Abstract:
The present disclosure describes using the JTAG Tap&#39;s TMS and/or TCK terminals as general purpose serial Input/Output (I/O) Manchester coded communication terminals. The Tap&#39;s TMS and/or TCK terminal can be used as a serial I/O communication channel between; (1) an IC and an external controller, (2) between a first and second IC, or (3) between a first and second core circuit within an IC. The use of the TMS and/or TCK terminal as serial I/O channels, as described, does not effect the standardized operation of the JTAG Tap, since the TMS and/or TCK I/O operations occur while the Tap is placed in a non-active steady state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of application Ser. No. 11/857,688, filed Sep. 19, 2007, currently pending; which is a divisional of application Ser. No. 11/015,816, filed Dec. 17, 2004, now U.S. Pat. No. 7,284,170, issued Oct. 16, 2007; which claims priority from Provisional Application No. 60/534,298, filed Jan. 5, 2004. 
         [0002]    This application is related to U.S. patent application Ser. No. 10/983,256, filed Nov. 4, 2004, now U.S. Pat. No. 7,284,170, issued Oct. 16, 2007, which is incorporated herein by reference. 
     
    
     BACKGROUND 
     Field 
       [0003]    This disclosure relates in general to circuit designs, and in particular to an improvement in the design of IEEE 1149.1 Tap interfaces of ICs and core circuits for improved communication of test, debug, emulation, programming, and general purpose I/O operations. 
         [0004]    Today&#39;s ICs may contain many embedded 1149.1 Tap architectures (Tap domains). Some of these TAP domains are associated with intellectual property (IP) core circuits within the IC, and serve as access interfaces to test, debug, emulation, and programming circuitry within the IP cores. Other TAP domains may exist in the IC which are not associated with cores but rather to circuitry in the IC external of the cores. Further, the IC itself will typically contain a TAP domain dedicated for operating the boundary scan register associated with the input and output terminals of the ICs, according to IEEE std 1149. 1. 
         [0005]      FIG. 1  illustrates a simple example of an IEEE 1149.1 Tap domain  102 . The Tap domain includes a Tap controller  104 , an instruction register (IR)  106 , at least two data registers (DR)  108 , and multiplexer circuitry  110 . The Tap domain interface consists of a TDI input, a TCK input, a TMS input, a TRST input, and a TDO output. In response to TCK and TMS control inputs to Tap controller  104 , the Tap controller outputs control to capture data into and shift data through either the IR  106  from TDI to TDO or a selected DR  108  from TDI to TDO. The data shifted into IR  106  is updated and output on bus  114  to other circuits, and the data shifted into a DR  108  is updated and output on bus  112  to other circuits. DR  108  may also capture data from other circuits on bus  112  and IR  106  may capture data from other circuits on bus  114 . In response to a TRST input to the Tap controller  104 , the TAP controller, IR and DR are reset to known states. The structure and operation of IEEE 1149.1 Tap domain architectures like that of  FIG. 1  are well known. 
         [0006]      FIG. 2  illustrates the state diagram of the Tap controller  104 . All IEEE 1149.1 standard Tap controllers operate according to this state diagram. State transitions occur in response to TMS input and are clocked by the TCK input. The IEEE 1149.1 Tap state diagram is well known. 
         [0007]      FIG. 3  illustrates an example system where a number of Tap domain  102  interfaces of ICs  306 - 312  or embedded cores  306 - 312  within ICs are connected together serially, via their TDI and TDO terminals, to form a scan path  302  from TDI  304  to TDO  306 . Each Tap domain  102  of the ICs/cores  306 - 312  are also commonly connected to TCK  314 , TMS  316 , and TRST  318  inputs. The scan path&#39;s TDI  304 , TDO  306 , TCK  314 , TMS  316 , and TRST  318  signals are coupled to a controller, which can serve as a test, debug, emulation, in-system-programming, and/or other application controller. While only four Tap domains  102  of ICs/cores  306 - 312  are shown, any number of IC/core Tap domains may exist in scan path  302 , as indicated by dotted line  322 . The scan path  302  arrangement of IC/core Tap domains is well known in the industry. 
         [0008]    As seen in  FIG. 3 , if data is to be input to Tap domain  102  of IC/core  312  from controller  320  it must serially pass through all leading Tap domains of ICs/cores  306 - 3   10 . Further, if data is to be output from Tap domain  102  IC/core  306  to controller  320  it must pass through all trailing Tap domains of ICs/cores  308 - 312 . Thus a data input and output latency exists between Tap domains of ICs/cores in scan path  302  and controller  320 . As will be seen later, the present disclosure provides a way to eliminate this data input and output latency by making use of the direct TMS  316  and/or TCK  314  connections between the Tap Domains of ICs/cores  306 - 312  and controller  320 . Having a direct connection for data input and output between the controller  320  and the Tap domains  102 , via the TMS and/or TCK connections, provides improved data communication throughput during test, debug, emulation, in-circuit-programming, and/or other type of operations. Further, using the direct TCK and/or TMS connections for data input and output between controller  320  and Tap domains  102  only involves the controller and the targeted Tap domain. Non-targeted Tap domains are not aware of or affected by the direct TMS and/or TCK communication. 
       SUMMARY 
       [0009]    The present disclosure provides a method and apparatus of communicating data between; (1) an IC in a scan path and a controller of the scan path using the standard direct TMS and/or TCK connections that exists between the IC and controller, (2) a first IC of a scan path and a second IC of the scan path using the direct TMS and/or TCK connections between the ICs, (3) a first core circuit of a scan path in an IC and second core circuit of the scan path of the IC using the direct TMS and/or TCK connections between the cores. The TMS and/or TCK data I/O communication occurs while the Tap controller of the Tap domains of the IC/core are in a non-active state. Thus the TMS and/or TCK I/O communication does not disturb or modify the state of Tap domains of the IC/core in a scan path. The TMS and/or TCK I/O communication is achieved by adding circuitry to the IC/core and coupling the circuitry to the TMS and/or TCK terminals of the IC&#39;s/core&#39;s Tap domain. When enabled by control output from the IC&#39;s/core&#39;s Tap domain, the added circuitry becomes operable to input data from the Tap domain&#39;s TMS and/or TCK terminal or output data onto the Tap domain&#39;s TMS and/or TCK terminal. Conventional controllers  320  coupled to the TMS and TCK signals are improved, according to the present disclosure, such that they can input data from a Tap domain&#39;s TMS and/or TCK terminal and output data to a Tap domain&#39;s TMS and/or TCK terminal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  illustrates a conventional IEEE  1149 . 1  Tap domain architecture. 
           [0011]      FIG. 2  illustrates the state diagram of a conventional IEEE 1149.1 Tap controller. 
           [0012]      FIG. 3  illustrates a conventional arrangement of ICs or cores within ICs with their Tap domains connected in a scan path and the scan path coupled to a controller. 
           [0013]      FIG. 4  illustrates the scan path and controller arrangement of  FIG. 3  adapted for TMS I/O communication according to the present disclosure. 
           [0014]      FIG. 5  illustrates TMS I/O communication circuitry coupled to a Tap domain according to the present disclosure. 
           [0015]      FIG. 6  illustrates the TMS I/O Circuit of  FIG. 5  according to the present disclosure. 
           [0016]      FIG. 7  illustrates circuitry and timing for receiving Manchester encoded TMS input data according to the present disclosure. 
