Abstract:
All feedback cycles in a circuit network which cross only non-scannable memory elements are detected in linear run time. The method models a circuit network as a directed graph, then attributes network elements so that a single feedback cycle may be found in constant time. In the breadth first version, feedback is detected by traversing at most a constant distance back to the last scannable memory element. In the depth first version, graph nodes are not FINISHED until all predecessors are FINISHED. Feedback is found immediately if a node runs into another node that is NOT — FINISHED. This feedback is illegal if both nodes are in a zone defined by the same scannable memory element. The resulting identification and removal of feedback loops crossing only non-scannable memory elements significantly reduces the subsequent complexity of test pattern generation. This ensures a faster, more reliable, and more accurate test process after circuit fabrication.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the testing of digital circuits and, more particularly, to identifying feedback loops crossing non-scannable memory elements. 
   SUMMARY OF THE INVENTION 
   The present invention provides for finding a feedback loop that has no clearing elements. A first node, having a distance count, is employable to record the distance count associated with at least one ancestor node of the first node. There is a loop connecting the first node to a second node. A count analyzer is employable to determine whether the second node, having its own distance count, is the same node as the recorded ancestor of the first node. In one aspect, the loop is flagged as unallowable if the second node is the same node as the ancestor node of the first node. 
   BACKGROUND 
   As modern circuit networks shrink in physical dimensions and element counts (transistors, gates, wires) increase exponentially over time, testing a chip for defects after fabrication has become increasingly important and complex. Current state of the art incorporates automated testers which scan computer or hand-generated test cases (known as test vectors) into scan-latches within the circuit network. Generally, scannable memory elements, such as scan-latches, are those elements whose values can be directly observed during a test process. These scannable memory elements are employable to reveal logic values of the circuit blocks under test. 
   Through employment of known input values, fed into a particular logic function, an output measured by the automated tester can be compared to an expected result. Correspondence of measured results with expected results verifies correct operation of the device. 
   The physical topology of the circuit network, particularly the arrangement and operation of scannable memory elements, helps to determine the complexity of the test patterns used for verification, the time required for testing each chip, and the quality of those tests in terms of potential faults covered. 
   Feedback that crosses a particular class of network elements, however, is a physical topology which makes this verification process extremely difficult. Feedback crossing a network element can be generally defined as a feedback loop containing the network element. Feedback across non-scan latches (NSLS)—memory elements whose stored values cannot be directly observed or altered during the test process—pose problems to test pattern generation, test time and test quality. 
   When a network of logic elements driven by an NSL is fed back to the input of that same NSL, a loop is created that can make testing the circuit networks involved very difficult. Generally, the output value of the NSL at time t n , known as F(t n ), depends on the input F(t n−1 ) because of feedback. F(t n−1 ) in turn depends on F(t n−2 ) in a regression all the way back to the initial state F(0). If this initial state is unknown, testing can be impossible. Even if the initial state is known, however, all values F(0) to F(t n ) (or at a minimum, all the values before some known pattern begins to repeat) should be known by the test pattern generation program in order to determine the next expected output of the circuit network F(t n+1 ). Obtaining and retaining the output values is very costly in terms of compute time and memory, respectively. However, not retaining this information can be even more costly. Not retaining the output values means that the node of logic involved in the loop cannot be tested and, as a result, may have errors that will go unnoticed until use by a customer. 
   Feedback crossing scannable latches (SLs), however, effectively short-circuits this infinite loop. With SLs, known values can be injected into the circuit network loop at any time and the output can be tested for correspondence to these known inputs. Because it does not pose any special difficulty for test pattern generation, feedback across a scannable element is generally permitted in circuit networks. 
   A circuit network can be modeled as a directed graph, wherein the nodes or vertices of the graph are the circuit elements (transistors, gates, and so on), while the edges of the graph are the wires, buses or nets of the circuit network. Generally, directed graph is traversed by entering an input pin to the circuit network. A breadth first traversal can be performed from the input. Generally, a breadth first traversal can be defined as visiting and marking all unvisited elements a distance of one from the input, then all unvisited elements a distance of two from the input, and so on. Unvisited elements can generally be defined as a state wherein the element has not yet been visited by the directed graph during a traversal. 
   However, in the run time of such breadth first traversal algorithms, quadratic O(N 2 ) time is typically required to find feedback cycles in a directed graph. Here, N equals the number of network elements plus the number of wires or nets connecting those elements. With circuit networks where N is in the range of hundreds of millions to billions of elements, this run time is unacceptably slow. Enormous run time in the case of the O(N 2 ) breadth first traversal algorithm means that few, if any, of the detrimental feedback cycles crossing non-scan latches will be detected. This in turn makes the circuit network harder to test, diminishes test coverage and increases test time. 
   As feedback loops cross non-scannable elements, it is desirable to find and eliminate such loops prior to fabrication. Therefore, what is needed is a feedback detector that overcomes the limitations of conventional feedback detectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1A  schematically depicts a directed map comprising scannable and non-scannable elements; 
       FIG. 1B  schematically depicts a directed map comprising a hidden feedback loop; 
       FIG. 2A  schematically depicts a circuit network with feedback loops crossing scannable and non-scannable latches; 
       FIG. 2B  schematically depicts the circuit network expressed as a directed graph; 
       FIGS. 3A and 3B  illustrate a method for checking for illegal feedback loops; 
       FIG. 4  schematically depicts two coupled networks; 
       FIG. 5A  illustrates pseudo-code for a BFS traversal; and 
       FIG. 5B  illustrates pseudo-code for a DFS traversal. 
   

