Abstract:
An optimized JTAG interface is used to access JTAG Tap Domains within an integrated circuit. The interface requires fewer pins than the conventional JTAG interface and is thus more applicable than conventional JTAG interfaces on an integrated circuit where the availability of pins is limited. The interface may be used for a variety of serial communication operations such as, but not limited to, serial communication related integrated circuit test, emulation, debug, and/or trace operations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of application Ser. No. 14/494,787, filed Sep. 24, 2014, currently pending; 
         [0002]    Which was a divisional of application Ser. No. 13/855,970, filed Apr. 3, 2013, now U.S. Pat. No. 8,880,966, granted Nov. 4, 2014; 
         [0003]    Which was a divisional of application Ser. No. 13/551,167, filed Jul. 17, 2012, now U.S. Pat. No. 8,433,962, granted Apr. 30, 2013; 
         [0004]    Which was a divisional of application Ser. No. 13/197,000, filed Aug. 3, 2011, now U.S. Pat. No. 8,250,421, granted Aug. 21, 2012; 
         [0005]    Which was a divisional of application Ser. No. 13/012,117, filed Jan. 24, 2011, now U.S. Pat. No. 8,020,059, granted Sep. 13, 2011; 
         [0006]    Which was a divisional of application Ser. No. 12/887,672, filed Sep. 22, 2010, now U.S. Pat. No. 7,900,110, granted Mar. 1, 2011; 
         [0007]    Which was a divisional of application Ser. No. 12/640,941, filed Dec. 17, 2009, now U.S. Pat. No. 7,823,037, granted Oct. 6, 2010; 
         [0008]    Which was a divisional of application Ser. No. 12/182,605, filed Jul. 30, 2008, now U.S. Pat. No. 7,669,099, granted Feb. 23, 2010; 
         [0009]    Which was a divisional of application Ser. No. 11/370,017, filed Mar. 7, 2006, now U.S. Pat. No. 7,421,633, granted Sep. 2, 2008; 
         [0010]    And this application claims priority from Provisional Application No. 60/663,953, filed Mar. 21, 2005, and is related to the following patent applications or patents: 
         [0011]    Application Ser. No. 11/292,643, “Reduced Signal Interface Method and Apparatus,” now U.S. Pat. No. 7,308,629, issued Dec. 11, 2007; 
         [0012]    Application Ser. No. 11/293,061, “Selectable Pin Count JTAG,” now U.S. Pat. No. 7,328,387, issued Feb. 5, 2008; 
         [0013]    Application Ser. No. 11/258,315, “2 Pin Bus”, now U.S. Pat. No. 8,412,853, issued Apr. 2, 2013; 
         [0014]    Application Ser. No. 08/918,872, U.S. Pat. No. 6,073,254 “Selectively Accessing IEEE 1149.1 Taps in a Multiple Tap Environment,” issued Jun. 6, 2000; and 
         [0015]    Application Ser. No. 11/292,597, “Multiple Test Access Port Protocols Sharing Common Signals”, now U.S. Pat. No. 7,571,366, issued Aug. 4, 2009. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0016]    This disclosure relates in general to IC signal interfaces and in particular to IC signal interfaces related to test, emulation, debug, and trace operations. 
       DESCRIPTION OF THE RELATED ART 
       [0017]      FIG. 1  illustrates a conventional 5 wire JTAG interface  106  between an external JTAG controller  100  and Tap Domains  104  within a target IC  102 . Modern day ICs typically have a Tap Domain associated with the IC&#39;s JTAG boundary scan test operations and/or one or more Tap Domains associated with each one or more core circuits designed into the IC. The interface couples the TDO output of JTAG controller to the IC&#39;s TDI pin input, the TMS output of the JTAG controller to the IC&#39;s TMS pin input, the TCK output of the JTAG controller to the IC&#39;s TCK pin input, the TDI input of the JTAG controller to the IC&#39;s TDO pin output, and the TRST output of the JTAG controller to the IC&#39;s TRST pin input. The IC&#39;s TDI, TDO, TMS, TCK, and TRST pins  108  are dedicated for interfacing to the JTAG controller and cannot be used functionally. 
         [0018]    In response to the TMS and TCK signals, the Tap Domains  104  of IC  102  communicates data to and from the JTAG controller via the TDO to TDI connections. A low output on the JTAG controller&#39;s TRST output causes the Tap Domains of IC  102  to enter a reset state. The JTAG controller receives a clock input (CKIN) from a clock source  110 . The CKIN input times the operation of the JTAG controller, which in turn times the operation of the Tap Domains in IC  102 . The JTAG controller can be used to perform test, emulation, debug, and trace operations in the target IC by accessing the embedded Tap Domains via the 5 wire interface. The arrangement between the JTAG controller and the target IC and its use in performing test, emulation, debug, and trace operations is well known in the industry. 
         [0019]      FIG. 2  illustrates an alternate arrangement whereby a JTAG controller  200  is interfaced to a target IC  202  via the JTAG bus  108  and a Debug/Trace bus  204 . The JTAG controller  200  differs from the JTAG controller of  FIG. 1  in that it includes additional circuitry and input/outputs for interfacing to the IC&#39;s Debug/Trace circuitry  204 . As in  FIG. 1 , the JTAG bus  108  is coupled to Tap Domains  104  within the IC via IC pins  108 . The Debug/Trace bus  204  is coupled to Debug/Trace circuitry  206  within the IC via N IC pins  208 . The JTAG bus is used to input commands and data that enable the Debug/Trace circuitry to perform debug and/or trace operations. The Debug/Trace bus signals can be used for a myriad of operations including but not limited to; (1) importing and/or exporting data between the JTAG controller  200  and Debug/Trace circuitry  206  during debug and/or trace operations, (2) operating as a communications bus between the JTAG controller  200  and Debug/Trace circuitry  206 , and ( 3 ) inputting and/or outputting trigger signals between the JTAG controller  200  and Debug/Trace circuitry  206  during debug and trace operations. 
         [0020]    One of the key advantages of the debug/trace bus  204  is that it increases the data input/output bandwidth between the JTAG controller and target IC during debug/trace operation over what is possible using only the 5 wire JTAG bus  106 . For example, the data input/output bandwidth of the JTAG bus is limited to the amount of data that can flow between the JTAG controller and IC over the single TDO to TDI signal wire connections. Since the debug/trace bus can have N signal wire connections between the JTAG controller and IC (N), its data bandwidth can be much greater than the JTAG bus bandwidth. Increased data bandwidth between the JTAG controller and IC facilitates debug/trace operations such as; (1) monitoring real time code execution, (2) accessing embedded memories, (3) uploading/downloading code during program debug, and (4) triggered output trace functions. 
         [0021]    With the current trend towards smaller IC packaging to allow more ICs to be placed on smaller assemblies used in mobile applications, such as cell phones and personal digital assistants, the number of IC pins is being reduced. It is therefore a benefit of the present disclosure to provide a reduced pin count interface on ICs for test, emulation, debug, and trace operations, as this will allow more IC pins to be available for functional purposes. While it is advantageous to reduce the pin counts of both the JTAG and Debug/Trace buses of  FIGS. 1 and 2 , this application focuses on reducing the JTAG bus pins of an IC. 
         [0022]    In addition to reducing the JTAG bus pins of an IC, a second benefit of the present disclosure is to maintain a high communication bandwidth over the reduced JTAG pins. As will be shown, the present disclosure provides a data communication bandwidth using the reduced JTAG pins that is equal to one half the data communication bandwidth using a full set of JTAG pins. For example, if the JTAG controller  100  can communicate data to and from Tap Domains  104  of  FIG. 1  at 100 Mhz using the full JTAG bus  106 , a JTAG controller adapted according to the present disclosure can communicate data to and from Tap Domains  104  of an IC, also adapted according to the present disclosure at 50 Mhz. 
         [0023]    One prior art technique, referenced herein, is called the J-Link System. The J-Link system provides a way to reduce the JTAG pins of an IC from the standard five pins to a reduced set of one or two pins. In a chart shown in the J-Link reference, it is seen that the J-Link interface provides a data communication bandwidth that is one sixth that of the conventional JTAG 5 pin interface. For example and as stated in the J-Link reference, if the standard 5 pin JTAG interface can operate at 48 Mhz, the J-Link interface operates at one sixth of the 48 Mhz frequency, or at 8 Mhz. In comparison and as will be shown herein, if the standard 5 pin JTAG interface can operate at 48 Mhz, the reduce pin approach of the present disclosure can operate at one half the 48 Mhz frequency, of at 24 Mhz. Thus the present disclosure provides a three times improvement in operating frequency over the referenced J-Link approach. The present disclosure is therefore capable of performing operations related to IC test, debug, emulation, and trace at three times the bandwidth of the referenced J-Link approach. 
       SUMMARY OF THE DISCLOSURE 
       [0024]    The present disclosure provides a reduced pin interface for JTAG based test, emulation, debug, and trace transactions between a JTAG controller and a target IC. 
     
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates a conventional 5 signal interface between a JTAG controller and target IC. 
           [0026]      FIG. 2  illustrates a conventional JTAG controller interfaced to a target IC via a 5 signal JTAG bus and an N signal Debug/Trace bus. 
           [0027]      FIG. 3  illustrates a JTAG controller interfaced to a target IC via a 2 signal JTAG bus according to the present disclosure. 
           [0028]      FIGS. 4A-4C  illustrate various conventional Tap Domain arrangements within a target IC. 
           [0029]      FIG. 5A  illustrates a circuit example of the parallel to serial controller (PSC) circuit of the present disclosure. 
           [0030]      FIG. 5B  illustrates a timing diagram of the operation of the PSC circuit of  FIG. 5A . 