           [0017]      FIG. 8A  illustrates a Manchester decoder state machine for receiving encoded TMS data according to the present disclosure. 
           [0018]      FIG. 8B  illustrates a state diagram of the operation of the Manchester decoder state machine of  FIG. 8A . 
           [0019]      FIG. 9  illustrates circuitry and timing for transmitting Manchester encoded TMS output data according to the present disclosure. 
           [0020]      FIG. 10A  illustrates a Manchester encoder state machine for transmitting encoded TMS data according to the present disclosure. 
           [0021]      FIG. 10B  illustrates a state diagram of the operation of the Manchester encoder state machine of  FIG. 10A . 
           [0022]      FIG. 11  illustrates how TMS I/O communication can occur while a Tap controller is in the Run Test/Idle state according to the present disclosure. 
           [0023]      FIG. 12  illustrates how TMS I/O communication may occur while a Tap controller is in other states according to the present disclosure. 
           [0024]      FIG. 13  illustrates TMS I/O communication occurring between and IC and a controller. 
           [0025]      FIG. 14  illustrates TMS I/O communication occurring between two ICs. 
           [0026]      FIG. 15  illustrates TMS I/O communication occurring between two core circuits within an IC. 
           [0027]      FIG. 16  illustrates the scan path and controller arrangement of  FIG. 3  adapted for TCK I/O communication according to the present disclosure. 
           [0028]      FIG. 17  illustrates TCK I/O communication circuitry coupled to a Tap domain according to the present disclosure. 
           [0029]      FIG. 18  illustrates the TCK I/O Circuit of  FIG. 17  according to the present disclosure. 
           [0030]      FIG. 19  illustrates circuitry and timing for receiving Manchester encoded TCK input data according to the present disclosure. 
           [0031]      FIG. 20A  illustrates a Manchester decoder state machine for receiving encoded TCK data according to the present disclosure. 
           [0032]      FIG. 20B  illustrates a state diagram of the operation of the Manchester decoder state machine of  FIG. 20A . 
           [0033]      FIG. 21  illustrates circuitry and timing for transmitting Manchester encoded TCK output data according to the present disclosure. 
           [0034]      FIG. 22A  illustrates a Manchester encoder state machine for transmitting encoded TCK data according to the present disclosure. 
           [0035]      FIG. 22B  illustrates a state diagram of the operation of the Manchester encoder state machine of  FIG. 22A . 
           [0036]      FIG. 23  illustrates how TCK I/O communication can occur while a Tap controller is in the Run Test/Idle state according to the present disclosure. 
           [0037]      FIG. 24  illustrates how TCK I/O communication may occur while a Tap controller is in other states according to the present disclosure. 
           [0038]      FIG. 25  illustrates TCK I/O communication occurring between and IC and a controller. 
           [0039]      FIG. 26  illustrates TCK I/O communication occurring between two ICs. 
           [0040]      FIG. 27  illustrates TCK I/O communication occurring between two core circuits within an IC. 
       
    
    
     DETAILED DESCRIPTION  
       [0041]      FIG. 4  illustrates a scan path system  402  of ICs/cores that include Tap domains plus additional TMS I/O circuitry. The combination of the Tap domain and TMS I/O circuitry is referred to as TAPIO  416 .  FIG. 4  is similar to  FIG. 3  in regard to the way the TDI, TDO, TCK, TMS, and TRST signals are coupled between the TAPIOs  416  and controller  420 . Controller  420  is different from controller  320  in that it has been improved according to the present disclosure to include the capability of communicating data to and from the TAPIOs  416  via the TMS connection. Controller  420  maintains the conventional ability of controller  320  to communicate the Tap domains of the TAPIOs  416  using the standard IEEE 1149.1 serial protocol. As seen, the TMS connection between controller  420  and TAPIOs  416  is shown as a bidirectional signal path, as opposed to the unidirectional signal path of the TMS connection in  FIG. 3 . When a TAPIO  416  is selected for sending data to the controller  420  according to the present disclosure, the TMS connection will become an output from the TAPIO and an input to the controller. When a TAPIO  416  is selected for receiving data from the controller  420  according to the present disclosure, the TMS connection will become an output from the controller and an input to the TAPIO. As can be seen in  FIG. 4 , data is transferred directly between a selected TAPIO  416  and controller  420 . Therefore the data latency problem mentioned in regard with  FIG. 3  does not exist in  FIG. 4 . 
         [0042]    Additionally, according to the present disclosure, one TAPIO of an IC/core in the scan path may communicate to another TAPIO of an IC/core in the scan path via the common bidirectional TMS connection. To achieve this mode of operation, the controller  420  selects one TAPIO to transmit and another TAPIO to receive. The controller then disables its TMS output driver so that the transmitting TAPIO can output on its TMS terminal to send data to the TMS terminal of the receiving TAPIO. Again, the data is directly transferred between the TAPIOs without the aforementioned latency problem. 
         [0043]      FIG. 5  illustrates the TAPIO circuit  416  in more detail. As seen the TAPIO  416  consists of a Tap domain  502 , a TMS communication circuit  514 , And gates  506 - 508 , and a Clock Source circuit  528 . The Clock Source  528  can be a clock producing circuit within the IC or it can come from a pin of the IC. Tap domain  502  is similar to Tap domain  102  with the exception that it includes And gate  504  for detecting when the Tap controller  104  is in the Run Test/Idle (RTI) state  202  of  FIG. 2 . The Tap controller  104  is a four bit state machine defining the  16  unique states shown in  FIG. 2 . Each of the  16  Tap states is defined by a unique one of the four bit state machine codes. While not shown, the four inputs of the And gate  504  are inverted or not inverted to allow the And gate to detect, with a logic high output, when the Tap controller is in the Run Test/Idle state. For example, if the Run Test/Idle state has a four bit code of 0101, then the “0” inputs to And gate  504  will be inverted such that the And gate will receive all “1&#39;s” at its inputs so that it outputs a logic one when the Tap controller is in the Run Test/Idle state. This will be the case throughout the remainder of this specification for all And gates that are described for use in detecting Tap controller states. Also while And gates are shown being used to detect Tap controller states, other gating circuits may be used as well. 
         [0044]    Further, Tap domain  502  differs from Tap domain  102  in that it includes an Enable TMS Output signal  510  and an Enable TMS Input signal  512 . The Enable TMS Output signal is set whenever the TMS communication circuit  514  is to perform a data output operation on TMS. The Enable TMS Input signal is set whenever the TMS communication circuit  514  is to perform a data input operation on TMS. As seen, the Enable TMS Input or Enable TMS Output signals can come, by design choice, from either the IR  106  via bus  114  or from a DR  108  via bus  112 . 
         [0045]    When Enable TMS Output is set high and when the Tap controller  104  is in the Run Test/Idle (RTI) state  202 , the output of And gate  506  will go high to enable the TMS communications circuit  514  to perform a TMS output operation. When Enable TMS Input is set high and when the Tap controller  104  is in the Run Test/Idle (RTI) state, the output of And gate  508  will go high to enable the TMS communications circuit  514  to perform a TMS input operation. During either TMS communication operation, the Tap controller  104  remains in the Run Test/Idle state  202 . 