   DETAILED DESCRIPTION 
   In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning network communications, electro-magnetic signaling techniques, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art. 
   It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combination thereof. In a preferred embodiment, however, the functions are performed by a processor, such as a computer or an electronic data processor, in accordance with code, such as computer program code, software, and/or integrated circuits that are coded to perform such functions, unless indicated otherwise. 
   Turning to  FIG. 1A , disclosed is a directed map of circuit network  100  comprising scannable, or “clearing,” elements  110  and non-scannable, or “non-clearing,” elements  120 . Generally, the network  100  is tested for non-allowable feedback loops between different logic blocks. 
   In one embodiment, the scannable and non-scannable elements  110 ,  120  are pairs of flip-flops. In the network  100 , clearing elements, such as scan latches  110 , are interposed in electrical paths through the circuit network  100 .  FIG. 1  further comprises logic blocks  115 ,  125  and  135 . Generally, a node comprises circuit network elements, such as memory elements, logic gates and logic blocks. 
   The network  100  further comprises a distance incrementer  117 , an attribute assignor  119  and a count analyzer  122  within a feedback detector engine  116 . Generally, the distance incrementer  117  increments a count associated with a node when traversing from node to node or from node to non-scannable flip-flop. For instance, the count is incremented by one when traversing from node  125  to non-scannable element  120 . In one embodiment, the count set by the distance incrementer  117  is reset to zero when encountering a scannable element. For example, when traversing from node  115  to scannable element  110 , the count is reset to zero. 
   In the network  100 , feedback lines can cross either clearing elements or non-clearing elements. Turning briefly to  FIG. 2A , disclosed is a network  200  with feedback crossing clearing and non-clearing elements. Turning back to  FIG. 1 , typically, feedback lines are allowed in the circuit network  200  if they cross a clearing element. However, there are design constraints if the feedback loop crosses non-clearing elements. Generally, denoting clearing and non-clearing elements in the circuit network  100  allows for detection of unallowable feedback loops. Generally, feedback loops which cross scan-latches  110  are allowed. Feedback cycles which cross only non-scan latches  120  are not allowed. 
   In the circuit  100 , these clearing elements are placed a maximum distance D from the last clearing element and a maximum distance D from the circuit network input pins. The parameter D is generally defined by design criteria. For example, a maximum of D=30 logic blocks are allowed to be interposed between scan latches. If a feedback loop contains a clearing element, and meets the design criteria, then the feedback loop is allowable. If a feedback loop does not contain the clearing element within the specified parameter D, then the feedback loop is unacceptable. 
   In  FIG. 1A , for each node  115  in the network  100 , graph information is stored by an attribute assignor  119 . 
   The network  100  is tested for unacceptable feedback loops through employment of a traversal of the network  100 . When traversing the network  100  from input to output, the logic blocks  115  have one of their attributes assigned to them by the attribute assignor  119 . In one embodiment, these attributes are color-coded. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Information for Breadth First Traversal 
             