           [0031]      FIG. 6A  illustrates a circuit example of the controller within the PSC circuit of  FIG. 5A . 
           [0032]      FIG. 6B  illustrates a timing diagram of the operation of the controller of  FIG. 6A . 
           [0033]      FIG. 7A  illustrates a circuit example of the serial to parallel controller (SPC) circuit of the present disclosure. 
           [0034]      FIG. 7B  illustrates a timing diagram of the operation of the SPC circuit of  FIG. 7A . 
           [0035]      FIG. 8A  illustrates a circuit example of the controller within the SPC circuit of  FIG. 7A . 
           [0036]      FIG. 8B  illustrates a timing diagram of the operation of the controller of  FIG. 8A . 
           [0037]      FIG. 9A  illustrates a circuit example of the master reset and synchronizer (MRS) circuit within the SPC circuit of  FIG. 7A . 
           [0038]      FIG. 9B  illustrates a state diagram of the operation of the MRS circuit of  FIG. 9A . 
           [0039]      FIG. 9C  illustrates a timing diagram of the operation of the MRS circuit of  FIG. 9A . 
           [0040]      FIG. 10  illustrates the state diagram of the IEEE standard 1149.1 Tap controller state machine. 
           [0041]      FIG. 11A  illustrates a circuit example of the input/output (I/O) circuits within the PSC and SPC circuits. 
           [0042]      FIG. 11B  illustrates the signaling cases for the I/O circuits of  FIG. 11A . 
           [0043]      FIG. 12  illustrates each signaling case of  FIG. 11B  in more detail. 
           [0044]      FIG. 13A  illustrates an example circuit for determining the appropriate TDI or IN signal output of the I/O circuits of  FIG. 11 . 
           [0045]      FIG. 13B  illustrates the truth table used for determining the appropriate TDI or IN signal output based on the voltage level of the data I/O (DIO) signal. 
           [0046]      FIG. 14A  illustrates the  2  signal connection between the PSC of the JTAG controller and the SPC of the target IC according to the present disclosure. 
           [0047]      FIG. 14B  illustrates a timing diagram of the operation of the PSC and SPC circuits of  FIG. 14A  performing JTAG transactions between the JTAG controller and the Tap Domains of the target IC. 
           [0048]      FIG. 14C  illustrates a timing diagram of the operation of the PSC and SPC circuits of  FIG. 14A  performing a single bit data register scan between the JTAG controller and the Tap Domains of the target IC. 
           [0049]      FIG. 15  illustrates a Texas Instruments SN74ACT8990 JTAG bus controller chip operating to compensate for cable delays. 
           [0050]      FIG. 16  illustrates a 2 pin realization of the present disclosure whereby the CLK signal is driven by a clock source within the JTAG controller. 
           [0051]      FIG. 17  illustrates a 2 pin realization of the present disclosure whereby the CLK signal is driven by an internal clock source of the target IC. 
           [0052]      FIG. 18  illustrates a 1 pin realization of the present disclosure whereby the CLK signal is driven by an external clock source that functionally inputs to the target IC. 
           [0053]      FIG. 19  illustrates a 1 pin realization of the present disclosure whereby the CLK signal is driven by an internal clock source of the target IC that functionally outputs from the IC. 
           [0054]      FIG. 20  illustrates a 2 pin realization of the present disclosure whereby the CLK signal is driven by an clock source external of the JTAG controller and target IC. 
           [0055]      FIG. 21A  illustrates an alternate circuit example of the parallel to serial controller (PSC) circuit of the present disclosure. 
           [0056]      FIG. 21B  illustrates a timing diagram of the operation of the alternate PSC circuit of  FIG. 5A . 
           [0057]      FIG. 22A  illustrates an alternate circuit example of the serial to parallel controller (SPC) circuit of the present disclosure. 
           [0058]      FIG. 22B  illustrates a timing diagram of the operation of the SPC circuit of  FIG. 7A . 
           [0059]      FIG. 23A  illustrates the 3 signal connection between the  FIG. 21A  alternate PSC of the JTAG controller and the  FIG. 22A  alternate SPC of the target IC of according to the present disclosure. 
           [0060]      FIG. 23B  illustrates a timing diagram of the operation of the alternate  FIG. 21A  PSC and  FIG. 22A  SPC circuits performing JTAG transactions between the JTAG controller and the Tap Domains of the target IC. 
           [0061]      FIG. 24  illustrates a 3 pin realization of the alternate version of the present disclosure whereby the CLK signal is driven by a clock source within the JTAG controller. 
           [0062]      FIG. 25  illustrates a 3 pin realization of the alternate version of the present disclosure whereby the CLK signal is driven by an internal clock source of the target IC. 
           [0063]      FIG. 26  illustrates a 2 pin realization of the alternate version of the present disclosure whereby the CLK signal is driven by an external clock source that functionally inputs to the target IC. 
           [0064]      FIG. 27  illustrates a 2 pin realization of the alternate version of the present disclosure whereby the CLK signal is driven by an internal clock source of the target IC that functionally outputs from the IC. 
           [0065]      FIG. 28  illustrates a 3 pin realization of the alternate version of the present disclosure whereby the CLK signal is driven by an clock source external of the JTAG controller and target IC. 
       
    
    
     DETAILED DESCRIPTION 
       [0066]      FIG. 3  illustrates the approach of the present disclosure to reduce the number of JTAG pins on an IC  300  and the number of JTAG bus signal connections between the IC  300  and JTAG controller  100 . IC  300  and others illustrated in this disclosure could represent any type of integrated circuit including but not limited to, a microcontroller IC, a microprocessor IC, a digital signal processor IC, a mixed signal IC, an FPGA/CPLD IC, an ASIC, a system on chip IC, a peripheral IC, a ROM memory IC, or a RAM memory IC. In  FIG. 3 , the JTAG controller  100  is interfaced to a Parallel to Serial Controller (PSC) circuit  302  via TDO, TMS, CKIN, TDI, and TRST signals. The PSC  302  may be a separate circuit from the JTAG controller  100  or the PSC  302  and JTAG controller  100  may be integrated to form a new JTAG controller  304 . The PSC  302  is interfaced to a Serial to Parallel Controller (SPC) circuit  306  in IC  300  via a bus comprising a data I/O (DIO) signal  308  and a clock (CLK) signal  310 . The SPC  306  is interfaced to Tap Domains  104  in the IC  300  via TDI, TMS, TCK, TDO, and TRST signals. As will be described later in regard to  FIGS. 16-20 , the CLK signal  310  may be driven by a clock source associated with the JTAG controller  100 , a clock source associated with the IC  300 , or a clock source not associated with the JTAG controller  100  or IC  300 . 
         [0067]      FIG. 4A  illustrates that the Tap Domain block  104  of IC  300  may consist of a single 1149.1 Tap architecture. 
         [0068]      FIG. 4B  illustrates that the Tap Domain block  104  of IC  300  may consist of a series of daisy-chained Tap architectures 1−N. 
         [0069]      FIG. 4C  illustrates that the Tap Domain block  104  of IC  300  may consist of a group of Tap architectures 1−N that may be selected individually or linked serially together in various daisy-chain arrangements using linking circuitry  400 . An example of such linking circuitry  400  has been described in referenced U.S. Pat. No. 6,073,254. 
         [0070]      FIG. 5A  illustrates the PSC circuit  302  in more detail. The PSC consists of a controller  500 , a parallel input serial output (PISO) register  502 , and an input/output (I/O) circuit  504 . PISO  502  inputs parallel TMS and TDO signals from the JTAG controller  100 , the TRST signal from the JTAG controller  100 , a load (LD) signal from controller  500 , and outputs a serial output (OUT) signal to I/O circuit  504 . 
         [0071]    A simplified view of PISO  502  shows it containing two serially connected FFs  503  and  505 . While the TRST signal from the JTAG controller is low, FFS  503  and  505  are asynchronously set to logic ones and do not respond to the CLK or LD inputs. This can be achieved, for example, by connecting the TRST signal to the Set input of FFs  503  and  505 . The OUT signal is therefore high while TRST is low. When TRST goes high FFS  503  and  505  are enabled to respond to the CLK and LD inputs. In response to the LD input, FFs  503  and  505  asynchronously load TMS and TDO output from the JTAG controller, respectively. Once loaded, the FFs are shifted by CLK  310  to output TMS then TDO signals to I/O circuit  504  via the OUT signal. 
         [0072]    Controller  500  inputs the CLK signal  310 , the TRST signal from the JTAG controller  100 . Controller  500  outputs the asynchronous LD signal to the PISO and a clock signal to the CKIN input of JTAG controller  100 . While TRST is low, the controller is reset and does not respond to the CLK input. While reset the LD and CKIN outputs from the controller are low. When TRST goes high, the controller is enabled to respond to the CLK input and output LD and CKIN output signals. 
         [0073]    I/O circuit  504  inputs the OUT signals from the PISO and outputs them on DIO  308 . The I/O circuit  504  also inputs signals from DIO  308  and outputs them to the TDI input of JTAG controller  100 . I/O circuit  504  is designed to allow the output of OUT signals to DIO  308  and the input of TDI signals from DIO  308  to occur simultaneously. The simultaneous input and output operation of I/O circuit  504  will be described in detail later in regard to  FIGS. 11A ,  11 B,  12 ,  13 A, and  13 B. 
         [0074]    The operation of PSC  302  (while TRST is high) is illustrated in the timing diagram of  FIG. 5B . In response to the CLK input  310 , the controller  500  operates to periodically output the LD signal to PISO  502  and the CKIN signal to JTAG controller  100 . Also the CLK input  310  times the PISO  502  to shift data from its OUT output to the I/O circuit  504 . The I/O circuit passes the OUT signal to the DIO  308  signal. The CKIN signal times the operation of the JTAG controller  100 . The LD signal causes the PISO to asynchronously load the TMS and TDO signal pattern from JTAG controller  100 . Once loaded, the TMS and TDO pattern is shifted out of the PISO to the I/O circuit in response to the CLK signal. 