         [0046]    TMS communication circuit  514  consists of a Frame Counter  516 , And gate  520 , TMS I/O Circuit  526 , Data Source  522 , and Data Destination  524 . The Frame Counter  516  is a data register  108  that can be scanned via TDI and TDO by the Tap controller  104  to load a count of the number of data frames that are to be sent from the Data Source  522  during a TMS output operation. A data frame in this disclosure is defined by a fixed number of transmitted data bits. After being scanned, and when enabled by the output of And gate  506 , the Frame Counter operates as a counter to count the number of data frames output on TMS. After all data frames have been sent out on TMS, the count complete (CC) output from the Frame Counter will go low to halt the TMS output operation of TMS I/O Circuit  526 , via And gate  520 . And gate  520  is gated on and off by the Enable TMS Output signal being high and low respectively. When gated off, the CC output from the Frame Counter cannot inadvertently, say during an 1149.1 operation that passes through the Run Test/Idle state, enable the TMS I/O Circuit  526 . The Frame Counter receives IR &amp; Tap Control input via bus  530  for scanning in the count, control input  518  from the TMS I/O Circuit  526  for knowing when to count a frame, and a clock input from the Clock Source circuit  528 . 
         [0047]    When enabled for inputting data from TMS, the TMS I/O Circuit  526  receives the TMS data and transfers it to the Data Destination circuitry  524 . Data Destination circuitry  524  may be any circuitry within an IC including but not limited to; (1) an address bus, (2) a data bus, (3) a Ram memory, (4) a Cache memory, (5) a register file, (6) a FIFO, (7) a register, (8) a processor, (9) a peripheral circuit, or (10) a bus coupled to circuitry external to the IC. 
         [0048]    When enabled for outputting data on TMS, the TMS I/O Circuit  526  receives data from the Data Source circuitry  522  and outputs the data on TMS. Data Source circuitry  522  may be any circuitry within an IC including but not limited to; (1) an address bus, (2) a data bus, (3) a Ram memory, (4) a Rom memory, (5) a Cache memory, (6) a register file, (7) a FIFO, (8) a register, (9) a processor, (10) a peripheral circuit, or (11) a bus coupled to circuitry external to the IC. 
         [0049]      FIG. 6  illustrates TMS I/O Circuit  526  in more detail. TMS I/O Circuit consists of a Data &amp; Clock Decoder  604 , Input Register  602 , Data &amp; Clock Encoder  614 , and Output Register  612 . As will be described in more detail later, the TMS communication is based on Manchester data communication whereby the clock and data signals are combined and transmitted together on TMS. 
         [0050]    The function of the Data &amp; Clock Decoder  604  is to receive a frame of Manchester encoded data on TMS terminal  316 , extract the data  606  and clock (CK)  608  components from the encoded data, and input the data  606  serially to Input Register  602  in response to the extract CK signal  608 . Enable (EN) signal  628  enables Input Register  602  to receive the data  606 . Input Register  602 , once filled with a complete serial data frame, outputs the data frame in parallel to Data Destination  524  via data bus  622 . CK signal  608  and Data In Ready control signal  606  controls the Data Destination to receive the parallel data from bus  622 . This process of receiving Manchester encoded serial data frames from TMS terminal  316 , decoding the serial data frames into parallel data patterns, and inputting the parallel data patterns to Data Destination  524  is repeated until the TMS input communication operation is completed. 
         [0051]    The function of the Data &amp; Clock Encoder  614  is to control the Output Register  612 , via Enable (EN)  626 , CK  618  and Data Output Ready  616  signals, to receive parallel data patterns from the Data Source  522  via bus  624  and output the data serially, via Data signal  620 , to the Data &amp; Clock Encoder  614 . The Data &amp; Clock Encoder  614  encodes the serial input data  620  with a clock from Clock Source  528  to produce a frame of serial Manchester encoded data to be output on TMS terminal  316 . This process of receiving a parallel data pattern from the Data Source  522 , converting the parallel data pattern into a frame of serial Manchester encoded data, and outputting the frame of serial Manchester encoded data onto TMS terminal  316  is repeated until all the parallel data patterns from Data Source  522  have been serially transmitted from TMS terminal  316 . As seen in  FIG. 6 , the Data Out Ready signal  616 , which controls the input of parallel data patterns from the Data Source to the Output Register is also input to Frame Counter  516  to control the frame count. The count value in the Frame Counter  516  controls the number of parallel data patterns that are output as encoded serial frames from TMS  316 . The Frame Counter  516  decrements once per each Data Out Ready signal. As seen in  FIG. 5 , when the frame count in Frame Counter  516  expires, the Frame Counter halts the TMS serial output operation by setting the count complete (CC) signal low. 
         [0052]      FIG. 7  illustrates a timing example of the Data &amp; Clock Decoder circuit  604  receiving Manchester encoded data on TMS terminal  316 . Manchester encoding, is well known and operates by sending an encoded signal as a pair of opposite bits. In the timing diagram, each pair of opposite bits are shown within boxes  708 . Each box represents a Manchester encoded signal. In one example of Manchester encoding, an encoded logic one is represented by a logic zero bit followed by a logic one bit, and an encoded logic zero is represented by a logic one bit followed by a logic zero bit. An alternate Manchester encoding reverses the polarity of the bit pair for an encoded logic one and encoded logic zero. 
         [0053]    As seen the Manchester Decoder circuit  702  in circuit  604 , when enabled by Input Enable, becomes operable to receive Start signals  704 , four logic ones in this example, from TMS  316 . More than two consecutive logic ones is an illegal Manchester bit encode, therefore more than two logic ones can be used as an indication to initialize the Manchester Decoder for receiving serial frames of encoded TMS data. While two Start signals, each comprising two logic ones, are shown in this example, more Start signals may be used if desired. After recognizing the Start signals, the Manchester Decoder receives frames 1-N of Manchester encoded serial data from TMS  316 . The Manchester Decoder extracts the Data and CK components from each Manchester encoded bit in the frame and shifts the extracted Data into the Serial Input Parallel Output (SIPO) Register  602 . The Enable output from the Manchester Decoder enable the SIPO Register  602  to receive data. After each frame is decoded and shifted into SIPO Register  602 , the Manchester Decoder outputs the Data In Ready signal to Data Destination  524 . In response to the Data In Ready signal the Data Destination receives (stores and/or processes) the parallel output of Register  602 . This process continues until the Manchester Decoder receives Stop signals  706 , four logic zeros in this example, from TMS  316 . More than two consecutive logic zeros is an illegal Manchester bit encode, therefore more than two logic zeros can be used as an indication to cause the Manchester Decoder to stop receiving serial frames of encoded TMS data. While two Stop signals, each comprising two logic zeroes, are shown in this example, more Stop signals may be used if desired. 
         [0054]      FIG. 8A  illustrates a more detail example of Manchester Decoder circuit  702 . The Manchester Decoder  702  consists of a Manchester Decoder State Machine  802  and a Bit Counter  806 . The state machine  802  receives the TMS signal from TMS terminal  316 , a clock signal from Clock Source  528 , the Input Enable signal from And gate  508 , and a count complete (CC) signal from Bit Counter  804 . The state machine outputs a Data signal to SIPO Register  602 , a clock (CK) signal to SIPO Register  602  and Data Destination  524 , an Enable signal to SIPO Register  602 , the Data In Ready signal to Data Destination  524 , count control to Bit Counter  804 . 