           
        
         
             
                 
               Characteristic 
               Description 
             
             
                 
                 
             
             
                 
               Attribute 
               First, Second or Third 
             
             
                 
               (Color) 
               (RED, GREEN, or BLACK) 
             
             
                 
               Distance 
               The distance away from the last 
             
             
                 
                 
               clearing element if the color of this node 
             
             
                 
                 
               115 is GREEN or RED, or the distance away 
             
             
                 
                 
               from the input if the color of this node 115 
             
             
                 
                 
               is BLACK. (“DIST”). 
             
             
                 
               Predecessor 
               The unique element from which the 
             
             
                 
                 
               directed map first reached this logic block. 
             
             
                 
                 
               Also known as the parent element. By 
             
             
                 
                 
               definition, a parent element is a distance 
             
             
                 
                 
               of 1 away from its children. 
             
             
                 
                 
             
           
        
       
     
   
   Nodes inherit the color of their predecessor node except for the following initial and special conditions. In one embodiment, the attribute assignor  119  performs the attribute assignment of inheritance. Nodes are assigned a first attribute by the attribute assignor  119  when they are first traversed as an input to the circuit network  100 . In one embodiment, the first attribute is the color BLACK. Nodes are assigned a second attribute by the attribute assignor  119 , after a path passes through a clearing element  110 , such as a scan latch between a parent node and a child logic block. In one embodiment, the second attribute is the color GREEN. Nodes are assigned a third attribute after a traversal crosses a non-clearing element. In one embodiment, the third attribute is the color RED. 
   The distance characteristic DIST associated with each element is incremented by the distance incrementer  117  with each node  115  or a non-scannable element  120  of the network  100  from a starting point of zero at the input. The increment occurs with each traversal of a node except after passing through a clearing element  110 , whereupon the distance is reset to zero by the distance incrementer  117 . Hence, the distance characteristic DIST associated with each node  115  measures the distance to the last clearing element  110 , if the node  115  color is RED or GREEN. The distance characteristic DIST also measures distance to the input, if the node  115  color is BLACK. If this distance characteristic DIST ever exceeds the predetermined constant value D, set by the architectural design criteria, a design violation occurs. 
   In  FIG. 1A , feedback cycles across logic blocks  115  have design restraints. Again, feedback loops which cross logic element paths having scan latches  110  are allowed, if they meet the other design criteria. Feedback cycles which cross only non-scan latches are not allowed. 
   In  FIG. 1A , logic blocks  115  are previously visited if they have the first, second or third attribute. In traversing a directed graph of the network  100 , encountering a first node from a second node can indicate a feedback line, or it can denote the convergence of two paths. The test is then performed by the count analyzer  122 . 
   For instance, if the attribute color associated with an originating, or first logic element, is GREEN, a potential feedback loop, to a potential predecessor second node, is automatically permitted. The count analyzer  122  performs no further testing of this loop and flags it as an allowable connection. This is because feedback originating from a GREEN node would necessarily cross a scannable element  110 . Therefore, determining whether the second node is a predecessor of the first node is not necessary and the operation completes in constant time. This type of scannable element  110  is known as a “clearing element” because after passing through it, the DIST characteristic is reset to zero by the distance incrementer  117 . A check for feedback within its cleared, or GREEN, zone is not necessary. 
   However, if the second, or intersected, node has the third, or RED, attribute, an illegal feedback loop could be present. An illegal feedback loop is detected by the count analyzer  122  if the second node that the first node intersects is a predecessor of the current logic block, and if there is no scannable element between the first node and the second node that would make the feedback legal. 
   In order to determine if an illegal feedback cycle has been found, the count analyzer  122  traverses the list of predecessors of the first, or generating, node back to the distance corresponding to the intersected second node. If the second node is an ancestor of the first node, then an illegal feedback loop has occurred. If the ancestor having the same DIST value is not the second node, an illegal feedback loop has not occurred, and the count analyzer  122  flags an allowable feedback loop. In other words, by the count analyzer  122  enquiring to, at most, the last scannable element  110 , but not beyond it, it can be discovered that no scannable element  110  exists between the first node and the second node that would make the feedback allowable. If feedback is not allowable, then the design can be altered to either eliminate the feedback loop or make it allowable by inserting a scannable element. 
   Following is pseudo-code to ensure that a predecessor logic element  115  contains or does not contain a scannable element. 
   
