (FIG. 8) the signal bsr --  oe and the inverse of the high impedance control signal hiz (i.e., bsr --  hiz --  n). The output lead of the AND gate 906 is connected to the select input leads of the multiplexers 902 and 904. Accordingly, the output signal generated by the AND gate 906 serves as the select signal for the multiplexers 902 and 904. 
     In addition, the multiplexer 902 is connected to receive a logic zero signal on one input lead. In this embodiment, the logic zero signal is hardwired. The multiplexer 902 is also connected to receive at its other input lead the signal bsr --  data from the CSUS 110 (FIG. 8). The output signal of the multiplexer 902 serves as the signal bsr --  q --  up. Similarly, the multiplexer 904 is connected to receive a hardwired logic one signal at one input lead and the bsr --  data signal at the other input lead. The output signal of the multiplexer 904 serves as the signal bsr --  q --  dn. Both multiplexers are configured to select the signal bsr --  data when the select signal from the AND gate 906 is at a logic one level. Consequently, the multiplexers select the hardwired signal when the AND gate 906 outputs a logic high level signal. 
     This embodiment of the BSR-LIC logic circuit 802 operates as follows. When both the signal bsr --  hiz --  n and the signal bsr --  oe are at logic one levels (i.e., the driver is enabled and the &#34;global&#34; high impedance state is not selected), the AND gate 906 will output the select signal with a logic one level to the multiplexers 902 and 904. As a result, the multiplexers select the signal bsr --  data. Conversely, when either signal bsr --  oe or signal bsr --  hiz is at a logic zero level (i.e., the driver is disabled or the &#34;global&#34; high impedance state is selected), the AND gate 906 will generate the select signal with a logic zero level. Thus, the BSR-LIC logic circuit 802 outputs the signals bsr --  q --  up and bsr --  q --  dn with logic zero and logic one levels, respectively. Consequently, the LIC driver 202 will enter the high impedance state. Accordingly, this embodiment of the BSR-LIC logic circuit 802 implements the truth table of Table 8. 
     FIG. 10 is a circuit diagram of another embodiment of the BSR-LIC logic circuit 802 (FIG. 8), in accordance with the present invention. This embodiment includes a three-input AND gate 1001 and a three-input NAND gate 1003. The AND gate 1001 is connected to receive the signals bsr --  data, bsr --  oe and bsr --  hiz --  n. The output signal generated by the AND gate 1001 serves as the signal bsr --  q --  up received by the multiplexer 112 (FIG. 8). 
     The NAND gate 1003 is connected to receive the signals bsr --  data --  n (i.e., the complement of the signal bsr --  data), bsr --  oe and bsr --  hiz --  n. Of course, in other embodiments, an inverter may be included to generate the signal bsr --  data --  n from the signal bsr --  data. The output signal generated by the NAND gate 1003 serves as the signal bsr --  q --  dn received by the multiplexer 116 (FIG. 8). 
     This embodiment of the BSR-LIC logic circuit 802 operates as follows. When either the signal bsr --  oe or the signal bsr --  hiz --  n is at a logic low level, (i) the AND gate 1001 will output the signal bsr --  q --  up at a logic zero level, and (ii) the NAND gate 1003 will output the signal bsr --  q --  dn at a logic one level. As a result of the signals bsr --  q --  up and bsr --  q --  dn being at logic zero and logic one levels, respectively, the LIC driver 202 (FIG. 8) will enter a high impedance state. 
     In contrast, when the signals bsr --  oe and bsr --  hiz --  n are both at logic one levels, the AND gate 1001 is, in effect, equivalent to a non-inverting buffer receiving the signal bsr --  data. Thus, in this situation, the signal bsr --  q --  up is equivalent to the signal bsr --  data. Likewise, the NAND gate 1003 is, in effect, equivalent to an inverter receiving the signal bsr --  data --  n. Because of the two inversions, the NAND gate 1003 outputs the signal bsr --  q --  dn having the same logic level as the signal bsr --  data. Accordingly, this embodiment of the BSR-LIC logic circuit 802 implements the truth table of Table 8. 
     The embodiments of the circuit described above are illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. For example, in light of the present disclosure, those skilled in the art of boundary scan circuits can implement other embodiments adapted for use with other boundary scan standards without undue experimentation. In addition, switching devices other than the multiplexers described may be used in other embodiments. Accordingly, while the preferred embodiment of the invention has been illustrated and described, it will be appreciated that in view of the present disclosure, various changes can be made therein without departing from the spirit and scope of the invention.