         [0075]    The following describes the PSC&#39;s repeating load and shift out sequence. A TMS and TDO pattern  510  is asynchronously loaded into the PISO in response to LD signal  512 . CLK signal  514  shifts out the TMS signal portion of pattern  510  on the OUT output of the PISO, then CLK signal  516  shifts out the TDO signal portion of pattern  510  on the OUT output of the PISO. CKIN signal  518  advances the JTAG controller to output the next TMS and TDO pattern  520 . LD signal  522  asynchronously loads the next TMS and TDO pattern  520  into the PISO. CLK signal  524  shifts out the TMS signal portion of pattern  520  on the OUT output of the PISO, then CLK signal  526  shifts out the TDO signal portion of pattern  520  on the OUT output of the PISO. CKIN signal  528  advances the JTAG controller to output the next TMS and TDO pattern  530  which is asynchronously loaded into the PISO by LD signal  532  and shifted out by CLK signals  534  and  536 . The JTAG controller is advanced to output the next TMS and TDO pattern  540  during CKIN  538 . The above described pattern load, pattern shift, and JTAG controller advancement process repeats as long as the CLK input  310  is active. 
         [0076]    When the JTAG controller  100  receives a CKIN input it will output a new TMS and TDO signal pattern to PISO  502  and input the TDI signal from I/O circuit  504 . The TMS signal output will control the Tap state machine of the target IC&#39;s Tap Domain  104  according to  FIG. 10 , the TDO signal will provide the TDI input signal to the target IC&#39;s Tap Domain (if in the Shift-DR/IR state), and the TDI input signal will input data to the JTAG controller from the target IC&#39;s Tap Domain (if in the Shift-DR/IR state). 
         [0077]      FIG. 6A  illustrates an example implementation of controller  500 . Controller  500  consists of FF  600 , FF  602 , AND gates  604 - 608 , and delay inverter  610 . While the TRST input from the JTAG controller  100  is low, FFs  600  and  602  are reset and the LD and CKIN outputs are low. When TRST goes high, FFs  600  and  602  are enabled to respond to the CLK input  310 . FF  600  toggles its load enable (LDENA) output during each rising edge of CLK input  310 . FF  602  stores the LDENA output of FF  600  at its clock enable (CKENA) output on each falling edge of CLK input  310 . AND gate  604  outputs a high when LDENA is high and CLK is low. AND Gate  606  and delay inverter  620  operate together to produce a high going pulse on the LD output whenever the output of AND gate  604  goes high. 
         [0078]    The duration of the high going pulse on the LD signal is determined by the input to output signal delay through delay inverter  610 . The duration of the LD pulse should be long enough to asynchronously load the PISO with the TMS and TDO pattern but not long enough to interfere with the shifting operation of the PISO. For example, the high going LD pulse should return low for a sufficient amount of time prior to the next rising edge of the shifting CLK input so as to not interfere with the shift operation. The CKENA output of FF  602  enables AND gate  608  to pass the CLK signal  310  to the CKIN output. CKENA changes state on the falling edge of CLK  310  to allow a AND gate  608  to be enabled prior to the rising edge of CLK  310  to allow for good clock gating operation at the CKIN output. 
         [0079]    The operation of controller  500  is illustrated in the timing diagram of  FIG. 6B . In response to the CLK input  310 , the controller  500  operates to periodically output the LD and CKIN signals. As mentioned, the CKIN signal times the operation of the JTAG controller  100  and the LD signal causes the PISO to asynchronously load the TMS and TDO pattern from the JTAG controller  100 . On each rising edge of CLK  310  the LDENA output of FF  600  toggles its state. On each falling edge of CLK  310  the CKENA output of FF  602  is set to the state of the LDENA input to FF  602 . A LD pulse output occurs each time LDENA is high and the CLK goes low. A CKIN output occurs each time CKENA is high and the CLK is high. 
         [0080]      FIG. 7A  illustrates the SPC circuit  306  in more detail. The PSC consists of a controller  700 , a serial input parallel output (SIPO) register  702 , update register  704 , Tap state machine (TSM)  706 , master reset and synchronizer (MRS) circuit  708 , input/output (I/O) circuit  710 , and power on reset circuit (POR)  712 . 
         [0081]    POR circuit  712  produces a temporary low active power on reset pulse whenever the target IC is first power up. This power on reset pulse is used to initialize the MRS circuit. When initialized, the MRS circuit  708  outputs a low on the master reset (MRST) signal to initialize other circuitry within the SPC  306  and to set TRST input of the connected Tap Domains  104  low. When TRST is low, the Tap Domains  104  are forced to the Test Logic Reset state. The Test Logic Reset state is a state of the 1149.1 Tap state machine and is shown in the Tap state machine diagram of  FIG. 10 . The POR circuit  712  may exist in the SPC  306  as shown or it may exist external to the SPC, i.e. as a separate circuit within the target IC. The function of the POR circuit to initialize the MRS circuit  708  may be achieved by other means. For example a reset pin of the IC may be substituted for the POR circuit  712  and used to initialize the MRS circuit  708 . 
         [0082]    Controller  700  inputs the CLK signal  310 , a controller enable (CENA) signal from MRS  708 , a reset (RST) signal from TSM  706 . The controller outputs an update clock (UCK) to update register  704  and a TCK signal to Tap Domains  104  and TSM  706 . A detail description of controller  700  will be given in  FIGS. 8A and 8B . 
         [0083]    I/O circuit  710  inputs an output enable (OE) signal from TSM  706 . The OE signal is used to enabled or disable the output drive of I/O circuit  710 . I/O circuit  710  inputs signals from DIO  308  and outputs them to SIPO  702  via the IN signal. If the OE is set to enable the output drive of I/O circuit  710 , TDO signals input from Tap Domains  104  are output on DIO. If the OE is set to disable the output drive of I/O circuit  710 , TDO signals are not output on DIO and the I/O circuit operates to only input DIO signals to SIPO  702  via the IN signal. I/O circuit  504  is designed to allow the output of TDO signals to DIO  308 , if enabled by OE, and the input of IN signals from DIO  308  to occur simultaneously. The simultaneous input and output operation of I/O circuit  710  will be described in detail later in regard to  FIGS. 11A ,  11 B,  12 ,  13 A, and  13 B. 
         [0084]    SIPO  702  inputs the serialized TMS and TDO signal patterns from the IN output of I/O circuit  710  in response to the CLK input  310  and outputs them to update register  704 . The update register  704  inputs the TDO and TMS outputs from the SIPO and outputs them as TDI and TMS signals to Tap Domains  104 . The update register also inputs the MRST signal from the MRS circuit  708 . While the MRST signal is active low the TDO and TMS outputs of the update register  704  are set high. While the MRST signal is inactive high the update register can respond to the update clock (UCK) signal from controller  700  to load TDO and TMS signals from the SIPO  702 . 
         [0085]    A more detail view of SIPO  702  and update register  704  shows the SIPO containing two serially connected FFs  703  and  705 . In response to the CLK signal  310 , FFs  703  and  705  shift in the serialized TMS and TDO signals from the IN output of I/O circuit  710 . Once the TMS and TDO signals are shifted in they are transferred in parallel to FFs  707  and  709  in the update register  704  in response to the UCK signal where they are input to the TDI and TMS inputs of Tap Domains  104 . The update register serves to provide the current TDI and TMS input pattern to the Tap Domains  104  while the SIPO operates to serially input the next TDO and TMS pattern to be input to the Tap Domains  104 . As mentioned, the outputs of FFs  707  and  709  are asynchronously forced high in response to a low on the MRS signal, which results in highs being input to the TDI and TMS inputs of Tap Domain  104 . This can be achieved, for example, by connecting the MRS signal to the Set input of FFs  707  and  709 . 
         [0086]    TSM circuit  706  inputs the TMS output from the update register, the TCK output of controller  700 , and the MRST output from MRS circuit  708 . TSM circuit  706  outputs a reset (RST) signal to controller  700  and MRS circuit  708 , and the OE signal to I/O circuit  710 . The TSM is simply the Tap state machine defined in IEEE standard 1149.1. The MRST input from MRS circuit  708  is connected to the standard “TRST” input of 1149.1 TSM, the TCK input from controller  700  is connected to the standard “TCK” input of the 1149.1 TSM, the TMS input from controller  700  is connected to the standard “TMS” input of the 1149.1 TSM, the RST output from TSM is connected to the standard “Reset*” output of the 1149.1 TSM, and the OE output of the TSM is connected to the standard “Enable” output of the 1149.1 TSM. 
         [0087]    The TSM circuit is used by the present disclosure to allow the SPC to track the Tap states of the connected Tap Domains, especially the states that control the OE and RST outputs. The operation of the 1149.1 Tap state machine is defined in the 16 states shown in  FIG. 10 . While it is possible to actually use signals from the Tap state machine(s) of the connected Tap Domains  104  for tracking, instead of implementing a dedicated TSM circuit  706  in the SPC  306 , the required signals (OE and RST) may not always be available from the Tap Domains  104 . For example, connected Tap Domains  104  of hard cores (i.e. cores that are fixed and cannot be modified) may not provide OE and RST output signal terminals for connection to the SPC&#39;s OE and RST terminals. Further, Tap Domains  104  having linking arrangements as shown in  FIG. 4C  may present OE and RST signal switching complexities between the SPC  306  and linked Taps within Tap Domains  104 . Therefore, the SPC  306  preferably includes a TSM circuit  706  to insure simplicity in tracking the states of connected Tap Domains  104 . 