     
       
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               CurrNode = Y 
             
             
                 
               WHILE Distance[CurrNode] &gt; Distance[X] DO 
             
           
        
         
             
                 
               # Iterate back through predecessor list 
             
             
                 
               CurrNode = Predecessor[CurrNode] 
             
           
        
         
             
                 
               OD 
             
             
                 
               # Distance[CurrNode] = = Distance[X] 
             
             
                 
               IF CurrNode = = X THEN 
             
           
        
         
             
                 
               RETURN FeedbackCycleFound 
             
           
        
         
             
                 
               FI 
             
             
                 
               RETURN FeedbackCycleNotFound 
             
             
                 
                 
             
           
        
       
     
   
   As is understood by those of skill in the art, OD represents closing the statement that is started with DO, in a similar manner to FI closing a statement starting with IF. 
   Since the distance characteristic DIST measures the distance away from the last clearing element  110  or the initial input, and this distance can be, at most, a constant distance D away, the above operation takes O(D) constant time. That is, at most, D predecessors will be queried to determine if that predecessor matches the requested DIST value of the intersected node X. Run time depends upon the design criteria which set D at a constant value. For example, there can be no more than D=30 logic gates between scan latches. 
   When the search for a particular input is complete, the attribute assignor  119  and the distance incrementer  117  are applied to the next input pin to the circuit network, and so on, until all of the inputs to the network are traversed. Distance, color and predecessor characteristics are retained on node  115  elements during this process. That is, nodes  115  that were visited and assigned an attribute during the traversal on one input pin will still remain colored via the previous traversal on the traversal of the next input pin. 
   Turning now to  FIG. 1B , disclosed is a hidden feedback loop. In the case of two converging paths of different colors (such as the attributes of GREEN and RED or BLACK and RED), the BFS traversal could proceed in a way that hides feedback crossing an NSL. Typically, this occurs only when a RED path and a non-RED path converge at an element. Whichever path reached the node first will color it and be listed as the predecessor. 
   In  FIG. 1B , DIST=distance as measured by the BFS. The top path reaches node P at DIST=2 and colors it GREEN. Since node P is GREEN, the algorithm does not check output leading to the BLACK node at distance  1  as a potential feedback. Therefore, the BFS determines there is no unallowable feedback loop from P with respect to the top path. However, there could be an unallowable feedback loop with respect to another path leading to node P if that other path is RED. When the bottom path reaches node P, RED dominates the converging paths and is propagated through. This allows for the detection of any illegal feedback from nodes P and beyond leading back to the bottom path. 
   Since the RED path dominates GREEN and BLACK, each path in the network is traversed, at most, twice; once when coloring GREEN or BLACK, and then potentially a second time if RED dominates a converging path. Hence, the overall run time 2*n=O(n) stays linear. 
   Turning again to  FIG. 2A , illustrated is a circuit network  200  with feedback loops crossing scannable and non-scannable latches. The network  200  comprises inputs IN 0 , IN 1  and IN 2 . The IN 0  is input into AND gate  210 . The output of AND gate  210  is input into a scannable latch (SL 1 )  220  and a non-scannable latch (NSL 1 )  230 . The NSL output is then fed into a 2-way AND gate  231 . The output of the 2-way AND gate is fed into an input of the AND gate  210 , thereby creating a feedback loop. The output of the scannable latch  220  is inverted by the inverter  225 . The output of the inverter  225  is input into a scan latch  235  and a non-scannable latch  237 . The output of scan latch  235  and the non-scannable latch  237  are OUT 0  and OUT 1 , respectively. The output of inverter  225  is also fed into an input of the 3-way AND gate  210 , thereby creating another feedback loop. 
   IN 1  and IN 2  are input into two signal buffers  240  and  241 , respectively. The buffers are then fed into a 3-way AND gate  242 . The output of the AND gate  242  is fed into a scannable latch  244 . The output of the scannable latch  244  is fed into a 2-way AND gate  246 . The output of AND gate  246  is fed into the 2-way input AND gate  231 , and also fed into NSL 3   248 . The output of NSL  248  is input into the 3-way AND gate  242 , thereby creating a feedback loop. Furthermore, the output of NSL  248  is input into the AND gate  246 , thereby creating another feedback loop. Finally, the output of NSL  248  is also fed to OUT 2 . 