         [0055]      FIG. 8B  illustrates the operation of state machine  802 . When the Input Enable signal is set high, the state machine begins sampling the TMS input for Start signals  704 . The frequency of the Clock Source is set sufficiently high to allow over-sampling of the TMS input signal. After Start signals are detected, the state machine begins sampling the TMS input to decode the Manchester encoded bit pairs  708 . Each time a bit pair is decoded, the appropriate Data value is clocked into SIPO Register  602  by the CK signal and the Bit Counter is clocked by counter control outputs. During the decode operation, the state machine monitors the CC input from the counter  804 . When a CC signal is detected, indicating that the number of bits received is equal to a full frame of bits, the state machine sets the Data In Ready signal high to enable the Data Destination to receive the full frame of bits from the parallel output from SIPO Register  602 . This process continues until the state machine receives the Stop signals  706  on the TMS signal, indicating the end of the transmission of Manchester encoded data frames. The state machine transitions to the Stop state and waits for the Input Enable signal to be set low by the Tap controller  104  exiting the Run Test/Idle state. A subsequent JTAG scan operation to either the DR  108  or the IR  106  register (i.e. the register from which it came) can set the Enable TMS input signal  512  low. When Input Enable goes low, the state machine  802  transitions back to the Input Enable state. 
         [0056]      FIG. 9  illustrates a timing example of the Data &amp; Clock Encoder circuit  614  outputting Manchester encoded data on TMS terminal  316 . In the timing diagram, each Start  704 , Data  708 , and Stop  706  bit signals are again illustrated as they were in  FIG. 7 . As seen the Manchester Encoder circuit  902  in circuit  614 , when enabled by Output Enable, becomes operable to transmit Start signals  704 , four logic ones in this example, onto TMS  316 . Since the TMS terminal of an IC or Core is normally driven by a controller  420 , the controller must disable its drive of the TMS terminal to allow the TMS terminal of the IC or Core to become an output to drive the TMS of the controller during TMS output modes of operation. The disabling of the TMS controller output is indicated a “Z” in the timing diagram. 
         [0057]    As seen, a 3-state buffer  904  inside the Manchester Encoder  902  becomes enabled during TMS output operation to drive the TMS terminal of the IC or core. After transmitting the Start signals, the Manchester Encoder loads parallel data into the Parallel Input Serial Output (PISO) Register  612  from the Data Source  522  and starts shifting the PISO Register  612 . Each bit shifted from the PISO Register to the Manchester Encoder is appropriately encoded as a Manchester bit pair signal  708  and transmitted out of the IC or core via the TMS terminal  316 . As seen the transmission of the Manchester bit pairs begins following the last transmitted Start signal. The Manchester Encoder combines the data and clock components together such that a Manchester Decoder  702  in the receiving controller  420  or other IC/core can extract the components back into separate data and clock signals. The Enable output from the Manchester Encoder enables the PISO Register  612  to load and shift out data. The serial data shifted out from one parallel load of the PISO Register forms one serial bit frame. After each frame is shifted out of the PISO Register  612 , the Manchester Encoder outputs the Data Out Ready signal to PISO Register  612  and Data Source  522 . In response to the Data Out Ready signal the PISO Register  612  inputs parallel data from Data Source  522  to began the next serial output frame that is encoded and output on TMS  316 . This process continues until the Output Enable input to the Manchester Encoder goes low, at which time the Manchester Encoder outputs Stop signals  706 , four logic zeros in this example, onto TMS  316  and disables the output buffer  904 , ending the TMS output operation. 
         [0058]      FIG. 10A  illustrates a more detail example of Manchester Encoder circuit  902 . The Manchester Encoder  902  consists of a Manchester Encoder State Machine  1002 , Bit Counter  1004 , TMS buffer  904 , and Clock Divider (CD)  906 . The state machine  1002  receives the Data output signal from PISO Register  612 , a clock signal from Clock Source  528  via Clock Divider  906 , the Output Enable signal from And gate  520 , and a count complete (CC) signal from Bit Counter  1004 . The state machine outputs a clock (CK) signal to PISO Register  612  and Data Source  522 , an Enable signal to PISO Register  612 , a Data Out Ready signal to PISO Register  612  and Data Source  522 , count control to Bit Counter  1004 , and encoded data to TMS  316  via buffer  904 . 
         [0059]      FIG. 10B  illustrates the operation of state machine  1002 . When the Output Enable signal is set high, the state machine enables the output buffer  904  and outputs Start signals  704  onto TMS  316 . Also the first parallel data pattern from Data Source  522  is loaded into PISO Register  612 . After sending the Start Bits, the state machine begins encoding the serial output data shifted from PISO Register  612  into Manchester encoded outputs on TMS  316 . The frequency of the CK output from the state machine  1002  is sufficiently less than the frequency of the clock output from the Clock Divider  906  to allow each data bit shifted out of PISO Register  612  to be encoded into the appropriate Manchester bit pair signal  708 . Each time an encoded bit pair is output on TMS  316 , the Bit Counter counts in response to control inputs from state machine  1002 . During the encoding operation, the state machine monitors the CC input from the Bit Counter  1004 . When a CC signal is detected, indicating that the last bit of the current bit frame is being shifted out of PISO Register  612 , the Data Out Ready signal is set to cause the next parallel data pattern from Data Source  522  to be loaded into PISO Register  612  to allow starting the next frame of data bit outputs from the Register  612 . Each data bit of each new frame of data loaded and shifted out of PISO Register  612  is encoded into Manchester bit pairs and output on TMS  316 . This process continues until the state machine  1002  detects the Output Enable signal going low, as a result of the count in Frame Counter  516  expiring and setting its frame count complete CC signal low, which in turn sets the Output Enable signal low via And gate  520 . When Output Enable is detected low, the state machine  1002  outputs the Stop signals  706  to indicate to controller  420  or other IC/core that the transmission of Manchester encoded data frames has come to an end. The state machine then disables output buffer  904  from driving the TMS terminal  316  and transitions to the Output Enable state of the diagram. When the TMS output operation is ended, the controller  420  enables its TMS output, transitions the Tap  104  from the Run Test/Idle state to set the Enable TMS Output signal low by a JTAG scan operation to either the DR  108  or the IR  106  register from whence it came. 
         [0060]    Preferably the Clock Sources  528  in the transmitting and receiving devices (i.e. controller  420 , IC, or core) are of the same frequency. This would ensure that, by the use of Clock Divider  906  of Manchester Encoder  902 , the data encoded and output from a transmitting device&#39;s Manchester Encoder  902  will be at a bit rate easily over-sampled and decoded by the non-divided Clock Source  528  driving the Manchester Decoder  702  in a receiving device. A test that TMS data transmitted from Encoder  902  at the divided Clock Source rate is properly received by Decoder  702  at the full Clock Source rate can be achieved by simply enabling both circuits in the same device to operate simultaneously to transmit and receive data over their common TMS connection, then check that the data received in the Data Destination circuit  524  is correct. 
         [0061]    As seen in the TMS Manchester communication timing diagrams of  FIGS. 7 and 9 , the format of a TMS output operation is the same as the format a TMS input operation. Both formats include a header of at least two Start signals  704  followed by frames of Manchester data signals  708  followed by a trailer of at least two Stop signals  706 . This allows simple and standardized data communication between a TMS transmitting device (i.e. controller  420 , IC, or core) and a TMS receiving device (i.e. controller  420 , IC, or core). Other more complex formats may also be implemented as the need arises, such as formats that include frames for addressing and commanding operations to support more sophisticated communication needs. 