         [0088]    MRS circuit  708  inputs the IN output of I/O circuit  710 , the CLK signal  310 , the RST signal from TSM  706 , and the power on reset output of POR circuit  712 . MRS circuit  708  outputs the MRST signal to Tap Domains  104 , TSM  706 , and update register  704  and the CENA signal to controller  700 . The purposes of the MRS circuit  708  are; (1) to maintain the SPC and connected Tap Domains  104  in a reset state when the target IC is operating normally in a system with no JTAG controller  100  and PSC  302  connected to the SPC&#39;s DIO  308  and CLK  310  signals, and (2) to allow synchronizing the operation of the SPC  306  to the operation of a JTAG controller  100  and PSC  302  when the JTAG controller and PSC are connected to the SPC&#39;s DIO and CLK signals. Synchronizing the operation of the SPC to the operation of the JTAG controller and PSC is important since it allows the serialized TMS and TDO patterns output from PSC to be correctly input as serialized TMS and TDO patterns to the SPC. A detail description of MRS circuit  708  will be given in regard to  FIGS. 9A-9C . 
         [0089]    The operation of SPC  306  is illustrated in the timing diagram of  FIG. 7B . In response to the CLK input  310 , the controller  700  operates to periodically output the UCK signal to the update register  704  and the TCK signal to Tap Domains  104  and TSM  706 . Also the CLK input  310  times the SIPO  702  to shift in data from the IN output of the I/O circuit  710 . The I/O circuit passes DIO input signals to the IN output. The TCK signal times the operation of the Tap Domains  104 . The UCK signal causes the update register  704  to load the parallel TDO and TMS signal pattern output of the SIPO  702 . Once loaded, the TDO and TMS signal pattern is applied to the TDI and TMS inputs of Tap Domains  104 . The Tap Domains  104  respond to the TDI and TMS signal pattern in response to the TCK. 
         [0090]    The following describes the SPC&#39;s repeating shift in and update sequence. A serial TMS and TDO bit stream  718  is shifted into SIPO  702  in response to CLK signals  720  and  722 . The shifted in TMS and TDO signals form a parallel TDO and TMS output pattern  724  from SIPO  702  that is clocked into to the update register  704  in response to UCK signal  726 . The TDO and TMS pattern  724  in the update register  704  is applied to the TDI and TMS inputs of Tap Domains  104 . TCK signal  728  clocks the Tap Domains  104  to respond to the TDI and TMS pattern  724  from update register  704 . The next serial TMS and TDO bit stream  730  is shifted into SIPO  702  in response to CLK signals  732  and  734 . The shifted in TMS and TDO signals form a parallel TDO and TMS output pattern  736  from SIPO  702  that is clocked into to the update register  704  in response to UCK signal  738 . The TDO and TMS pattern  738  in the update register  704  is applied to the TDI and TMS inputs of Tap Domains  104 . TCK signal  740  clocks the Tap Domains  104  to respond to the TDI and TMS pattern  730  from update register  704 . The above described serial pattern shift in, parallel pattern update, and Tap Domain clock operation repeats as long as the CLK input  310  is active. 
         [0091]    When the Tap Domain  104  receives a TCK input, the Tap state machine of the Tap Domain responds to the TMS input to perform state transitions as seen in  FIG. 10 . Also the Tap Domain  104  will input data from its TDI input and output data on its TDO output in response to a TCK input, if the Tap state machine is in the Shift-DR/IR state of  FIG. 10 . 
         [0092]      FIG. 8A  illustrates an example implementation of controller  700 . Controller  700  consists of FF  800 , FF  802 , AND gates  804  and  806 , and OR gate  808 . FF  800  toggles its update enable (UPENA) output during each rising edge of CLK  310 . FF  802  stores the UPENA output of FF  800  at its clock enable (CKENA) output on each falling edge of CLK  310 . AND gate  804  outputs a high on its UCK output when UPENA is high, CLK is low, and the controller reset (CRST) output of OR gate  808  is high. AND gate  806  is gated on to pass its CLK  310  input to its TCK output whenever CKENA and CRST are high, otherwise the TCK output is forced low. OR gate  808  outputs a high on CRST whenever the CENA input from CS circuit  708  is high and/or the RST input from TSM  706  is high, otherwise CRST outputs a low. CKENA changes state on the falling edge of CLK  310  to allow AND gate  806  to be enabled prior to the rising edge of CLK  310  to allow for good clock gating operation at the TCK output. 
         [0093]    The operation of controller  700  is illustrated in the timing diagram of  FIG. 8B . While the CRST output of OR gate  808  is high, the controller  700  operates to periodically output the UCK and TCK signals in response to the CLK input  310 . As mentioned, the TCK signal times the operation of the Tap Domains  104  and the UCK signal causes the update register to load the parallel TDO and TMS pattern from SIPO  702 . On each rising edge of CLK  310  the update enable (UPENA) output of FF  800  toggles its state. On each falling edge of CLK  310  the CKENA output of FF  802  is set to the state of the UPENA input to FF  802 . An UCK output occurs each time LDENA is high and the CLK goes low. A CKIN output occurs each time CKENA is high and the CLK is high. If CENA and RST are both low, the CRST output of OR gate  808  will be low to reset controller  700 . While CRST is low, the UPENA output of FF  800  is set high, the CKENA output of FF  802  is set low, the UCK output of AND gate  804  is set low, and the TCK output of AND gate  806  is set low. 
         [0094]      FIG. 9A  illustrates an example implementation of the MRS circuit  708 . MRS circuit  708  consists of a state machine  900  and a FF  902 . The state machine  900  operates on the rising edge of CLK  310  and FF  902  operates on the falling edge of CLK  310 . The state machine  900  inputs the IN signal from I/O circuit  710 , the RST signal from TSM  706 , a clock signal from CLK  310 , and a power on reset signal from POR  712 . The state machine  900  outputs the previously mentioned MRST signal and a controller enable (CE) signal. The CE signal is connected to the D input of FF  902 . The Q output of FF  902  drives the previously mentioned CENA signal. The reset input of the FF  902  is connected to the power on reset output of POR  712 . 
         [0095]    As previously mentioned the purposes of the MRS circuit  708  are to maintain the SPC and Tap Domains in a reset condition when the SPC&#39;s DIO  308  signal is not externally driven and to synchronize the operation of the SPC with an external circuit driving the SPC&#39;s DIO  308  signal. 
         [0096]    The operation of state machine  900  is shown in the state diagram of  FIG. 9B . In response to a low active power on reset input from POR  712  or in response to the RST output of TSM  706  going low, the state machine  900  will enter “Set MRST Low &amp; Poll IN” state  904 . In state  904  the state machine will output a low on the MRST output signal. The state machine will remain in state  904  while the IN input from I/O circuit  710  is high. The state machine will transition to “Poll IN” state  906  if the IN input goes low. The MRST output remains low in state  906 . The state machine will return to state  904  from state  906  if the IN input goes high, otherwise the state machine will transition from state  906  to “Poll IN” state  908 . The MRST output remains low in state  908 . The state machine will return to state  904  from state  908  if the IN input goes low, otherwise the state machine will transition from state  908  to “Poll IN” state  910 . The MRST output remains low in state  910 . The state machine will return to state  904  from state  910  if the IN input goes low, otherwise the state machine will transition from state  910  to “Set MRST &amp; CE High” state  912 . 
         [0097]    In state  912 , the state machine sets the MRST and CE signals high. On the falling edge of CLK  310 , FF  902  clocks in the high CE output from state machine  900  which sets the CENA output of FF  902  high. The state machine will remain in state  912  while the RST input is low. When the RST input goes high, the state machine will transition to the “Set CE Low” state  914 . In state  914 , the state machine sets the CE signal low. On the falling edge of CLK  310 , FF  902  clocks in the low CE output from state machine  900  which sets the CENA output of FF  902  low. The state machine will remain in state  914  while the RST input is high and will transition to state  904  when the RST input goes low. 
         [0098]    The state machine is designed to enter state  904  when it receives a power on reset input from POR  712  or a low input on the RST output of TSM  706 . The state machine will remain in state  904  as long as the IN input from I/O circuit  710  is high. As will be described later in regard to  FIG. 11A , I/O circuit is designed to output a high on the IN signal when the state machine outputs a low on the MRST signal and if the DIO input  308  to I/O circuit  710  is not being externally driven. The high on the IN signal maintains the state machine  900  in state  904  which maintains a low on the state machine MRST output. While MRST is low, SPC  306  circuitry and Tap Domains  104  are held in an inactive reset state that cannot interfere with the normal operation of the target IC. 
         [0099]    When the JTAG controller  100  and PSC circuit  302  of  FIG. 5A  are first connected to the DIO signal of the target IC&#39;s SPC circuit  306  of  FIG. 7A , the operation of the PSC and SPC circuits need to be synchronized such that the serialized TMS and TDO patterns from the PSC are correctly input as serialized TMS and TDO patterns to the SPC. The states within section  916  of the state diagram of  FIG. 9B  provide one example of how this required synchronization step may be achieved. A timing diagram depicting this synchronization process is shown in  FIG. 9C . 
