FIELD OF THE INVENTION 
     The present invention relates to the identification of faults within electronic circuitry and more particularly to fault identification by voltage potential signature. 
     BACKGROUND OF THE INVENTION 
     To operate properly, electronic circuitry (e.g., digital logic) must be fault free. Accordingly, numerous techniques have been developed to identify circuit faults such as level sensitive scan design (LSSD) testing, quiescent current (Iddq) measurements and delay fault measurements. 
     During LSSD testing of a circuit, a logical voltage pattern is applied to the circuit and the resulting logical circuit outputs are observed. The resulting logical circuit outputs then are compared to the logical expected values for the circuit, and a fault is identified by a discrepancy therebetween. LSSD testing thus employs a simple binary comparison that provides no information about the internal voltage potentials of a circuit. Faults which degrade a circuit&#39;s internal voltage potentials without affecting the circuit&#39;s logical outputs (i.e., potential faults), therefore, are unidentifiable by binary testing schemes such as LSSD testing. 
     During an Iddq measurement, a DC voltage pattern is applied to a circuit, the power supply current supplied to the circuit is measured and the resulting power supply current is compared to an expected power supply current in order to identify faults. Similarly, during a delay fault measurement, the voltage pattern applied to a circuit&#39;s inputs is changed from one voltage pattern to another, the time required for the circuit&#39;s outputs to change states in response thereto (i.e., the circuit delay) is measured and the resulting circuit delay is compared to an expected circuit delay in order to identify faults. While both Iddq and delay fault measurements are analog in nature (e.g., measuring an analog power supply current and an analog circuit delay), neither measurement provides information about a circuit&#39;s internal voltage potentials. Additionally, as electronic circuits progress into the deep sub-micron regime, larger sub-threshold leakage currents result diminishing the usefulness of Iddq measurements. Accordingly, a need exists for an improved method and apparatus for identifying circuit faults. 
     SUMMARY OF THE INVENTION 
     To address the needs of the prior art, an inventive logic gate is provided that comprises a sensing circuit coupled to a test output (e.g., a test output of the logic gate or a test output of an integrated circuit (IC) chip employing the logic gate). As used herein, “coupled” means coupled directly or indirectly so as to operate. The sensing circuit also is coupled to an internal node of the logic gate (i.e., a node other than an output of the logic gate) and is adapted to sense a voltage on the internal node and to output a signal indicating a level of the voltage (i.e., a voltage potential signature). The sensing circuit is not used during normal operation of the logic gate and preferably comprises only a single field-effect-transistor (FET) (e.g., a p-channel metal-oxide-semiconductor FET or “PFET”) that is directly coupled to both the internal node and to the test output. 
     The inventive logic gate preferably comprises a pre-charge circuit for pre-charging the test output to a predetermined voltage level prior to testing (i.e., a pre-test voltage level). The pre-charge circuit may, for example, comprise an FET such as an n-channel metal-oxide-semiconductor FET or “NFET”. 
     An IC chip may be formed from a plurality of the inventive logic gates wherein each logic gate comprises a sensing circuit coupled to a test output and to an internal node of the logic gate. Each sensing circuit may be coupled to the same test output (e.g., a “common” test output for the IC chip) or to a unique test output for the sensing circuit&#39;s logic gate. Each logic gate&#39;s sensing circuit thus senses a voltage present on an internal node of the logic gate and outputs a signal indicating a level of the voltage. The sensing circuits are not used during normal operation of the IC chip. The IC chip preferably comprises one or more pre-charge circuits for pre-charging the common test output or each logic gate&#39;s test output to a pre-test voltage level. 
     By thus providing logic gates and integrated circuits that are testable for both the presence of and the location of potential faults IC quality assurance and IC testing/troubleshooting are greatly enhanced. 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears. 
     FIG. 1 is a schematic diagram of an integrated circuit configured in accordance with a first aspect of the present invention; and 
     FIG. 2 is a schematic diagram of an integrated circuit configured in accordance with a second aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of an integrated circuit (IC)  101  configured in accordance with a first aspect of the present invention. The inventive IC  101  comprises a plurality of dynamic logic circuits including a first AND gate  103 , a second AND gate  105 , and a first OR gate  107  as shown. The first AND gate  103  has a first input (IN 1 ), a second input (IN 2 ), and a reset input (RESET), and an output coupled to a first input of the second AND gate  105 . The second AND gate  105  has a second input coupled to another portion of the inventive IC  101  (not shown) and an output coupled to a first input of the first OR gate  107 . The first OR gate  107  has both a second input and an output coupled to another portion of the inventive IC  101  (not shown). 