         [0062]      FIG. 11  illustrates how a TMS I/O operation is initiated by the Tap controller transitioning into the Run Test/Idle state  202 . Prior to transitioning into the Run Test/Idle state, a scan operation will have been performed to set the Enable TMS Output  510  or Enable TMS Input  512  signal high, depending upon whether an TMS input or TMS output is desired. Also if it is a TMS output operation, the Frame Counter  516  is loaded with the frame count value. Once this setup procedure is accomplished, the Tap controller  104  is transitioned into the Run Test/Idle state as shown in  FIG. 11 . Once in the Run Test/Idle state, the TCK clock is halted at a low logic level. The RTI signal is set which enables And Gates  506  and  508  to pass the Enable TMS Input or Output signals  510  and  512  to TMS communication circuit  514 . The selected TMS input or TMS output operation begins at time  1102  and is executed during time  1104  as shown in  FIG. 11  while the Tap is in the Run Test/Idle state with the TCK halted. When the TMS input or TMS output operation is competed at time  1106 , the TMS and TCK signal may once again be operated by the controller  420  to transition the Tap controller  104  through its states. As can be seen, with the Tap controller in the Run Test/Idle state and with the TCK halted, TMS input and TMS output operations are completely transparent to the Tap controller and all IEEE 1149.1 circuitry connected to the Tap controller. While this example halts the TCK at a low logic level, the TCK could be halted at a high logic level as well. 
         [0063]      FIG. 12  is provided to illustrate that other Tap controller states, other than Run Test/Idle, may be used to perform TMS input and TMS output operations. For example, the Shift-DR state  1202 , the Pause-DR state  1204 , the Shift-IR state  1206 , and the Pause-IR state  1208  all may be used in addition to the Run Test/Idle state for TMS input and output operation modes. To use these additional Tap controller states as states in which TMS input and output operations may be performed is simply a matter of providing And gates  1210  to decode when the Tap controller is in one of the states, as And gate  504  did for detecting the Run Test/Idle state, and providing an Or gate  1212  for indicating when any of the And gate  1210  outputs are high. The output of the Or gate  1212  would be substituted for the RTI output of And gate  504  in  FIG. 5  and input to And gates  506  and  508 . With this substitution made, the TCK could be halted in any one of these states to allow the TMS I/O operation to be started, executed, and stopped, as was shown and described in regard to the Run Test/Idle state of  FIG. 11 . While And/Or gating is shown in this example, other gating circuitry types could be used as well to detect the other Tap states. 
         [0064]      FIG. 13  illustrates an example of the TMS I/O operation of the present disclosure being performed between an IC  1302  of a scan path  402  and a controller  420  as shown in  FIG. 4 . If the controller is performing a TMS output operation, the IC will be performing a TMS input operation to receive the data from the controller via the TMS connection. If the IC is performing a TMS output operation, the controller will be performing a TMS input operation to receive the data from the IC via the TMS connection. 
         [0065]      FIG. 14  illustrates an example of the TMS I/O operation of the present disclosure being performed between a first  1402  and second  1404  IC of a scan path  402 . If IC  1402  is performing a TMS output operation, IC  1404  will be performing a TMS input operation to receive the data from IC  1402  via the TMS connection. If IC  1404  is performing a TMS output operation, IC  1402  will be performing a TMS input operation to receive the data from IC  1404  via the TMS connection. During these operations, the TMS output of controller  420  of  FIG. 4  will need to be disabled to allow the TMS terminal of the outputting IC to drive the TMS connection between the ICs. 
         [0066]      FIG. 15  illustrates an example of the TMS I/O operation of the present disclosure being performed between a first  1502  and second  1504  core circuit within an IC of scan path  402 . If core  1502  is performing a TMS output operation, core  1504  will be performing a TMS input operation to receive the data from core  1502  via the TMS connection. If core  1504  is performing a TMS output operation, core  1502  will be performing a TMS input operation to receive the data from core  1504  via the TMS connection. During these operations, the TMS output of controller  420  of  FIG. 4 , or the output of an internal buffer in the IC being driven by the TMS output of controller  420  will need to be disabled to allow the TMS terminal of the outputting core to drive the TMS connection between the cores. 
         [0067]    In all of the examples in  FIGS. 13-15 , the TMS I/O data communication between the devices (IC and controller, IC and IC, core and core) is performed directly and without introducing any communication latency by having to pass the communicated data through any other devices. Further, devices not involved in the TMS I/O communication are not affected by the TMS I/O communication. 
         [0068]      FIG. 16  illustrates a scan path system  1602  of ICs/cores that include Tap domains plus additional TCK I/O circuitry. The combination of the Tap domain and TCK I/O circuitry is referred to as TAPIO  1616 .  FIG. 16  is similar to  FIGS. 3 and 4  in regard to the way the TDI, TDO, TCK, TMS, and TRST signals are coupled between the TAPIOs  1616  and controller  1620 .  FIG. 16  is different from  FIG. 4  in that communication is provided between the controller  1620  and TAPIO  1616  via the TCK signal  314  instead of via the TMS signal  316 . Controller  1620  is different from controller  420  of  FIG. 4  in that it has been improved according to the present disclosure to include the capability of communicating data to and from the TAPIOs  1616  via the TCK connection. As with controller  420 , controller  1620  maintains the conventional ability of controller  320  to communicate the Tap domains of the TAPIOs  1616  using the standard IEEE 1149.1 serial protocol. As seen, the TCK connection between controller  1620  and TAPIOs  1616  is shown as a bidirectional signal path, as opposed to the unidirectional signal path of the TCK connection in  FIG. 3 . When a TAPIO  1616  is selected for sending data to the controller  1620  according to the present disclosure, the TCK connection will become an output from the TAPIO and an input to the controller. When a TAPIO  1616  is selected for receiving data from the controller  1620  according to the present disclosure, the TCK connection will become an output from the controller and an input to the TAPIO. As can be seen in  FIG. 16 , data is transferred directly between a selected TAPIO  1616  and controller  1620 . Therefore the data latency problem mentioned in regard with  FIG. 3  does not exist in  FIG. 16 . 
         [0069]    Additionally, according to the present disclosure, one TAPIO of an IC/core in the scan path may communicate to another TAPIO  1616  of an IC/core in the scan path via the common bidirectional TCK connection. To achieve this mode of operation, the controller  1620  selects one TAPIO to transmit and another TAPIO to receive. The controller then disables its TCK output driver so that the transmitting TAPIO can output on its TCK terminal to send data to the TCK terminal of the receiving TAPIO. Again, the data is directly transferred between the TAPIOs without the aforementioned latency problem. 