         [0100]    Time reference  918  of  FIG. 9C  indicates a time period where the PSC  302  is not connected to SPC  306 , i.e. DIO  308  is not being externally driven. The circuitry in the SPC  306  and Tap Domains  104  of the target IC have been initialized as previously described and the state machine  900  is in state  904  polling the high output of the IN signal and outputting a low on the MRST output. Time  918  could be a time where the target IC in which the SPC  306  and Tap Domains  104  reside is operating normally in a system and the SPC&#39;s DIO signal is not being externally driven to perform test, emulation, debug, and/or trace operations. In this timing example it is assumed that CLK signal  310  is being actively driven by a clock source within the target IC. Thus state machine  900  state  904  is polling the high logic level of the IN signal during each rising edge of the active CLK signal  310 . It is worth noting that if the IN signal were to temporarily go low during a CLK cycle input for some unknown reason, the state machine would return to state  904  via state  906 . Further, the state machine would return to state  904  from states  908  and  910  in response to the IN signal having other temporarily low and high signal sequences for some unknown reason. 
         [0101]    Time reference  920  of  FIG. 9C  indicates a time period where the PSC  302  has been externally connected to the SPC  306  via the DIO  308  and CLK  310  signals. During the physical connection process there may be undesirable temporary signaling sequence on DIO  308  due to the electrical connection being formed between the PSC and SPC. These temporary signal sequences could prevent the successful synchronization between the PSC and SPC. The state transition mapping in section  916  of  FIG. 9B  is provided to filter out the following three types of temporary signal sequences on the DIO so that they do not affect the synchronization process between PSC and SPC. 
         [0102]    (1) As seen in the state diagram, a temporary DIO signal sequence of 1-0-1 during the connection process would cause the state machine to transition from state  904  to state  906  and back to state  904 . Thus this temporary DIO connection sequence is prevented from affecting the synchronization process. 
         [0103]    (2) As seen in the state diagram, a temporary DIO signal sequence of 1-0-0-0-1 during the connection process would cause the state machine to transition from state  904  to state  906  to state  908  and back to state  904 . Thus this temporary DIO connection sequence is prevented from affecting the synchronization process. 
         [0104]    (3) As seen in the state diagram, a temporary DIO signal sequence of 1-0-0-1-0-1 during the connection process would cause the state machine to transition from state  904  to state  906  to state  908  to state  910  and back to state  904 . Thus this temporary DIO connection sequence is prevented from affecting the synchronization process. 
         [0105]    It should be understood that while the example state machine has been designed to filter out the above three types of temporary DIO sequences, it could be designed to filter out a greater number of DIO sequences if desired. 
         [0106]    Time reference  922  of  FIG. 9C  indicates the start of a time period where the connection between the PSC  302  and SPC  306  has been made and the state machine is in state  904  with the IN signal driven high by DIO input from the connect PSC  302 . The PSC  302  begins the synchronization process by serially inputting a pattern of two logic 0&#39;s  924  on the SPC&#39;s IN signal via DIO  308 , which causes the state machine  900  to transition from state  904  to state  906  to state  908 . As seen in  FIG. 5A , the PSC outputs the two logic 0&#39;s by loading the PISO  502  with a TMS value of 0 and a TDO value of 0 using the LD signal, then shifting the PISO to output the two logic 0&#39;s using the CLK signal  310 . Next the PSC  302  serially inputs a pattern of two logic 1&#39;s  926  on the SPC&#39;s IN signal via DIO  308 , which causes the state machine  900  to transition from state  908  to state  910  to state  912 . Again as seen in  FIG. 5A , the PSC outputs the two logic 1&#39;s by loading the PISO  502  with a TMS value of 1 and a TDO value of 1 using the LD signal, then shifting the PISO to output the two logic 1&#39;s using the CLK signal  310 . As seen, the state machine  900  can only transition from state  904  to state  912  in response to the exact input of a serial pattern of two logic 0&#39;s followed by a serial pattern of two logic 1&#39;s. 
         [0107]    As seen in the timing diagram, the MRST and CE signal outputs of state machine  900  are set high in state  912  at time  925 . MRST going high removes the reset condition from Tap Domains  104 , TSM  706 , and update register  704 . CE going high causes FF  902  to set CENA high at time  927 . When CENA goes high, the CRST signal of controller  700  is set high which enables the controller  700  to start outputting UCK and TCK signals at time  923 . The first UCK signal at time  923  loads the two logic 1&#39;s of pattern  926  into update register  704 . The enabling of the SPC&#39;s controller  700  at time  923  occurs such that the UCK and TCK signals of the SPC&#39;s controller  700  are synchronized with the LD and CKIN signals of the PSC&#39;s controller  500 , respectively. By synchronizing the UCK signal with the LD signal and the TCK signal with the CKIN signal the SPC  306  can correctly receive subsequent serialized two bit patterns from PSC  302  via DIO  308 . For example, when the PISO  502  is shifting out a two bit pattern the SIPO  702  is shifting in the two bit pattern, and when the PISO  502  is loading the next two bit pattern to be shifted the SIPO  702  is updating the current two bit pattern to the update register  704 . The synchronized operation of the UCK and LD signals and the TCK and CKIN signals will be seen more clearly in regard to the description of  FIG. 14A . 
         [0108]    While state machine  900  of the present disclosure has been designed to use a sequence of two serialized two bit patterns  924  and  926  for synchronization, it could be designed to use a longer sequence of serialized two bit patterns for synchronization if desired. Using a longer sequence of two bit patterns would further reduce the possibility of synchronization failure between the PSC and SPC due to the previously mentioned connection process during time  920 . Also a longer synchronization pattern sequence would improve the state machine&#39;s  900  ability to return to state  904 , when DIO is not externally driven, in the event unexpected signaling were to occur on the state machine&#39;s IN input. While the example two bit patterns  924  and  926  used two 0&#39;s and two 1&#39;s respectively, the two bits of a pattern may use any desired or necessary combinations of 0&#39;s and 1&#39;s as well. The TMS portion of the last two bit pattern of a pattern sequence will be the first TMS input the Tap Domains  104  and TSM circuit  706  respond to. In the  FIG. 9C  example, the TMS portion of pattern  926  was set to logic 1 to cause the Tap Domains  104  and TSM circuit  706  to remain in the TLR state following synchronization. If the TMS portion of pattern  926  had been set to logic 0, the Tap Domains  104  and TSM circuit  706  would have transitioned to the RTI state following synchronization. 
         [0109]    Following the above described PSC and SPC synchronization process, the PSC may begin inputting serialized TDO and TMS patterns to the SPC to scan JTAG instructions or data into the Tap Domains  104 . The following example describes the PSC inputting serialized TDO and TMS patterns to the SPC to cause the Tap Domains  104  to perform an instruction scan operation according to the Tap state diagram of  FIG. 10 . 
         [0110]    The SPC inputs a first serialized TDO (X) and TMS ( 0 ) pattern  928  from the PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  929 . The X in the TDO portion of the pattern indicates that TDO is a “don&#39;t care” signal. This first TDI and TMS pattern input to Tap Domains  104  and TSM  706  causes the Tap Domains and TSM to transition from the Test Logic Reset (TLR) state to the Run Test/Idle (RTI) state ( FIG. 10 ) in response to TCK  942 . On the falling edge of TCK  942  the TSM  706  sets its RST signal high to remove the reset condition at the input of OR gate  808  of controller  700 . In response to RST going high, state machine  900  transitions to state  914  on the next rising edge of CLK  310 . The state machine sets the CE output low in state  914  which causes FF  902  to output a low on CENA on the falling edge of CLK  310 . State machine  900  will remain in state  914  while the RST signal is high. 
         [0111]    The SPC inputs a second serialized TDO (X) and TMS ( 1 ) pattern  930  from PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  931 . This second TDI and TMS pattern causes the Tap Domains  104  and TSM to transition from the RTI state to the Select-DR (SLD) state in response to TCK  944 . 
         [0112]    The SPC inputs a third serialized TDO (X) and TMS ( 1 ) pattern  932  from PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  933 . This third TDI and TMS pattern causes the Tap Domains  104  and TSM to transition from the SLD state to the Select-IR (SLI) state in response to TCK  946 . 
         [0113]    The SPC inputs a fourth serialized TDO (X) and TMS ( 0 ) pattern  934  from PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  935 . This fourth TDI and TMS pattern causes the Tap Domains  104  and TSM to transition from the SLI state to the Capture-IR (CPI) state in response to TCK  948 . 
         [0114]    The SPC inputs a fifth serialized TDO ( 0 ) and TMS ( 0 ) pattern  936  from PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  937 . This fifth TDI and TMS pattern causes the Tap Domains  104  and TSM to transition from the CPI state to the Shift-IR (SHI) state in response to TCK  950 . When the TSM  706  transitions to the SHI state it&#39;s OE output is set to enable the output drive of I/O circuit  710  such that the first TDO output from the Tap Domains  104  can be output on DIO  308  to be input to the JTAG controller&#39;s TDI input via I/O circuit  504  of PSC controller  500 . TSM  706  sets its OE to enable the output drive of I/O circuit  710  whenever the TSM (and Tap Domains) is in the Shift-IR or Shift-DR states of  FIG. 10 . 
         [0115]    The SPC inputs a sixth serialized TDO ( 1 ) and TMS ( 0 ) pattern  938  from PSC which is input to SIPO  702  and applied to the TDI and TMS input Tap Domains  104  and the TMS input of TSM  706  via update register  704  during UCK  939 . This sixth TDI and TMS pattern causes the Tap Domains  104  and TSM to remain in the SHI state in response to TCK  952 . In pattern  938 , TDO is shown set to a 1 to indicate that the first TDI input to be shifted into the Tap Domains  104  is a logic 1. On the rising edge of TCK  952  the first TDI input (1) of the sixth pattern  938  is shifted into the Tap Domains  104 . Also the first TDO output from the TAP Domains  104  is input to the TDI input of the JTAG controller  100  on the rising edge of a CKIN input which is synchronized to TCK  952 . 