     Each dynamic logic gate  103 - 107  comprises a plurality of interconnected transistor elements as is known in the art. For example, the first AND gate  103  comprises a first p-channel metal-oxide-semiconductor field-effect-transistor (PFET)  109  having a source coupled to a positive voltage rail (V DD ), a gate which serves as the RESET input and a drain coupled to an internal node (e.g., dynamic node  111 ), and a second PFET  113  having a source coupled to V DD , a gate coupled to the dynamic node  111  via an inverter  115  and a drain coupled to the dynamic node  111 . The first AND gate  103  further comprises a first n-channel metal-oxide-semiconductor field-effect-transistor (NFET)  117  having a drain coupled to the dynamic node  111 , a gate which serves as the first input (IN 1 ) of the first AND gate  103  and a source coupled to a secondary node  119 , and a second NFET  121  having a drain coupled to the secondary node  119 , a gate which serves as the second input (IN 2 ) of the first AND gate  103  and a source coupled to ground. The second AND gate  105  is identically configured, while the first OR gate  107  is configured to perform an OR function as is known in the art. 
     In addition to the logic gates  103 - 107 , the inventive IC  101  comprises a plurality of sensing circuits that allow testing of the inventive IC  101  for potential faults. Specifically, the inventive IC  101  comprises a first sensing circuit formed from a third PFET  123  having a source coupled to V DD , a gate coupled to the dynamic node  111  and a drain coupled to a test output  125  of the inventive IC  101 , a second sensing circuit formed from a fourth PFET  127  having a source coupled to V DD , a gate coupled to an internal node  129  (e.g., a dynamic node) of the second AND gate  105  and a drain coupled to the test output  125 , and a third sensing circuit formed from a fifth PFET  131  having a source coupled to a reference potential (V REF ), a gate coupled to an internal node  133  (e.g., a dynamic node) of the first OR gate  107  and a drain coupled to the test output  125 . Each PFET  123 ,  127  and  131  has a threshold voltage (V TH ). If desired, additional sensing circuits may be provided within the inventive IC  101  for testing for potential faults at other locations. 
     The inventive IC  101  further comprises a test pre-charge circuit formed from a third NFET  135  having a drain coupled to the test output  125 , a gate which serves as a test pre-charge input, and a source coupled to ground. A latch  137  preferably is provided for latching the voltage level present on the test output  125  as described below. 
     During normal operation of the inventive IC  101 , the dynamic node  111  of the first AND gate  103  is pre-charged to V DD  by applying a low voltage (“low”) RESET signal to the gate of the first PFET  109 . With the gate of the first PFET  109  low, the first PFET  109  turns ON, and the dynamic node  111  charges toward V DD . As the dynamic node  111  charges toward V DD , the switching threshold of the inverter  115  is passed, the gate of the second PFET  113  is driven low and the second PFET  113  is turned ON to assist in the charging of the dynamic node  111 . The internal node  129  and the internal node  133  similarly are pre-charged. 
     With the dynamic node  111  properly pre-charged to V DD , the output of the first AND gate  103  is driven to a low logic state via the inverter  115 , and the third PFET  123  is OFF. Similarly, with the internal node  129  and the internal node  133  properly pre-charged to V DD , the fourth PFET  127  and the fifth PFET  131  are OFF. 
     Thereafter, the RESET signal is switched to a high voltage (“high”), the first PFET  109  is turned OFF, and the first AND gate  103 &#39;s output logic state is dictated by the voltage states present on the first and the second inputs (IN 1 ) , (IN 2 ) of the first NFET  117  and the second NFET  121 , respectively. For instance, if the second input (IN 2 ) is held low, the second NFET  121  is OFF and no path can be created between the dynamic node  111  and ground whether the first NFET  117  is ON or OFF (e.g., whether the first input (IN 1 ) is held high or low). Therefore, the dynamic node  111  remains charged at V DD , and the output of the first AND gate  103  remains low. Similarly, if the first input (IN 1 ) is held low, the first NFET  117  is OFF and no path can be created between the dynamic node  111  and ground whether the second NFET  121  is ON or OFF (e.g., whether the second input (IN 2 ) is held high or low). The dynamic node  111 , therefore, remains charged at V DD , and the output of the first AND gate  103  remains low. 
     Only when both the first input (IN 1 ) and the second input (IN 2 ) are driven high are both the first NFET  117  and the second NFET  121  turned ON, is a path created between the dynamic node  111  and ground, is the dynamic node  111  pulled low, and is the output of the first AND gate  103  switched from a low to a high logic state as summarized in TABLE 1. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 DYNAMIC 
                   