         [0070]      FIG. 17  illustrates the TAPIO circuit  1616  in more detail. As seen the TAPIO  1616  consists of a Tap domain  502 , a TCK communication circuit  1714 , And gates  506 - 508 , a Clock Source circuit  528 , and a D flip flop  1702 . The Clock Source  528  can be a clock producing circuit within the IC or it can come from a pin of the IC. Tap domain  502  is similar to Tap domain  102  with the exception that it includes the previously described And gate  504  for detecting when the Tap controller  104  is in the Run Test/Idle (RTI) state  202  of  FIG. 2 , and that it includes an Enable TCK Output signal  1710  and an Enable TCK Input signal  1712 . D flip flop  1702  has a data (D) input and a reset (R) input coupled to the output of And gate  504 , an inverted clock input coupled to TCK  314 , and a data (Q) output (RTI) coupled to an input of And gates  506  and  508 . The Enable TCK Output signal is set whenever the TCK communication circuit  1714  is to perform a data output operation on TCK. The Enable TCK Input signal is set whenever the TCK communication circuit  1714  is to perform a data input operation on TCK. As seen, the Enable TCK Input or Enable TCK Output signals can come, by design choice, from either the IR  106  via bus  114  or from a DR  108  via bus  112 . 
         [0071]    When the Tap controller  104  is in the Run Test/Idle state  202  the output of And gate  504  will be high, placing a logic one on the data input and the reset input of D flip flop  1702 . With the Tap controller in Run Test/idle, the RTI output of D flip flop  1702  will go high on the falling edge of TCK via the logic one output from And gate  504 . When the Tap controller exits from the Run Test/Idle state, the RTI output of And gate  504  goes low by the output of And gate  504  going low, which resets the RTI output of D flip flop  1702  to a logic zero. 
         [0072]    When Enable TCK Output is set high and the RTI output of D flip flop  1702  is high, the output of And gate  506  will go high to enable the TCK communications circuit  1714  to perform a TCK output operation. When Enable TCK Input is set high and the RTI output of D flip flop  1702  is high, the output of And gate  508  will go high to enable the TCK communications circuit  1714  to perform a TCK input operation. During either TCK communication operation, the Tap controller  104  remains in the Run Test/Idle state  202  since the TMS signal  316  input from controller  1620  will be held low. 
         [0073]    The structure and operation of TCK communication circuit  1714  is the same as TMS communication circuit  514  of  FIG. 5  with the exception that TCK I/O Circuit  1726  has been substituted for TMS I/O Circuit  526 . 
         [0074]    When enabled for inputting data from TCK, the TCK I/O Circuit  1726  receives the TCK data and transfers it to the Data Destination circuitry  524 . Data Destination circuitry  524  may be any circuitry within an IC including but not limited to; (1) an address bus, (2) a data bus, (3) a Ram memory, (4) a Cache memory, (5) a register file, (6) a FIFO, (7) a register, (8) a processor, (9) a peripheral circuit, or (10) a bus coupled to circuitry external to the IC. 
         [0075]    When enabled for outputting data on TCK, the TCK I/O Circuit  1726  receives data from the Data Source circuitry  522  and outputs the data on TCK. Data Source circuitry  522  may be any circuitry within an IC including but not limited to; (1) an address bus, (2) a data bus, (3) a Ram memory, (4) a Rom memory, (5) a Cache memory, (6) a register file, (7) a FIFO, (8) a register, (9) a processor, (10) a peripheral circuit, or (11) a bus coupled to circuitry external to the IC. 
         [0076]      FIG. 18  illustrates TCK I/O Circuit  1726  in more detail. TCK I/O Circuit  1726  is the same as TMS I/O Circuit  526  of  FIG. 6  with the exception that it uses the TCK signal  314  for communication instead of the TMS signal  316 . 
         [0077]    As described previously in regard to TMS I/O Circuit  526 , the function of the Data &amp; Clock Decoder  604  of  FIG. 18  is to receive a frame of Manchester encoded data on TCK terminal  314 , extract the data and clock (CK) components from the encoded data, and input the data serially to Input Register  602  in response to the extract CK signal. Input Register  602 , once filled with a complete serial data frame, outputs the data frame in parallel to Data Destination  524  via data bus  622 . CK signal and Data In Ready control signal controls the Data Destination to receive the parallel data from bus  622 . This process of receiving Manchester encoded serial data frames from TCK terminal  314 , decoding the serial data frames into parallel data patterns, and inputting the parallel data patterns to Data Destination  524  is repeated until the TCK input communication operation is completed. 
         [0078]    As described previously in regard to TMS I/O Circuit  526 , the function of the Data &amp; Clock Encoder  614  of  FIG. 18  is to control the Output Register  612  to receive parallel data patterns from the Data Source  522  via bus  624  and output the data serially to the Data &amp; Clock Encoder  614 . The Data &amp; Clock Encoder  614  encodes the serial input data  620  with a clock from Clock Source  528  to produce a frame of serial Manchester encoded data to be output on TCK terminal  314 . This process of receiving a parallel data pattern from the Data Source  522 , converting the parallel data pattern into a frame of serial Manchester encoded data, and outputting the frame of serial Manchester encoded data onto TCK terminal  314  is repeated until all the parallel data patterns from Data Source  522  have been serially transmitted from TCK terminal  314 . As seen in  FIG. 18 , the Data Out Ready signal  616 , which controls the input of parallel data patterns from the Data Source to the Output Register is also input to Frame Counter  516  to control the frame count. The count value in the Frame Counter  516  controls the number of parallel data patterns that are output as encoded serial frames from TCK  316 . The Frame Counter  516  decrements once per each Data Out Ready signal. As seen in  FIG. 17 , when the frame count in Frame Counter  516  expires, the Frame Counter halts the TCK serial output operation by setting the count complete (CC) signal to And gate  520  low. 
         [0079]      FIG. 19  illustrates a timing example of the Data &amp; Clock Decoder circuit  604  receiving Manchester encoded data on TCK terminal  314 . The timing example is the same as that described previously in  FIG. 7 , with the exception that the TCK signal  314  is used for communication instead of the TMS signal  316 . Also it is seen that the Input Enable goes high on the falling edge  1902  of TCK  314 . Referring back to  FIG. 17 , the output of D flip flop  1702  is set high on the falling edge of TCK  314  when the Tap is in the RTI state  202 , which in turn sets the Input Enable output of And gate  508  high if Enable TCK Input  1712  is high. Use of the falling edge of TCK to initiate the TCK input operation allows the operation to start after TCK has transitioned to a low logic state which allows the Start signals  704  (four logic one&#39;s in this example) on TCK to be more easily recognized by the Data &amp; Clock Decoder circuit  604 . 
         [0080]    As seen the Manchester Decoder circuit  702  in circuit  604 , when enabled by Input Enable, becomes operable to receive Start signals  704 , four logic ones in this example, from TCK  316 . After recognizing the Start signals, the Manchester Decoder receives frames 1-N of Manchester encoded serial data from TCK  314 . The Manchester Decoder extracts the Data and CK components from each Manchester encoded bit  708  in the frame and shifts the extracted Data into the Serial Input Parallel Output (SIPO) Register  602 . The Enable output from the Manchester Decoder enable the SIPO Register  602  to receive data. After each frame is decoded and shifted into SIPO Register  602 , the Manchester Decoder outputs the Data In Ready signal to Data Destination  524 . In response to the Data In Ready signal the Data Destination receives (stores and/or processes) the parallel output of Register  602 . This process continues until the Manchester Decoder receives Stop signals  706 , four logic zeros in this example, from TCK  314 , to cause the Manchester Decoder to stop receiving serial frames of encoded TCK data. 