   Turning now to  FIG. 2B , illustrated is the network  200  of  FIG. 2A  expressed as a directed graph  270 . For purposes of illustration, the various nodes of the directed graph  270  have been expressed as a first, second and third attribute, such as RED, GREEN and BLACK, but those of skill in the art understand that other coloring schemes are within the scope of the present invention. 
   When testing for feedback loops that violate design parameters, the directed graph  270  is characterized. Starting at IN 0 , the node “a,” corresponding to a node  115 , is colored “BLACK,” as it has not intersected an SL or non-SL latch. The DIST count at node “a” is 1. Then, each path  211  and  212  is traversed in the next round of the BFS, which looks at elements a distance of two away from IN 0 , and memory elements, such as latches, are encountered. SL ( 220 ) is scannable, so it is colored GREEN. Furthermore, the distance variable is set back to zero. However, path  212  encounters a non-scannable latch NSL ( 230 ), so a color transition is made from BLACK to RED, and the Distance variable is incremented and stored in the non-scannable latch. SL  220  is then traversed to node “b” which is GREEN due to the GREENness of its immediate predecessor, and the Distance variable for this node is incremented to “1.” NSL  230  is then traversed into node “c.” Node “c” is RED due to the REDness of its immediate predecessor, and its associated Distance is incremented to “2.” 
   The output of node “b” is then in a feedback loop to node “a.” Because the color of NSL 3  is RED, a check is performed to determine whether a scannable element exists between the NSL 3  and the nodes “e” and “f” which are found to have already been visited. It is determined that “f” having DIST count 1, is the ancestor of NL 3  having DIST count 1. Therefore, the feedback loop to node “f” is not allowable. It is determined that “e” having DIST count 2 is not the ancestor of NSL 3  having DIST count 2. For this reason, there is a scannable element between NSL 3  and node “e.” Indeed, SL 3  is between NSL 3  and node “e,” so the feedback is allowable. Furthermore, in  FIG. 2B , the distance is within the allowable design constraint D, so the second design feature is met. Therefore, feedback loop  213  is allowable. 
   In  FIG. 2B , because node “a” already has an associated attribute, node “c” checks to see whether an ancestor node that has the same distance count, before the last scannable element, is the same as the intersected node. In the illustrated embodiment of  FIG. 2B , node “a” is the same node as the predecessor node having the same distance count, so the feedback is illegal. 
   Starting from input IN 1  and IN 2 , the input nodes “d” and “g” are both BLACK. They are, in turn, traversed into node “e,” which also inherits the color attribute, in this case BLACK, from its immediate predecessor. The output of BLACK is then fed into SL 3  ( 244 ), which resets the count to zero and changes the inheritable attribute to the second attribute, GREEN. This second, or GREEN, attribute is inherited by node “f.” The output of node “f” is input into node “c” and NSL 3 . 
   Because node “f,” the originating node, has the second attribute, any feedback that occurs between node “f” and node “c” is an allowable feedback. However, node “f” is also input into NSL 3 , thereby changing the color from GREEN to RED from this element and after. The output of RED is then fed into the node “e.” 
   Because the color input into node “e” is RED, a check is performed to determine whether a scannable element exists between the NSL 3  and the node “e.” It is then determined whether or not a node “e,” having its associated distance count, is the same node as the ancestor node of NSL  244 . These two nodes are not the same, so this is a legal loop. 
   In a further embodiment, it is determined whether the total number of logic elements between scannable elements exceeds D. This is typically performed as part of the same traversal as determining for an illegal feedback loop. In one embodiment, this is performed by determining if the DIST value for the tested node is greater than the D allowable value. 
   In a further embodiment, combinational logic feedback loops are discovered. Combinational logic loops are detrimental to chip testing for similar reasons that non-scan latch feedback loops are. A combinatorial logic loop can be generally defined as a logic loop where the loop does not involve any memory elements. Typically, combinational logic loops create infinite loops whose values cannot be directly observed or altered by the test equipment. 