         [0116]    For as long as serialized patterns ( 940 ,  942 , . . . ) are input to cause the Tap Domains  104  (and TMS  706 ) to remain in the SHI state (i.e. TMS portion of the patterns=0), the TDI input portion of each pattern will be input to the Tap Domains  104  while TDO outputs from the Tap Domains will be input to the JTAG controller  100 . When the shifting in and out of TDI and TDO is complete, the PSC will input serialized patterns with the TMS portion of the patterns set to move the Tap Domains  104  and TMS  706  from the Shift-IR state (SHI) to the Exit1-IR state, then to any other state according to the Tap state diagram of  FIG. 10 . 
         [0117]    While the above process described performing an instruction scan operation between the JTAG controller and Tap Domains of the target IC, data scan operations may be similarly performed. Instruction and data scan operations using serialized TDI and TMS inputs from the JTAG controller and TDO outputs from the Tap Domains can be used to perform test, emulation, debug, trace, and/or other operations via the two signal DIO  308  and CLK  310  interface between the PSC and SPC. 
         [0118]    When an operation is complete, the JTAG controller can output a string of serialized TDO and TMS patterns with the TMS portion of each pattern set to a logic one to cause the Tap Domains  104  and the TSM circuit  706  to transition into the Test Logic Reset state of  FIG. 10 . As seen in  FIG. 10 , the Tap state machine is designed to transition from any of its states to the Test Logic Reset state whenever it receives at least 5 logic high inputs on TMS. Therefore 5 serialized TDO and TMS patterns each with TMS high will cause the Tap Domains  104  and TSM  706  to enter the Test Logic Reset state. 
         [0119]    When the TSM  706  enters the Test Logic Reset state it will set the RST output low which will reset the controller  700  and cause the MRS  708  state machine  900  to enter state  904 , which will result in the signal levels shown during time reference  918  of the timing diagram of  FIG. 9C . After the SPC circuitry has been reset by the RST signal the DIO and CLK connection between the PSC and SPC can be removed. During the PSC and SPC disconnect step, temporary signal glitching/bounce may occur on the DIO signal. The previously described state machine  900  states in section  916  of  FIG. 9B  come into play once again to filter the IN input to the state machine such that the state machine remains in or returns to state  904  following any undesired temporary DIO signaling that may occur during the disconnect step. Following the disconnect step, the state machine will be in state  904  with the MRST output low, which maintains a reset condition on controller  700 , TSM  706 , and Tap Domains  104 . 
         [0120]      FIG. 11A  illustrates an example of a JTAG controller  100  and PSC  302  arrangement  1100  interfaced the SPC  306  and Tap Domains  104  of target IC  300  via DIO  308  signal connections between I/O circuit  504  of arrangement  1100  and I/O circuit  710  of the target IC. For simplification, the CLK  310  signal that accompanies the DIO signal  308  is not shown in this example. Also for simplification and ease of description, the I/O circuits  504  and  710  are shown to exist outside the PSC  302  and SPC  306  respectively, instead of inside as previously shown in  FIGS. 5A and 7A . I/O circuit  504  is coupled to the PSC  302  via the OUT signal and to the JTAG controller  100  via the TDI signal. I/O circuit  710  is coupled to the Tap Domains  104  via the TDO signal and to the SPC via the IN and OE signals. 
         [0121]    I/O circuit  504  consists of an input circuit  1102 , an output buffer  1104 , and a resistor  1106 . The OUT signal is coupled to the input of buffer  1104  and to a first input of the input circuit  1102 . The output of the buffer  1104  is coupled to the DIO signal via resistor  1106 . The DIO signal is coupled to a second input of the input circuit  1102 . The output of the input circuit  1102  is coupled to the TDI input of the JTAG controller  100 . 
         [0122]    I/O circuit  710  consists of an input circuit  1108 , an output buffer  1110 , a resistor  1112 , and a pull up (PU) circuit  1114 . The TDO signal is coupled to the input of buffer  1110  and to a first input of the input circuit  1108 . The output of the buffer  1110  is coupled to the DIO signal via resistor  1112 . The DIO signal is coupled to a second input of the input circuit  1108  and to the PU circuit  1112 . The output of the input circuit  1108  is coupled to the IN input of SPC  306 . 
         [0123]    The PU circuit  1114  is used to set the DIO signal input to input circuit  1108  high when the DIO signal is not being driven by either buffer  1104  or  1110 . For example, when the JTAG controller and PSC arrangement  1100  is not connected to the DIO of the target IC and while the output drive of buffer  1110  of the target IC is disabled by the OE signal, the PU circuit  1114  will set the DIO signal high so that logic ones are input to the SPC  306  from the IN signal output of input circuit  1108  high. The high on the IN signal will cause the state machine  900  of MRS circuit  708  to remain in state  904  of  FIG. 9B , as previously described. 
         [0124]    The output buffer  1104  of I/O circuit  504  and the output buffer  1110  of I/O circuit  710  will preferably be designed to have approximately the same current sink/source drive strength. Also the resistors  1106  and  1112  of I/O circuits  504  and  710  will have approximately the same resistance. 
         [0125]      FIG. 11B  illustrates timing waveforms for the four cases A-D in which simultaneous data communication occurs between the I/O circuits  504  and  710  via DIO  308 . Each case A-D is indicated in the timing diagram by vertical dotted line boxes.  FIG. 12  illustrates the current flow on the DIO signal wire during each of the four cases A-D. In these examples, the OE input to buffer  1110  is set to enable the buffer  1110  to drive the DIO signal. 
         [0126]    Case A shows PSC  302  driving OUT low and Tap Domains  104  driving TDO low. As seen in Case A of  FIG. 12 , with lows being output from both buffers  1104  and  1110  only a small amount of current flows on the DIO signal wire. This small current flow does not develop a significant voltage drop across resistors  1106  and  1112 . Thus the DIO signal input to the input circuits  1102  and  1108  will be easily detectable as being a low signal input. In response to this OUT and TDO output condition the DIO signal is driven low. With OUT and DIO low, the input circuit  1102  inputs a low on the TDI input to JTAG controller  100 . With TDO and DIO low, the input circuit  1108  inputs a low on the IN input to SPC  306 . 
         [0127]    Case B shows PSC  302  driving OUT low and Tap Domains  104  driving TDO high. As seen in Case B of  FIG. 12 , with a low being output from buffer  1104  and a high being output from buffer  1110  a larger current flows between the buffers on the DIO signal wire. The resistors  1106  and  1112  serve to limit this larger current flow and the voltage drops developed across them establish mid level voltage on the DIO wire that is easily detectable by the input circuits  1102  and  1108  from being either high or low. In response to this OUT and TDO output condition the DIO signal is driven to a mid voltage level. With OUT low and DIO at a mid voltage, the input circuit  1102  inputs a high on the TDI input to JTAG controller  100 . With TDO high and DIO at a mid voltage, the input circuit  1108  inputs a low on the IN input to SPC  306 . 
         [0128]    Case C shows PSC  302  driving OUT high and Tap Domains  104  driving TDO low. As seen in Case C of  FIG. 12 , with a high being output from buffer  1104  and a low being output from buffer  1110  a larger current flows between the buffers on the DIO signal wire. The resistors  1106  and  1112  serve to limit this larger current flow and the voltage drops developed across them establish mid level voltage on the DIO wire that is easily detectable by the input circuits  1102  and  1108  from being either high or low. In response to this OUT and TDO output condition the DIO signal is driven to a mid voltage level. With OUT high and DIO at a mid voltage, the input circuit  1102  inputs a low on the TDI input to JTAG controller  100 . With TDO low and DIO at a mid voltage, the input circuit  1108  inputs a high on the IN input to SPC  306 . 
         [0129]    Case D shows PSC  302  driving OUT high and Tap Domains  104  driving TDO high. As seen in Case D of  FIG. 12 , with highs being output from both buffers  1104  and  1110  only a small amount of current flows on the DIO signal wire. This small current flow does not develop a significant voltage drop across resistors  1106  and  1112 . Thus the DIO signal input to the input circuits  1102  and  1108  will be easily detectable as being a high signal input. In response to this OUT and TDO output condition the DIO signal is driven high. With OUT and DIO high, the input circuit  1102  inputs a high on the TDI input to JTAG controller  100 . With TDO and DIO high, the input circuit  1108  inputs a high on the IN input to SPC  306 . 
         [0130]      FIG. 13A  illustrates one example of how to design an input circuit  1300  that can be used as either an input circuit  1102  or  1108 . The input circuit  1300  includes a voltage comparator circuit  1302 , a multiplexers  1304 , an inverter  1306 , and a buffer  1308 . The voltage comparator circuit  1302  inputs voltages from DIO and outputs digital control signals S 0  and S 1  to multiplexer  1304 . As seen, a first voltage (V) to ground (G) leg  1310  of voltage comparator circuit  1302  comprises a series P-channel transistor and a current source and a second voltage to ground leg  1312  comprises a series N-channel transistor and a current source. As seen, S 1  is connected at a point between the P-channel transistor and current source of the first leg  1310  and S 0  is connected at a point between the N-channel transistor and current source of the second leg  1312 . The gates of the transistors are connected to DIO to allow voltages on DIO to turn the transistors on and off. 
         [0131]    The operation of the voltage comparator circuit  1302  and multiplexer  1304  is shown in the truth table of  FIG. 13B  and described herein. If the voltage on DIO is low, the S 0  and S 1  outputs are set high, which causes the multiplexer  1304  to select its low input  1314  and output the low input on the TDI/IN (TDI for circuit  1102  and IN for circuit  1108 ) signal via buffer  1308 . If the voltage on DIO is at a mid level, the SO is set low and the S 1  is set high, which causes the multiplexer  1304  to select its inverted OUT/TDO (OUT for circuit  1102  and TDO for circuit  1108 ) input signal  1316  and output the inverted OUT/TDO signal to the TDI/IN signal via and buffer  1308 . If the voltage on DIO is high, the S 0  and S 1  outputs are set low, which causes the multiplexer  1304  to select its high input  1318  and output the high input to the TDI/IN signal via and buffer  1308 . 