         [0081]      FIG. 20A  illustrates a more detail example of Manchester Decoder circuit  702 , which is the same as that described in  FIG. 8A  with the exception that the TCK signal  314  is substituted for the TMS  316  signal. The Manchester Decoder  702  consists of a Manchester Decoder State Machine  802  and a Bit Counter  806 . The state machine  802  receives the TCK signal from TCK terminal  314 , a clock signal from Clock Source  528 , the Input Enable signal from And gate  508 , and a count complete (CC) signal from Bit Counter  804 . The state machine outputs a Data signal to SIPO Register  602 , a clock (CK) signal to SIPO Register  602  and Data Destination  524 , an Enable signal to SIPO Register  602 , the Data In Ready signal to Data Destination  524 , count control to Bit Counter  804 . 
         [0082]      FIG. 20B  illustrates the operation of state machine  802 , which is the same as described previously in regard to  FIG. 8B . When the Input Enable signal is set high, the state machine begins sampling the TCK input for Start signals  704 . The frequency of the Clock Source is set sufficiently high to allow over-sampling of the TCK input signal. After Start signals are detected, the state machine begins sampling the TCK input to decode the Manchester encoded bit pairs  708 . Each time a bit pair is decoded, the appropriate Data value is clocked into SIPO Register  602  by the CK signal and the Bit Counter is clocked by counter control outputs. During the decode operation, the state machine monitors the CC input from the counter  804 . When a CC signal is detected, indicating that the number of bits received is equal to a full frame of bits, the state machine sets the Data In Ready signal high to enable the Data Destination to receive the full frame of bits from the parallel output from SIPO Register  602 . This process continues until the state machine receives the Stop signals  706  on the TCK signal, indicating the end of the transmission of Manchester encoded data frames. The state machine transitions to the Stop state and waits for the Input Enable signal to be set low by the Tap controller  104  exiting the Run Test/Idle state. A subsequent JTAG scan operation to either the DR  108  or the IR  106  register (i.e. the register from which it came) can set the Enable TCK input signal  1712  low. When Input Enable goes low, the state machine  802  transitions back to the Input Enable state. 
         [0083]      FIG. 21  illustrates a timing example of the Data &amp; Clock Encoder circuit  614  outputting Manchester encoded data on TCK terminal  314 . The timing example is the same as that described previously in  FIG. 9 , with the exception that the TCK signal  314  is used for communication instead of the TMS signal  316 . Also it is seen that the Output Enable goes high on the falling edge  2102  of TCK  314 . Referring back to  FIG. 17 , the output of D flip flop  1702  is set high on the falling edge of TCK  314  when the Tap is in the RTI state  202 , which in turn sets the Output Enable output of And gate  506  high if Enable TCK Output  1710  is high. Use of the falling edge of TCK to initiate the TCK output operation allows the operation to start after TCK has transitioned to a low logic state and the controller  1620  has disabled (“Z”) its TCK output driver. 
         [0084]    In the timing diagram, the Start  704 , Data  708  (of frames 1-N), and Stop  706  signals are again illustrated as they were in  FIG. 9 . As seen, the Manchester Encoder circuit  902  in circuit  614 , when enabled by Output Enable, becomes operable to transmit Start signals  704 , four logic ones in this example, onto TCK  314 . As mentioned above, the controller  1620  will have disabled its TCK output driver to allow the output buffer  904  of the Manchester Encoder circuit  902  to drive the TCK  314  terminal. 
         [0085]    After transmitting the Start signals, the Manchester Encoder loads parallel data into the Parallel Input Serial Output (PISO) Register  612  from the Data Source  522  and starts shifting the PISO Register  612 . Each bit shifted from the PISO Register to the Manchester Encoder is appropriately encoded as a Manchester bit pair signal  708  and transmitted out of the IC or core via the TCK terminal  314 . The Manchester Encoder combines the data and clock components together such that a Manchester Decoder  702  in the receiving controller  420  or other IC/core can extract the components back into separate data and clock signals. The Enable output from the Manchester Encoder enables the PISO Register  612  to load and shift out data. The serial data shifted out from one parallel load of the PISO Register forms one serial bit frame. After each frame is shifted out of the PISO Register  612 , the Manchester Encoder outputs the Data Out Ready signal to PISO Register  612  and Data Source  522 . In response to the Data Out Ready signal the PISO Register  612  inputs parallel data from Data Source  522  to began the next serial output frame that is encoded and output on TCK  314 . This process continues until the Output Enable input to the Manchester Encoder goes low, at which time the Manchester Encoder outputs Stop signals  706 , four logic zeros in this example, onto TCK  314  and disables the output buffer  904 , ending the TCK output operation. 
         [0086]      FIG. 22A  illustrates a more detail example of Manchester Encoder circuit  902 , which is the same as that described in  FIG. 1  OA with the exception that the TCK signal  314  is substituted for the TMS  316  signal. The Manchester Encoder  902  consists of a Manchester Encoder State Machine  1002 , Bit Counter  1004 , TCK buffer  904 , and Clock Divider (CD)  906 . The state machine  1002  receives the Data output signal from PISO Register  612 , a clock signal from Clock Source  528  via Clock Divider  906 , the Output Enable signal from And gate  520 , and a count complete (CC) signal from Bit Counter  1004 . The state machine outputs a clock (CK) signal to PISO Register  612  and Data Source  522 , an Enable signal to PISO Register  612 , a Data Out Ready signal to PISO Register  612  and Data Source  522 , count control to Bit Counter  1004 , and encoded data to TCK  314  via buffer  904 . 
         [0087]      FIG. 22B  illustrates the operation of state machine  1002 . When the Output Enable signal is set high, the state machine enables the output buffer  904  and outputs Start signals  704  onto TCK  314 . Also the first parallel data pattern from Data Source  522  is loaded into PISO Register  612 . After sending the Start Bits, the state machine begins encoding the serial output data shifted from PISO Register  612  into Manchester encoded outputs on TCK  314 . The frequency of the CK output from the state machine  1002  is sufficiently less than the frequency of the clock output from the Clock Divider  906  to allow each data bit shifted out of PISO Register  612  to be encoded into the appropriate Manchester bit pair signal  708 . Each time an encoded bit pair is output on TCK  314 , the Bit Counter counts in response to control inputs from state machine  1002 . During the encoding operation, the state machine monitors the CC input from the Bit Counter  1004 . When a CC signal is detected, indicating that the last bit of the current bit frame is being shifted out of PISO Register  612 , the Data Out Ready signal is set to cause the next parallel data pattern from Data Source  522  to be loaded into PISO Register  612  to allow starting the next frame of data bit outputs from the Register  612 . Each data bit of each new frame of data loaded and shifted out of PISO Register  612  is encoded into Manchester bit pairs and output on TCK  314 . This process continues until the state machine  1002  detects the Output Enable signal going low, as a result of the count in Frame Counter  516  expiring and setting its frame count complete CC signal low, which in turn sets the Output Enable signal low via And gate  520 . When Output Enable is detected low, the state machine  1002  outputs the Stop signals  706  to indicate to controller  1620  or other IC/core that the transmission of Manchester encoded data frames has come to an end. The state machine then disables output buffer  904  from driving the TCK terminal  314  and transitions to the Output Enable state of the diagram. When the output operation is ended, the controller  1620  enables its TCK output and sets the Enable TCK Output signal low by a JTAG scan operation to either the DR  108  or the IR  106  register from whence it came. 