               
               
                   
                 IN 1   
                 IN 2   
                 NODE 111 
                 OUT 
               
               
                   
                   
               
             
             
               
                   
                 0 
                 0 
                 1 
                 0 
               
               
                   
                 0 
                 1 
                 1 
                 0 
               
               
                   
                 1 
                 0 
                 1 
                 0 
               
               
                   
                 1 
                 1 
                 0 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     The output of the first AND gate  103  propagates to the first input of the second AND gate  105 , and affects the internal node  129  and the output of the second AND gate  105  accordingly. Thereafter, the output of the second AND gate  105  propagates to the first input of the first OR gate  107 , and affects the internal node  133  and the output of the first OR gate  107  accordingly. The output of the first OR gate  107  then propagates to another location within the inventive IC  101  (not shown). 
     The above described operation of the inventive IC  101  represents the “ideal” operation of the inventive IC  101  wherein the dynamic node  111 , the internal node  129  and the internal node  133  are properly pre-charged. As previously described, potential faults (e.g., short circuits, open circuits and resistive defects) may exist within the inventive IC  101  that do not affect the logical outputs of the first AND gate  103 , the second AND gate  105 , the first OR gate  107  or other components within the inventive IC  101  and are thus difficult to detect by conventional testing techniques. For example, one or more faults may exist that prevent the dynamic node  111 , the internal node  129  or the internal node  133  from properly pre-charging to V DD . The inventive IC  101  can identify such potential faults. 
     To detect potential faults within the inventive IC  101 , a test pattern (e.g., a DC speed test pattern) is applied to the inventive IC  101  via the first and the second inputs (IN 1 ) , (IN 2 ) and a high voltage pulse, test pre-charge signal is applied to the gate of the third NFET  135 . In response to the test pre-charge pulse, the third NFET  135  turns ON and the test output  125  is pulled to ground so as to “pre-discharge” the test output  125 . After a delay sufficient to allow the various potentials within the inventive IC  101  to settle, the voltage present on the test output  125  is evaluated (e.g., preferably by capturing the voltage present on the test output  125  via the latch  137  following the delay). This procedure preferably is repeated at least twice for the inventive IC  101  to facilitate the exercise of all circuit groups within the inventive IC  101  in both a pre-charge and evaluate condition (e.g., when the logic is in both a high and a low state). 
     As an example, assuming the test output  125  has been pre-discharged, if during testing of the inventive IC  101  the dynamic node  111  fails to pre-charge to a voltage greater than V DD −V TH , the third PFET  123  turns ON, pulling the test output  125  to V DD  so as to identify the presence of a potential fault within the inventive IC  101 . Similarly, if the internal node  129  fails to pre-charge to a voltage greater than V DD −V TH , the fourth PFET  127  turns ON, pulling the test output  125  to V DD  so as to identify the presence of a potential fault within the inventive IC  101 . If the internal node  133  fails to pre-charge to a voltage greater than V REF −V TH , the fifth PFET  131  turns ON, pulling the test output  125  to V REF  so as to identify the presence of a potential fault within the inventive IC  101 . By employing a reference potential (V REF ) with the inventive sensing circuits  123 ,  127  or  131 , any potential fault value V REF −V TH  may be identified. 
     FIG. 2 is a schematic diagram of an integrated circuit  201  configured in accordance with a second aspect of the present invention. The inventive integrated circuit  201  is identical to the inventive IC  101  with the exception that the third PFET  123 , the fourth PFET  127  and the fifth PFET  131  are each provided within a unique test output  203 - 207 , a unique pre-charge NFET  209 - 213  and a unique latch  215 - 219  as shown. In this manner, by supplying unique test outputs  203 - 207  rather than a common test output as in the inventive IC  101  of FIG. 1, the location of any potential fault can be isolated to a particular faulted circuit (e.g., the first AND gate  103 , the second AND gate  105  and/or the first OR gate  107 ). 
     By thus providing logic gates and integrated circuits that are testable for both the presence of and the location of potential faults, both IC quality assurance and IC testing/troubleshooting are greatly enhanced. 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, while the present invention has been described with reference to pre-charge high dynamic logic circuitry, it will be understood that potential faults within other circuitry types including pre-charge low dynamic logic (e.g., by employing an NFET sensing circuit and a PFET pre-charge circuit) and static logic circuitry may be similarly identified. A similar structure (e.g., NFET sensing circuits) may be employed to identify low voltage or “low potential” faults, although dynamic logic circuits will detect erroneous low potential faults. Other circuits may be employed as sensing circuits and as test pre-charge circuits such as an operational amplifier or other analog circuit. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.
A logic gate is provided that comprises a sensing circuit coupled to a test output and to an internal node of the logic gate. The sensing circuit is adapted to sense a voltage on the internal node and to output a signal indicating a level of the voltage. The sensing circuit is not used during normal operation of the logic gate and preferably comprises only a single FET that is directly coupled to both the internal node and to the test output. The logic gate also preferably comprises a pre-charge circuit for pre-charging the test output to a predetermined voltage level prior to testing. An IC chip may be formed from a plurality of the logic gates wherein each logic gate comprises a sensing circuit coupled to a test output and to an internal node of the logic gate. Each sensing circuit may be coupled to the same test output or to a unique test output for the sensing circuit's logic gate. The sensing circuits are not used during normal operation of the IC chip.