   In one embodiment, to find combinational logic loops, the predecessor list is examined up to the value of DIST of the intersected node, regardless of the color of the source node. If the source node identifies the intersection node as a predecessor, then this loop is either a combinational logic loop or an illegal NSL feedback loop. Either way, the feedback loop is identified and reported for correction. The testing for a single combinatorial logic loop is O(D) constant time. The run time of this new method to find all such loops is O(D*N)=O(N), where D is a constant value independent of N, where N is equal to the number vertices (nodes) plus the number of edges (arcs) in a directed graph representation of the circuit network. Equivalently, this is the number of circuit elements plus the number of interconnections between those elements. 
   Turning now to  FIGS. 3A and 3B , illustrated is a method  300  for checking for illegal feedback loops in a circuit network using a BFS algorithm. In step  301 , the method starts. In step  302 , an input of the circuit network is entered. In step  305 , distance count (DIST) is set to zero. Generally, distance count at a node is the path length from that node back to the last scannable element. In step  307 , a default attribute is set to the third attribute. In one embodiment, the third attribute comprises the color BLACK. 
   In step  310 , there is an increment from either an input or a parent node to a child node. In other words, the next child node for the BFS traversal is examined. In step  314 , the method  300  determines whether the child node has been visited before. In one embodiment, this is performed through determining whether the child node has a first, second or third attribute assigned to it. In other words, if the child node has any of the RED, GREEN or BLACK colors assigned to it. 
   If the child node has been visited before, in step  320 , the method  300  determines whether the intersected node is RED. If the intersected node is RED, then the DIST value of the intersected node is determined in step  322 . Then, in step  325 , the parent/originating node predecessors are traversed to the same DIST as the intersected node in step  330 . In step  340 , the method  300  determines whether the intersected node is the same node as the node having the same DIST value in the predecessor list. If it is, then the feedback is unacceptable, and the method stops in step  345 . However, if the two nodes are not the same, the feedback is determined to be allowable in step  347 . 
   However, if the intersected node is determined not to be RED in step  320 , the method  300  determines if the parent node is RED in step  330 . If the parent node is not RED, then step  347  executes, and feedback is allowable. However, if the parent node is RED, then step  362  executes, and the color of the intersected node is set to the third attribute (RED). 
   If step  314  determines that the child node does not have an attribute of RED, GREEN or BLACK associated with it, that child node has not been visited before, and step  350  executes. Step  350  determines whether the child node is a memory element and, if so, whether it is a scannable memory element. If the child node is a scannable memory element, then the second attribute (GREEN) is set for the child node, in step  361 . Then, the DIST count is reset to zero in step  364 . If the child node is a non-scannable memory element, then the third attribute (RED) is set for the child node, in step  362 . If the child node of step  350  is not a memory element, the child node inherits its color from the parent node, in step  363 . In either case of step  362  or step  363 , the child DIST is set to equal the parent DIST+1 in step  365 . In either the case of step  364  or step  365 , in step  370 , the parent node indicia (that is, the first second or third attribute) is stored in the predecessor attribute of the child node, such as a pointer to the parent node in the child node&#39;s predecessor attribute. Typically, one such value is stored per node. 
   After either step  347  or step  370  executes, the method  300  then determines whether a parent node has any other child nodes to look at, in step  375 . If there are more child nodes to look at, step  310  re-executes, and the next child node is looked at. If not, in step  380 , the method  300  determines if there are any more nodes left to test. If there are more nodes to test for illegal feedback, then in step  382 , the BFS moves to the next node. Determination of the next node is implementation specific. In general, each node stores a list of pointers to its children. Once done iterating through this list of children, the next node can be obtained by popping the top element off a queue, and step  310  re-executes. If there are no nodes to be tested, then in step  385 , it is determined whether there are any more inputs to the circuit network. If there are more inputs, then the new input is entered again in step  302 , and the flow re-executes. If not, then in step  390 , the method indicates that it is done, and exits the program. 
   In a further embodiment, a depth first traversal is performed from an input to the circuit network. Depth first searches (DFS) go to the end of a given path before returning to go to the end of the next path. The attributes of the DFS are as follows. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               DEPTH FIRST EMBODIMENT 
             