         [0132]    From the above description it is clear that the input circuit  1300  will; (1) input a low on TDI/IN if the DIO signal is low, (2) input a high on TDI/IN if the DIO signal is high, and (3) will input the inverse of OUT/TDO on TDI/IN if the DIO signal is at a mid level voltage between high and low. 
         [0133]    Referring back to  FIG. 11A  and in reference to the above description of input circuit  1300  it is clear that, 
         [0134]    (1) If DIO is high, input circuits  1102  and  1108  will input highs to the JTAG controller  100  and SPC  306  respectively. 
         [0135]    (2) If DIO is low, input circuits  1102  and  1108  will input lows to the JTAG controller  100  and SPC  306  respectively. 
         [0136]    (3) If DIO is mid level and the OUT signal from PSC  302  is low, input circuit  1102  will know that the Tap Domain  104  is outputting a high on TDO to cause the mid level on DIO. Input circuit  1102  will therefore input a high to the TDI input of JTAG controller  100 . 
         [0137]    (4) If DIO is mid level and the OUT signal from PSC  302  is high, input circuit  1102  will know that the Tap Domain  104  is outputting a low on TDO to cause the mid level on DIO. Input circuit  1102  will therefore input a low to the TDI input of JTAG controller  100 . 
         [0138]    (5) If DIO is mid level and the TDO signal from Tap Domain  104  is low, input circuit  1108  will know that the PSC  302  is outputting a high on OUT to cause the mid level on DIO. Input circuit  1108  will therefore input a high to the IN input of SPC  306 ; and 
         [0139]    (6) If DIO is mid level and the TDO signal from Tap Domain  104  is high, input circuit  1108  will know that the PSC  302  is outputting a low on OUT to cause the mid level on DIO. Input circuit  1108  will therefore input a low to the IN input of SPC  306 . 
         [0140]      FIG. 14A  shows a complete arrangement where the JTAG controller  100  and PSC  302  are connected to and are communicating with the SPC  306  and Tap Domains  104  of target IC  300  via the DIO  308  and CLK  310  signals. For simplification only the circuit elements of the PSC  302  and SPC  306  that are involved with the communication process are shown. The timing diagram of  FIG. 14B  details the communication process. 
         [0141]    In the timing diagram of  FIG. 14B , both the controllers  500  and  700  of PSC and SPC, respectively, have been synchronized as previously described and are actively operating their respective LD and CKIN and UCK and TCK signals in response to the CLK signal  310 . As seen and previously mentioned, the LD signal of the PSC operates synchronous with the UCK signal of the SPC, and the CKIN signal of the PSC operates synchronous with the TCK signal of the SPC. For simplification the CKIN and TCK signals are shown as one clock signal. 
         [0142]    During LD signal  1402  TMS and TDO pattern N  1404  from JTAG controller  100  is loaded into PISO  502 . The TMS portion of the loaded pattern is shifted from PISO  502  to SIPO  702  during CLK  1406  and the TDO portion of the loaded pattern is shifted from PISO  502  to SIPO  702  during CLK  1408 . CKIN  1410  advances the JTAG controller to output the next TMS and TDO pattern N+1  1412  and to input the TDO output  1415  from the Tap Domains (if in the Shift-DR or Shift-IR state). TCK  1410  causes the TAP Domains  104  to respond to the previously transmitted TDI and TMS input pattern N−1  1414  input to the Tap Domains during UCK  1413 . Also during TCK  1410 , the Tap Domains will output the next TDO output to be input to the JTAG controller (if in the Shift-DR or Shift-IR state). 
         [0143]    During LD signal  1418  TMS and TDO pattern N+1  1412  from JTAG controller  100  is loaded into PISO  502 . The TMS portion of the loaded pattern is shifted from PISO  502  to SIPO  702  during CLK  1420  and the TDO portion of the loaded pattern is shifted from PISO  502  to SIPO  702  during CLK  1422 . CKIN  1424  advances the JTAG controller to output the next TMS and TDO pattern N+2  1426  and to input the TDO output  1428  from the Tap Domains. TCK  1424  causes the TAP Domains  104  to respond to TDI and TMS input pattern N  1416  input to the Tap Domains during UCK  1413 . Also during TCK  1424 , the Tap Domains will output the next TDO output  1432  to be input to the JTAG controller. 
         [0144]    The above described timing example of the communication between the JTAG controller  100  and Tap Domains  104 , via PSC and SPC, continues while a DIO and CLK connection exists between the PSC and SPC and while the CLK signal  310  is active. 
         [0145]      FIG. 14C  illustrates a timing example of the arrangement of  FIG. 14A  performing a single data register shift operation between the JTAG controller and Tap Domains. As seen the JTAG controller outputs a sequence of TMS and TDO patterns  1440 - 1454  that will control the Tap Domains to transition from the Run Test/Idle (RTI) state, to the Select-DR (SLD) state, to the Capture-DR (CPD) state, to the Select-DR (SLD) state, to the Exit1-DR (X1D) state, to the Update-DR (UPD) state, and back to the RTI state of  FIG. 10 . This Tap state sequence will cause a one bit data register shift operation to occur between the JTAG controller and Tap Domains. The sequence of patterns  1440 - 1454  output from the JTAG controller is serialized by the PSC and de-serialized by the SPC to be input to the Tap Domains as TDI and TMS pattern sequences  1454 - 1468 . As seen the process of serializing and de-serializing the patterns causes TDI and TMS patterns input to the Tap Domains to lag behind the TMS and TDO patterns output from the JTAG controller. 
         [0146]    If the JTAG controller were conventionally connected to the Tap Domains as seen in  FIG. 1 , the TDO to TDI data shift operation between them would occur on the rising edge of the CKIN and TCK at time  1470 , i.e. when the Tap Domains transition from the Shift-DR (SFD) state to the Exit1-DR (X1D) state. However due to the pattern lag, the TDO to TDI data shift operation between them occurs on the rising edge of the CKIN and TCK at time  1472 . The shift in of the TDO data output from the JTAG controller to the TDI input of the Tap Domains is not effected by the pattern lag since the TDO data remains in the TDI and TMS pattern input to the Tap Domains following the serialization and de-serialization process and is clocked into the Tap Domains on the rising edge of TCK  1472 . However, the JTAG controller will not input the correct TDO output from the Tap Domains on the rising edge of CKIN  1470  since, due to the pattern lag, the correct TDO output (shown as dark filled) from the Tap Domains is not output from the Tap Domains until the falling edge of TCK  1470 . Thus while TDO data from the JTAG controller is correctly input as TDI date to the Tap Domains, the TDO output from the Tap Domains is incorrectly input as TDI data to the JTAG controller. 
         [0147]    JTAG controllers that are designed using Texas Instruments SN74/54ACT8990 JTAG bus controller chips can resolve the above mentioned pattern lag problem. The SN74/54ACT8990 JTAG bus controller chips were designed to operate with cabling between JTAG controllers and target ICs that can register the TMS and TDO outputs from the JTAG controller to the TMS and TDI inputs of the target IC. 
         [0148]      FIG. 15  illustrates an arrangement whereby the ACT8990 JTAG controller chip  1502  is interfaced to a target IC  1520  via a cable  1514  that includes FFs  1516 - 1518  in the path between the ACT8990&#39;s TMS and TDO outputs and the target IC&#39;s TMS and TDI inputs. In this example the target IC sources the CKIN to the ACT8990 and also times the operation of FFs  1516  and  1518 . As seen, the FFs  1516  and  1518  cause the TMS and TDI inputs to the target IC to lag the TMS and TDO output from the ACT8990 similar to the way the PSC and SPC circuits of  FIG. 14A  cause the TMS and TDI inputs to IC  300  to lag the TMS and TDO output of the JTAG controller  100  in  FIG. 14A . 
         [0149]    A simplified block diagram of the ACT8990 shows it containing a circuit  1504  for transmitting the TMS signal, a circuit  1506  for transmitting the TDO signal, a circuit  1510  from receiving the TDI signal, and a circuit  1508  for delaying the TMS signal  1512  input to the TDI receiver circuit  1510 . The TDI receiver circuit responds to the TMS signal  1512 , as per the Tap state diagram of  FIG. 10 , to know when to input the TDI signal. In this example, all the circuits  1504 - 1510  are timed by the CKIN input from the TCK output of IC  1520 . 
         [0150]    If no FFs existed in the cable, i.e. TMS and TDO output of the ACT8990 were directly connected to TMS and TDI inputs of the target IC, the TMS delay circuit would be set to not delay the TMS signal input to the TDI receiver. In this case the TDI receiver  1510  operates in step with the Tap of the target IC  1520  such that TDI receiver  1510  inputs TDI data at the same time that the Tap of IC  1520  inputs TDI data. 
         [0151]    If the FFs existed in the path as shown, the TMS delay circuit is set to delay the operation of the TDI receiver for one CKIN cycle to allow the operation of the TDI receiver to be synchronized with the operation of the Tap of IC  1520 . By delaying the operation of the TDI receiver, the TDI receiver is made to operate in step with the delayed operation of the Tap of target IC  1520  such that TDI receiver  1510  inputs TDI data at the same time that the Tap of IC  1520  inputs TDI data. 
         [0152]    While the delay circuit  1508  of the ACT8990 JTAG bus controller chip was originally designed to compensate for delays associated with cables, the present disclosure utilizes the delay circuit  1508  feature to compensate for the delay associated with the serialization and de-serialization operation of the PSC and SPC circuits in  FIG. 14A . 
