Method of design for testability for integrated circuits

Abstract

A method of design for testability using scan FF identification of this invention eases generation of test sequences as compared with conventional technique. An FF relation graph is generated from an integrated circuit, FFs having self loops are recognized in the FF relation graph, and all FFs are replaced with scan FFs. FFs not having self loops are sorted in accordance with a predetermined evaluation function indicating the degree of relation with difficulty in generating test sequences. For example, a function indicating the degree of relation with a balanced reconvergence structure is used as the evaluation function. In a sort order thus obtained, with regard to each FF not having self loops, it is determined whether or not the integrated circuit has an n-fold line-up structure in assuming the FF is replaced with a non-scan FF, thereby identifying scan FFs.

What is claimed is:  
     
         1 . A method of design for testability for modifying design of an integrated circuit designed at gate level or register transfer level, in order to attain testability after manufacture, comprising: 
 a full scan process for temporarily determining about all flip-flops (FFs) or registers in said integrated circuit to replace with scan FFs or registers;    a sorting process for sorting said FFs or registers in accordance with a predetermined evaluation function indicating a degree of relation with difficulty in generating test sequences; and    a non-scan selecting process for examining for each of said FFs or registers temporarily determined to replace with scan FFs or registers in said full scan process, in a sort order obtained in said sorting process, whether or not said integrated circuit has an n-fold line-up structure in assuming said FF or register to replace with a non-scan FF or register, and temporarily identifying said FF or register as a non-scan FF or register when said integrated circuit has an n-fold line-up structure by said assumption,    wherein an FF or register temporarily identified as a scan FF or register as a result of executing said non-scan selecting process is defined as a scan FF or register.    
     
     
         2 . The method of design for testability of  claim 1 , 
 wherein said sort order is an ascending order of the degree of relation indicated by said predetermined evaluation function.    
     
     
         3 . The method of design for testability of  claim 1 , wherein said sorting process comprises the steps of: 
 recognizing a balanced reconvergence structure included in said integrated circuit; and    sorting said FFs or registers with using a function indicating a degree of relation with said recognized balanced reconvergence structure as said predetermined evaluation function.    
     
     
         4 . The method of design for testability of  claim 1 , wherein said sorting process comprises the step of: 
 sorting said FFs or registers with using a number of inputs/outputs of each FF or register in an FF relation graph or a register relation graph representing said integrated circuit as said predetermined evaluation function.    
     
     
         5 . The method of design for testability of  claim 1 , wherein said sorting process comprises the step of: 
 sorting said FFs or registers with using a result of a predetermined calculation using a maximum sequential input distance and a maximum sequential output distance as said predetermined evaluation function.    
     
     
         6 . The method of design for testability of  claim 1 , wherein said non-scan selecting process comprises the steps of: 
 omitting said examining step for an FF or register identified to have a self loop structure; and    maintaining temporary determination of said FF or register as to replace with a scan FF or register.    
     
     
         7 . The method of design for testability of  claim 1 , further comprising a process for searching a single-output FF or a single-output register in said integrated circuit, 
 wherein said non-scan selecting process comprises the steps of: 
 omitting said examining step for said searched single-output FF or single-output register; and  
 maintaining temporary determination of said searched single-output FF or single-output register as to replace with a scan FF or register.  
   
     
     
         8 . The method of design for testability of  claim 1 , further comprising a process for searching a single-input/output FF or a single-input/output register in said integrated circuit, 
 wherein said non-scan selecting process comprises the steps of: 
 omitting said examining step for said searched single-input/output FF or single-input/output register; and  
 maintaining temporary determination of said searched single-input/output FF or single-input/output register as to replace with a scan FF or register.  
   
     
     
         9 . A method of design for testability for modifying design of an integrated circuit designed at gate level or register transfer level, in order to attain testability after manufacture, comprising: 
 a step of identifying a scan FF or register in said integrated circuit so that said integrated circuit does not have a balanced reconvergence structure satisfying a predetermined condition.    
     
     
         10 . The method of design for testability of  claim 9 , 
 wherein said predetermined condition is having a depth of a predetermined number or more.    
     
     
         11 . The method of design for testability of  claim 9 , 
 wherein said predetermined condition is including a predetermined number or more paths.    
     
     
         12 . The method of design for testability of  claim 9 , 
 wherein said predetermined condition is including a predetermined number or more FFs.    
     
     
         13 . A method of design for testability for modifying design of an integrated circuit designed at gate level or register transfer level, in order to attain testability after manufacture, comprising: 
 a step of identifying a scan flip-flop (FF) or register in said integrated circuit so that said integrated circuit has an n-fold line-up structure,    wherein said identification step comprises the steps of: 
 sorting FFs or registers in said integrated circuit in accordance with a predetermined evaluation function indicating a degree of relation with difficulty in generating test sequences; and  
 determining for each of said FFs or registers in said integrated circuit whether or not to replace with a scan FF or register in a sort order obtained in said sorting process.  
   
     
     
         14 . The method of design for testability of  claim 13 , wherein said sorting process comprises the steps of: 
 recognizing a balanced reconvergence structure included in said integrated circuit; and    sorting said FFs or registers with using a function indicating a degree of relation with said recognized balanced reconvergence structure as said predetermined evaluation function.    
     
     
         15 . The method of design for testability of  claim 13 , wherein said sorting process comprises the step of: 
 sorting said FFs or registers with using a number of inputs/outputs of each FF or register in an FF relation graph or a register relation graph representing said integrated circuit as said predetermined evaluation function.    
     
     
         16 . The method of design for testability of  claim 13 , wherein said sorting process comprises the step of: 
 sorting said FFs or registers with using a result of a predetermined calculation using a maximum sequential input distance and a maximum sequential output distance as said predetermined evaluation function.    
     
     
         17 . A method of design for testability for modifying design of an integrated circuit designed at gate level or register transfer level, in order to attain for testability after manufacture, comprising: 
 a step of identifying a scan flip-flop (FF) or register in said integrated circuit so that said integrated circuit has an n-fold line-up structure,    wherein said identification step comprises the steps of: 
 searching a single-output FF or a single-output register in said integrated circuit; and  
 identifying said searched single-output FF or single-output register as a non-scan FF or register.  
   
     
     
         18 . A method of design for testability for modifying design of an integrated circuit designed at gate level or register transfer level, in order to attain testability after manufacture, comprising: 
 a step of identifying a scan flip-flop (FF) or register in said integrated circuit so that said integrated circuit has an n-fold line-up structure,    wherein said identification step comprises the steps of: 
 searching a single-input/output FF or a single-input/output register in said integrated circuit; and  
 identifying said searched single-input/output FF or single-input/output register as a non-scan FF or register.