         [0088]    As with the TMS I/O communication, it is preferable that the Clock Sources  528  in transmitting and receiving devices (i.e. controller  1620 , IC, or core) be at the same frequency. This would ensure that, by the use of Clock Divider  906  of Manchester Encoder  902 , the data encoded and output from a transmitting device&#39;s Manchester Encoder  902  will be at a bit rate easily over-sampled and decoded by the non-divided Clock Source  528  driving the Manchester Decoder  702  in a receiving device. A test that TCK data transmitted from Encoder  902  at the divided Clock Source rate is properly received by Decoder  702  at the full Clock Source rate can be achieved by performing the test previously describe with the TMS I/O communication. 
         [0089]    As seen in the TCK Manchester communication timing diagrams of  FIGS. 19 and 21 , the format of a TCK output operation is the same as the format a TCK input operation. Both formats include a header of Start signals  704  followed by frames of Manchester data signals  708  followed by a trailer of Stop signals  706 . This allows simple and standardized data communication between a TCK transmitting device (i.e. controller  1620 , IC, or core) and a TCK receiving device (i.e. controller  1620 , IC, or core). As with the previously described TMS communication of  FIGS. 7 and 9 , TCK communication may be expanded to include other more complex formats as the need arises, such as formats that include frames for addressing and commanding operations. 
         [0090]      FIG. 23  illustrates how a TCK I/O operation is initiated by the Tap controller transitioning into the Run Test/Idle state  202 . Prior to transitioning into the Run Test/Idle state, a scan operation will have been performed to set the Enable TCK Output  1710  or Enable TCK Input  1712  signal high, depending upon whether a TCK input or TCK output is desired. Also if it is a TCK output operation, the Frame Counter  516  is scanned to load the frame count value. Once this setup procedure is accomplished, the Tap controller  104  is transitioned into the Run Test/Idle state as shown in  FIG. 23 . Once in the Run Test/Idle state, and after the RTI output of D flip flop  1702  goes high, the selected TCK input or output operation can begin. 
         [0091]    The RTI signal is set high on the falling edge of TCK at time  2302  which enables And Gates  506  and  508  to pass the Enable TCK Input or Output signals  1710  and  1712  to TCK communication circuit  1714 . If a TCK output operation is to be performed, the controller  1620  will disable its TCK output driver after the falling edge of TCK at time  2302  and before time  2304  to allow a transmitting device to drive the TCK signal  314 . The selected TCK input or TCK output operation begins at time  2304  and is executed during time  2306  as shown in  FIG. 23  while the Tap is in the Run Test/Idle state. When the TCK input or TCK output operation is competed at time  2308 , the TCK signal may once again be driven by the controller  1720  to conventionally operate Tap controller  104  using IEEE 1149.1 protocols. As can be seen, with the Tap controller in the Run Test/Idle state with TMS held low, TCK input and TCK output operations are completely transparent to the Tap controllers and all IEEE 1149.1 circuitry connected to Tap controllers. 
         [0092]      FIG. 24  is provided to illustrate that other Tap controller states, other than Run Test/Idle, may be used to perform TCK input and TCK output operations. For example, the Pause-DR state  1204  or the Pause-IR state  1208  may be used in addition to the Run Test/Idle state  202  for TCK input and output operation modes. To use these additional Tap controller states as states in which TCK input and output operations may be performed is simply a matter of providing And gates  2402  to detect when the Tap controller is in one of the states, as And gate  504  did for detecting the Run Test/Idle state, and providing an Or gate  2404  for indicating when any of the And gate  2402  outputs are high. The output of the Or gate  2404  would be substituted for the output of And gate  504  in  FIG. 17  as input to D flip flop  1702 . The output of D flip flop  1702 , renamed “TCK I/O State” in  FIG. 24 , would maintain its connection to And gate  506  and  508  as shown in  FIG. 17 . With this substitution made, the Tap controller  104  could be transitioned into any one of these states, and held there by asserting a low on TMS, to allow a TCK I/O operation to be started, executed, and stopped, as was shown and described in regard to the Run Test/Idle state of  FIG. 23 . While it is possible to also use the Shift-DR state  1202  and Shift-IR state  1206  for TCK input and output operations, as was shown and described in the TMS input and output operations of  FIG. 12 , one must be aware that data will be shifting through the ICs/cores of scan path  1602  from TDI to TDO during the TCK input or output operations, since the TCK signal will be active. This may or may not be a desired situation and is therefore left up to the user of the disclosure to determined whether TCK input and output operations are also allowed in the Shift-DR and Shift-IR Tap states. If allowed, then additional And gates  2402  would be assigned to detect these additional Tap states and the Or gate  2404  would be equipped with additional inputs for receiving the outputs from the additional And gates  2402 . 
         [0093]      FIG. 25  illustrates an example of the TCK I/O operation of the present disclosure being performed between an IC  2502  of a scan path  1602  and a controller  1620  as shown in  FIG. 16 . If the controller is performing a TCK output operation, the IC will be performing a TCK input operation to receive the data from the controller via the TCK connection. If the IC is performing a TCK output operation, the controller will be performing a TCK input operation to receive the data from the IC via the TCK connection. 
         [0094]      FIG. 26  illustrates an example of the TCK I/O operation of the present disclosure being performed between a first  2602  and second  2604  IC of a scan path  1602 . If IC  2602  is performing a TCK output operation, IC  2604  will be performing a TCK input operation to receive the data from IC  2602  via the TCK connection. If IC  2604  is performing a TCK output operation, IC  2602  will be performing a TCK input operation to receive the data from IC  2604  via the TCK connection. During these operations, the TCK output of controller  1620  of  FIG. 16  will need to be disabled to allow the TCK terminal of the outputting IC to drive the TCK connection between the ICs. 
         [0095]      FIG. 27  illustrates an example of the TCK I/O operation of the present disclosure being performed between a first  2702  and second  2704  core circuit within an IC of scan path  1602 . If core  2702  is performing a TCK output operation, core  2704  will be performing a TCK input operation to receive the data from core  2702  via the TCK connection. If core  2704  is performing a TCK output operation, core  2702  will be performing a TCK input operation to receive the data from core  2704  via the TCK connection. During these operations, the TCK output of controller  1620  of  FIG. 16 , or the output of an internal buffer in the IC being driven by the TCK output of controller  1620  will need to be disabled to allow the TCK terminal of the outputting core to drive the TCK connection between the cores. 
         [0096]    In all of the examples in  FIGS. 25-27 , the TCK I/O data communication between the devices (IC and controller, IC and IC, core and core) is performed directly and without introducing any communication latency by having to pass the communicated data through any other devices. Further, devices not involved in the TCK I/O communication are not affected by the TCK I/O communication. 
         [0097]    While the Manchester encoding and decoding circuits described herein to achieve the TMS and TCK I/O communication have been described as being state machines operating synchronous to a clock source  528 , the disclosure is not limited to a particular type of Manchester encoding and decoding circuit. Indeed, other types of Manchester encoding and decoding circuits may be readily substituted for the example circuits shown herein and used to achieve the Manchester based TMS and TCK I/O communication objective of the present disclosure. 
         [0098]    While the TMS and TCK I/O communication circuit examples were shown as residing in ICs and/or cores, it should be clear that similar TMS and TCK I/O communication circuits or software that can emulate the TMS and TCK I/O communication circuit functionality also resides in the controllers that connect to the ICs and/or cores to enable the controllers to communicate with the ICs and/or cores during TMS and TCK I/O communication operations. 
         [0099]    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.