           
        
         
             
                 
               Characteristic 
               Description 
             
             
                 
                 
             
             
                 
               Attribute 
               First, Second or Third 
             
             
                 
               (Color) 
               (RED, GREEN, or BLACK) 
             
             
                 
               Distance 
               The distance away from the last 
             
             
                 
                 
               clearing element if the color of this node 
             
             
                 
                 
               115 is GREEN or RED, or the distance away 
             
             
                 
                 
               from the input if the color of this node 115 
             
             
                 
                 
               is BLACK. 
             
             
                 
               Predecessor 
               The unique element from which the 
             
             
                 
                 
               directed map first reached this logic block. 
             
             
                 
                 
               Also known as the parent element. By 
             
             
                 
                 
               definition, a parent element is a distance 
             
             
                 
                 
               of 1 away from its children 
             
             
                 
                 
               The unique ID assigned to the last 
             
             
                 
                 
               scannable latch encountered on the path 
             
             
                 
                 
               leading to this node. 
             
             
                 
               Status 
               NULL/NOT — VISITED, 
             
             
                 
                 
               NOT — FINISHED, 
             
             
                 
                 
               FINISHED. 
             
             
                 
               ZONE — ID 
               The unique element from which the 
             
             
                 
                 
               directed map first reached this logic block. 
             
             
                 
                 
               Also known as the parent element. By 
             
             
                 
                 
               definition, a parent element is a distance 
             
             
                 
                 
               of 1 away from its children. 
             
             
                 
                 
             
           
        
       
     
