Abstract:
A single conducting path provides communication between a JTAG unit and a JTAG TAP controller. The data is communicated between the two units using time-division multiplexing. Three time slots are allocated to data-in, to data-out and to JTAG control signals. Two of the time-division multiplexing slots exchange data by having one logic signal state defined by the supply voltage and the second logic signal state defined by the voltage level high than the supply voltage. The third time-division multiplexing slot has the logic signal state defined by the presence or absence of signal occurring within the time slot.

Description:
[0001]     This application claims the benefit of Provisional Application Ser. No. 60/699,938, entitled “Low Cost Single Wire Interface for Test and Emulation Purposes”, filed on Jul. 6, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to the test and emulation of integrated circuits and, more particularly, to the test and emulation of integrated circuits using the JTAG protocol.  
         [0004]     2. Background of the Invention  
         [0005]     the JTAG protocol has become one of the premier tools in the test, debug, and emulation of integrated circuits. In a process referred to as boundary scan, a host processor can initialize the state of an integrated circuit and can determine the state of the integrated circuit after a predetermined number of clock cycles or upon detection of a predetermined event.  
         [0006]     The JTAG protocol includes five signal groups that are exchanged between the emulation unit and the target processor. The TCK signals synchronize the internal state machine operations. The TCK, TMS, TDI, TDO single signals are mode select signals that are sampled on the rising edge of a TCK, TMS, TDI, TDO single signal to determine the next state. The TCK, TMS, TDI, TDO single signals are the test data-in signals that are at the rising edge a TCK, TMS, TDI, TDO single signals and are shifted into the target processor test or programming logic circuits when the internal state machine is the correct state. The TCK, TMS, TDI, TDO single signals are test data-out signals and are data shifted out of the target processor&#39;s test or programming logic and are valid on the falling edge of the TCK, TMS, TDI, TDO single signals when the internal state of the state machine is in the correct state. The TRST signals (optional) are reset signals that, when driven low, resets the internal state machine.  
         [0007]     Typically, four or five pins on the integrated circuit chip that includes the target processor are dedicated to transfer of signals between the JTAG unit and the target processor. Referring now to  FIG. 1 , a host processing unit (not shown) controls the activity of a JTAG unit  11 . The host processing unit controls the test procedures to be implemented and analyses the results of the test procedures. The JTAG unit  11  exchanges the TCK, TMS, TDI, TDO single signals, the TCK, TMS, TDI, TDO single signals, the TCK, TMS, TDI, TDO single signals, and the TCK, TMS, TDI, TDO single signals with the JTAG TAP (test access port) unit  121  in the target device  12 . These signals are transferred through pins forming part of the integrated circuit  12 . The JTAG TAP unit  121  exchanges signals with an emulation unit  122 . The emulation unit exchanges signals with a core logic portion  123 .  
         [0008]     As the number and complexity of components/gates in target device  12  has continued to increase, competition for use of the interface pins has expanded. The competition has only gotten more intense with each new product.  
         [0009]     A need has therefore been felt for apparatus and an associated method having the feature of providing additional pins associated with an integrated circuit for the exchange of signals between the integrated circuit and external apparatus. It would be yet another feature of the apparatus and associated method to provide an interface between a JTAG unit and an emulation unit in the target device. It would be still another feature of the apparatus and associated method to provide exchange JTAG signals over a single conductor using time-division multiplex protocols.  
       SUMMARY OF THE INVENTION  
       [0010]     The foregoing and other features are accomplished, according the present invention by providing an interface unit associated with the JTAG unit that creates a single set of multiplexed signals that can be exchanged with an interface device in the target device. The signals are formatted to provide all of the information needed for the JTAG TAP unit to test and debug the core logic using the JTAG boundary value protocols. In addition to the multiplexing of the JTAG control signals, the interface apparatus uses an additional voltage level to transmit the TDI from the JTAG unit to the interface unit in the target device.  
         [0011]     Other features and advantages of present invention will be more clearly understood upon reading of the following description and the accompanying drawings and the claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of apparatus for the test and debug of core logic of an integrated circuit using the JTAG protocol according to the prior art.  
         [0013]      FIG. 2  is a block diagram of apparatus for the test and debug of core logic of an integrated circuit according to the present invention.  
         [0014]      FIG. 3  illustrates the partition of the signals in the time-division multiplex protocol according to the present invention.  
         [0015]      FIG. 4  illustrates a circuit for the detection of selected signal levels in the time-division multiplex protocol of  FIG. 3  according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     1. Detailed Description of the Figures  
       [0016]      FIG. 1  has been described with respect to the related art.  
         [0017]     Referring next to  FIG. 2 , is the block diagram of a JTAG unit and target device test configuration  20  according to the present invention. The JTAG unit  211  exchanges signals with and is controlled by a host processing unit (not shown). The JTAG unit  211  exchanges the TMS signals, the TDO signals and the TDI signals with the JTAG interface unit  212 . The JTAG interface unit  212  exchanges SCK signals with the I.C. interface unit  220 , the T.D. interface unit  220  being a part of the target device  22 . The SCK signals are exchanged over a single conducting path and are coupled to the target device  22  by a single pin. The T.D. interface unit exchanges TCK signals, TMS signals, TDI signals and the TDO signals with the JTAG TAP controller  221 . The JTAG TAP controller  221  exchanges signals with the emulation unit  222 . The emulation unit  223  exchanges signals with the core logic  223 .  