         [0153]    For example, if the JTAG controller  100  of  FIG. 14A  used the ACT8990 chip to control the JTAG bus, the delay circuit  1508  of the ACT8990 could be set to delay the TDI input from the Tap Domains of IC  300  by one CKIN cycle such that the TDI input is correctly received on the rising edge of CKIN  1472 , as shown in the timing diagram of  FIG. 14C . Thus the previously mentioned lag problem, due to the serialization and de-serialization process of the PSC and SPC circuits, is remedied by using JTAG controllers  100  that incorporate the ACT8990 JTAG bus controller chip or other chips/circuits that can similarly delay the inputting of TDI data from the Tap Domains  104  of  FIG. 14A . 
         [0154]      FIG. 16  illustrates a first system example wherein a JTAG controller  100  and PSC  302  arrangement  1602  is coupled to the SPC  306  and Tap Domains  104  of a target IC  1604  via DIO  308  and CLK  310  signal wiring. In this example a clock source  1606  within arrangement  1602  is used to drive the CLK signal that times the operation of the PSC and SPC circuits. In this example the target IC  1604  requires two dedicated pins for the DIO and CLK signals. 
         [0155]      FIG. 17  illustrates a second system example wherein a JTAG controller  100  and PSC  302  arrangement  1702  is coupled to the SPC  306  and Tap Domains  104  of a target IC  1704  via DIO  308  and CLK  310  signal wiring. In this example a clock source  1706  within target IC  1704  is used to drive the CLK signal that times the operation of the PSC and SPC circuits. In this example the target IC  1704  requires two dedicated pins for the DIO and CLK signals. 
         [0156]      FIG. 18  illustrates a third system example wherein a JTAG controller  100  and PSC  302  arrangement  1702  is coupled to the SPC  306  and Tap Domains  104  of a target IC  1802  via a DIO  308  signal wire. In this example an external clock source  1804  used to input a functional clock to IC  1802  via a functionally required clock input pin. The external clock source also drives the CLK signal of PSC  302 . Since the SPC  306  CLK input is connected to and driven by the IC&#39;s functional clock, a dedicated pin for the CLK signal  310  is not required on IC  1802 . In this example the target IC  1802  requires only a dedicated pin for the DIO signal. 
         [0157]      FIG. 19  illustrates a fourth system example wherein a JTAG controller  100  and PSC  302  arrangement  1702  is coupled to the SPC  306  and Tap Domains  104  of a target IC  1802  via a DIO  308  signal wire. In this example a functional clock is output from IC  1902  to drive the clock input of a peripheral circuit  1904  via a functionally required clock output pin. Internal to the IC  1902 , the functional clock is connected to and drives the CLK input of SPC  306 . External of the IC  1902 , the functional clock is connected to and drives the CLK input of PSC  302 . Since the PSC  302  CLK input is connected to the external functional clock, a dedicated pin for the CLK signal  310  is not required on IC  1902 . In this example the target IC  1902  requires only a dedicated pin for the DIO signal. 
         [0158]      FIG. 20  illustrates a fifth system example wherein a JTAG controller  100  and PSC  302  arrangement  1702  is coupled to the SPC  306  and Tap Domains  104  of a target IC  1604  via DIO  308  and CLK  310  signal wiring. In this example a clock source  2002  external of both arrangement  1702  and IC  1604  is used to drive the CLK signal that times the operation of the PSC and SPC circuits. In this example the target IC  1604  requires two dedicated pins for the DIO and CLK signals. 
         [0159]    The above system examples of  FIGS. 16-20  have shown various ways to interface the PSC and SPC circuits together such that at most the interface requires two dedicated IC pins for DIO and CLK and at least the interface only requires one dedicated pin for DIO. Thus the present disclosure is seen to require only one or two dedicated pins on the target IC. 
         [0160]    The following Figures illustrate an alternate version of the present disclosure whereby the SPC  302  and PSC  306  circuits do not use I/O circuits  504  and  710 , respectively. 
         [0161]      FIG. 21A  illustrates a JTAG controller  100  interfaced to an alternate PSC circuit  2102 . The PSC circuit  2102  is identical to the PSC  302  of  FIG. 5A  with the exception that the I/O circuit  504  is not used in PSC circuit  2102 . As seen, without the I/O circuit  504  the OUT output from PISO  502  is directly output from the PSC via output buffer  1104 . Also as seen, without the I/O circuit  504  the TDO input goes directly to the TDI input of the JTAG controller  100  via an input buffer  1308 . As seen in  FIG. 21B , the operation timing of the alternate PSC  2102  and JTAG controller  100  is identical to the  FIG. 5B  timing operation of the PSC  302  and JTAG controller  100  of  FIG. 5A . 
         [0162]      FIG. 22A  illustrates an alternate SPC circuit  2202  interfaced to Tap Domains  104  of target IC  2204 . The SPC circuit  2202  is identical to the SPC  302  of  FIG. 7A  with the exception that the I/O circuit  710  is not used in SPC circuit  2202 . As seen, without the I/O circuit  710  the OUT input to SPC  2202  is directly input to the MRS  708  and SIPO  702  circuits via a second input buffer  1308 . Also as seen, without the I/O circuit  710  the TDO output from Tap Domains  104  is directly output from SPC  2202  via 3-state buffer  1110 . Buffer  2206  is enabled by the OE signal from TSM  706 . The pull up (PU) element  1114  is connected to the IN signal to pull the IN signal high when it is not being externally driven for reasons previously mentioned. As seen in  FIG. 22B , the operation timing of the alternate SPC  2202  and Tap Domains  104  is identical to the  FIG. 7B  timing operation of the SPC  302  and Tap Domains  104  of  FIG. 7A . 
         [0163]      FIG. 23A  shows a complete arrangement where the JTAG controller  100  and alternate PSC  2102  are connected to and are communicating with the alternate SPC  2202  and Tap Domains  104  of target IC  2302  via the OUT, CLK, and TDO signals. For simplification only the circuit elements of the alternate PSC  2102  and SPC  2202  that are involved with the communication process are shown. As seen the OUT output from PSC  2102  is directly input to the IN input of the SPC  2202  and the TDO output from Tap Domains  104  is directly input to the TDI input of JTAG controller  100 . As seen in  FIG. 23B , the operation timing of the  FIG. 3A  arrangement is identical to the  FIG. 14B  timing operation of the  FIG. 14A  arrangement. 
         [0164]      FIG. 24  illustrates the previously described clocking arrangement of the  FIG. 16  system. In  FIG. 24 , alternate PSC  2102  is used instead of PSC  302  and alternate SPC  2202  is used instead of SPC  306 . As seen, the IC  2402  requires three dedicated pins for OUT, TDO, and CLK. 
         [0165]      FIG. 25  illustrates the previously described clocking arrangement of  FIG. 17  system. In  FIG. 25 , alternate PSC  2102  is used instead of PSC  302  and alternate SPC  2202  is used instead of SPC  306 . As seen, the IC  2502  requires three dedicated pins for OUT, TDO, and CLK. 
         [0166]      FIG. 26  illustrates the previously described clocking arrangement of  FIG. 18  system. In  FIG. 26 , alternate PSC  2102  is used instead of PSC  302  and alternate SPC  2202  is used instead of SPC  306 . As seen, the IC  2602  requires two dedicated pins for OUT and TDO. 
         [0167]      FIG. 27  illustrates the previously described clocking arrangement of  FIG. 19  system. In  FIG. 27 , alternate PSC  2102  is used instead of PSC  302  and alternate SPC  2202  is used instead of SPC  306 . As seen, the IC  2702  requires two dedicated pins for OUT and TDO. 
         [0168]      FIG. 28  illustrates the previously described clocking arrangement of  FIG. 20  system. In  FIG. 28 , alternate PSC  2102  is used instead of PSC  302  and alternate SPC  2202  is used instead of SPC  306 . As seen, the IC  2402  requires three dedicated pins for OUT, TDO, and CLK. 
         [0169]    The above system examples of  FIGS. 24-28  have shown various ways to interface the alternate PSC  2102  and SPC  2202  circuits together such that at most the interface requires three dedicated IC pins for OUT, TDO and CLK, and at least the interface only requires two dedicated pin for OUT and TDO. Thus the alternate version of the present disclosure is seen to require only two or three dedicated pins on the target IC. 
         [0170]    In reference to  FIGS. 14A ,  14 B,  14 C,  23 A, and  23 B it is seen that the frequency of the CKIN and TCK signals is one half the frequency of the source driving the CLK signal. Therefore the JTAG controller and the Tap Domains operate together at one half the frequency of the CLK sources. For example, if the CLK frequency is 100 Mhz, the JTAG operations will occur at 50 Mhz. Thus the second benefit of the present disclosure, stated in the DESCRIPTION OF THE RELATED ART section, of providing a reduced pin interface capable of operating at one half the frequency of the standard 5 pin JTAG interface is achieved. 
         [0171]    It should be understood that while the SPC  306  and  2202  of the present disclosure has been shown as it would be used for accessing Tap Domains within ICs, the SPC is not limited to only accessing Tap Domains within ICs. Indeed, as the need may arise, the SPC can be used within embedded core circuits of an IC to allow accessing Tap Domains that exists within those embedded core circuits. The teaching in the present disclosure of how to use an SPC in an IC is sufficiently detailed to enable one skilled in the art to also use the SPC within an embedded core. 
         [0172]    Although the present disclosure has been described in detail, it should be understood that various changes, substitutions, and alterations may be made without departing from the spirit and scope of the disclosure as defined by the appended claims.