   
   The distance, predecessor and color attributes of the BFS of Table 1 substantially correspond to the DFS of Table 2. However, DFS have two additional characteristics. These are the “status” attribute and the “ZONE — ID” attribute. 
   Before being encountered in an attribute-assigning traversal, each node has the status of NULL/NOT VISITED. When first visited, a node is designated as NOT — FINISHED. A node is not marked FINISHED until all of its children nodes have completed their depth first traversals. 
   The ZONE — ID is a unique ID assigned to the last scannable latch seen on the path leading to a logic block. After passing through a scannable latch (SL), the logic blocks on the subsequent paths are inside the given zone until the paths pass through a different SL. 
   If during traversal a first node runs into a second node that already has an attribute associated therewith, it can be determined if the second node is a predecessor through reading the status of the second node. If the status of the second node equals NOT — FINISHED, then the second node is a predecessor and feedback has been found. To determine whether this is an illegal or permissible feedback loop, the ZONE — ID of the first and second nodes are compared. If the ZONE — ID of the second node is equal to the ZONE — ID for the first node, then a feedback loop which does not cross any scannable element has been found. Therefore, this feedback is not allowed (that is, it is illegal). 
   If the ZONE — ID for the second node is not equal to the ZONE — ID of the first node, there is a scannable element between the first and second node, and feedback is allowed. This follows from the definition of the ZONE — ID. This definition is that nodes along all paths originating from a scannable element inherit that scannable element&#39;s ZONE — ID. This continues until those paths pass through a different scannable element wherein the nodes will obtain a different ZONE — ID. 
   DFS and BFS are two fundamental and widely used ways to traverse graphs. By providing methods for detecting feedback crossing only NSLs using both traversals, the invention can more easily be integrated into existing microelectronics design tools and methodologies. 
   Whether DFS or BFS is used is implementation specific and may depend on some knowledge of the circuit network. For example, a circuit network with wide fan out relative to its depth would require a large BFS queue and a relatively small DFS stack. Conversely, a circuit network with low fan out and long relative depth would require a larger DFS stack than a BFS queue. This can be important if system memory is a limiting bottleneck. 
   Turning now to  FIG. 4 , illustrated are two networks  410  and  420 . Large integrated circuit designs, including system-on-a-chip designs, can consist of many interconnected modules, or networks. In  FIG. 4 , the networks  410  and  420 , each free of illegal NSL feedback loops, form an illegal NSL feedback loop when connected. 
   To avoid such problems, in a further embodiment, when networks  410  and  420  are verified to be free of detrimental feedback loops across NSLs, the resulting aggregate circuit network is also verified to be free of boundary violations. This can be enforced through the following boundary rules. 
   1) A scannable element is reached on any input path before a non-scannable element is reached. This is the “input” rule. 2) A scannable element is the last latch seen on an output path. This is the “output” rule. In other words, a non-scannable element cannot be seen last (that is, after a scannable element). 3) Finally, a latch is to be seen at some point on the path before being output. The is the “latch” rule. 
   If both the input and output rules are employed in the design of the plurality of networks  410  and  420 , a feedback loop will not cross only an NSL when networks  410  and  420  are connected. Generally, if either the input or output rules are applied, along with the latch rule, there cannot be any NSLs between the output of a first module and its connection to the input of a second module, and hence no illegal feedback loops are possible. 
   In this further embodiment, at least some of the boundary latches, such as  235 ,  237  and  248  are scannable between every large functional unit, such as networks  200  are scannable. This means that at least the first latch traversed along any input path, such as latches  220 ,  230 , and  244 , are scannable. In a further embodiment, the last latch seen on any output, such as latches  235 ,  237  and  248 , are scannable. In a still further embodiment, scan latches may be placed as both the first input and last output latches. This configuration allows control over the inputs of the network  200  and observation of all outputs to match with the expected results. Any one of these three boundary scan embodiments, if employed within a plurality of aggregated networks  200 , will ensure that no feedback loops crossing only NSLs can exist between network  200 .  FIG. 3  indicates how this could happen. If only the output rule is applied, this again is sufficient to guarantee no feedback loops crossing only NSLs can occur when networks  410  and  420  are connected, provided the latch rule is applied. 
   To enforce the boundary rules, it is necessary to ensure that, for the input rule, there is no color transition from the first attribute to the third attribute, for example, from BLACK to RED. Such a transition indicates that a non-clearing element was seen before a clearing element on an input path, which is not allowed under the input rule. To enforce the output rule, the attribute color of the last node (the node connected to the output pin) is not RED, the third attribute. If the attribute color is RED, then the last latch seen on the output path was not scannable. For the latch rule to be enforced, the color of the last node must be GREEN or RED. This ensures that the input to output path has a latch on it. 
   Those of skill in the art understand, however, that discovering illegal feedback loops can apply to communication networks, power networks, computer networks, control networks, and so on. In other words, the present invention can be applied to networks having feedback present within. 
   Turning now to  FIGS. 5A and 5B , disclosed are source and pseudo-code for a BFS and DFS implementation, respectively, of the methods described in this invention. Input and output boundary checking is performed. Design criteria validation is also performed. This ensures that the distance between non-scan latches never exceeds the predetermined value D=MAX — DIST. 
   In  FIG. 5A , the procedure Find — All — Feedback( ) is called on the directed graph representation of the circuit network. From there, a breadth first search (BFS) traverses the graph looking for feedback loops. All feedback loops crossing only non-scan latches are found in linear O(N) time. N=|V|+|E|, the number of nodes plus the number of edges or, equivalently, the number of circuit elements plus the number of interconnects/wires between them. In a further embodiment of  FIG. 5B , a depth search (DFS) traverses the graph looking for feedback loops. 
   It is understood that the present invention can take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. For example, different communications standards may be implemented, and the like. 
   Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.