         [0018]     Referring to  FIG. 3 , the basic timing for the serial interface according to the present invention is shown. The serial communication uses time division multiplexing. Three time slots are allocated TMS_SLOT, TDI_SLOT, and TDO_SLOT. These slots are indicated under the clock cycle diagram in  FIG. 3 . Data from JTAG unit is written between two logic high states during the high time of the SCK clock cycle. These two levels are referred to as V IH  and V IHH . The V IH  level never exceeds the logic high level for normal device operation. In the case of CMOS operation, this voltage is typically the provided by the upper supply rail. The V IHH  level can exceed the upper supply rail voltage. These voltages are modulated by the incoming data stream. To write a logic “1”, the JTAG unit drives the SCK line to the V IHH  voltage level during the high cycle. To write a logic “0”, the JTAG unit drives the SCK line to the logic high (V IH ) level. Circuitry in the T.D. interface unit determines when the V IHH  signal is present and will store the associated data on the falling edge of the SCK signal. Because the SCK is clocked by the JTAG unit, a good time-base is formed with each falling edge, thereby allowing for relatively high speed data transfer.  
         [0019]     The SCK signal is set to input both on the TMS slot and the TDI slot. Internally to the target device, the serial SDATA signal stream is shifted into a two bit register at each falling edge of the SCK clock. During the TDO_SLOT, the shift register is disabled. At the end of three cycles, the serial shift register is transferred into a two bit register. The contents of this register are then entered in the JTAG TAP controller.  
         [0020]      FIG. 3  also includes some of the internal signals in the target device. The SHIFT_COUNT signal keeps track of the current clock cycle. The JTAG_REG signal is the two bit register that stores the parallel JTAG commands. This register is updated each time SHIFT_COUNT reaches zero. Note that the TCK signal for the JTAG TAP controller is delayed one cycle from the JTAG_REG update as required. Therefore, the JTAG TAP controller state changes at each rising edge of the TCK signal. The TCLK signal represents the test clock signal. The value of the TCLK signal value is determined by writing a “1” or a “0” during the TDI_SLOT during the Run-Test-Idle of the JTAG algorithm.  
         [0021]     Referring once again to  FIG. 3 , the TDO signal stream is allocated one time slot. The JTAG interface unit releases SCK signal at the end of the TDI slot, the release being triggered by the falling edge of the SCK signal. Once the SCK signal is released, TDO data can be placed on the SCK conductor from the T.D. interface device. Two possibilities can be considered. The first case occurs when TDO data is a high logic level, referred to in  FIG. 3  as “SLAVE READ  1 . In this case, the T.D. interface unit will drive the SCK conductor to a logic signal high. The SCK conducting path is driven actively for a predetermined period of time, the period of time determined by the parameters of one-shot multi-vibrator. The use of a one shot component allows for a fast transition of the SCK signal under heavy loads. The one shot component is required because no SCK signal is available during the TDO_SLOT to indicate when the T.D. interface unit is to be disabled. The one shot component automatically disables the T.D. interface unit after a set period of time. The JTAG interface unit has a weak signal holder to prevent the SCK line form floating. Therefore, once the signal on the SCK conductor is released, the last previously driven value will be maintained. As shown in  FIG. 3 , the JTAG interface unit is enabled at the end of the TDO_SLOT. For the case when TDO data is a logic high signal, this signal causes the SCK conductor to transition from high to a logic low signal. For the case when the TDO signal is a logic low signal, (“SLAVE READ”), the T.D. interface unit does not actually drive the SCK conducting path. The bus holder simply maintains the logic low signal imposed by the JTAG interface unit. In this case, no high to low transition is imposed on the SCK conducting path during the TDO_SLOT. Apparatus inside the T.D. interface unit handles both cases to ensure proper shifting of the data into the internal two bit register.  
         [0022]     Referring to  FIG. 4 , detection apparatus  40  for the SCK signals by the T.D. interface unit is shown. As discussed before, the lower voltage is the supply (rail) voltage V DD . The SCK conductor is coupled through a strong p-channel FET  42  and a weak n-channel FET  43  to ground potential. An amplifier  44  is coupled between the p-channel FET  42  and the n-channel FET  43 . The V DD  voltage is coupled to the control terminal of both FET devices. At normal input voltages the p-channel FET  42  is not turned on. Once the signal on the SCK conductor exceeds the threshold voltage above V DD , the p-channel begins to conduct. The process for fabricating the components must be selected to provide for the higher voltages. For mixed mode components, e.g., 3.3V/1.8V systems, this interface circuit is easily implemented. The SCK conductor can be switched between 1.8V and 3.3V. The I/O components are implemented with 3.3V tolerant transistors.  
       2. Operation of the Preferred Embodiment  
       [0023]     While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, the scope of the invention being defined by the following claims.