Scheme for Masking Output of Scan Chains in Test Circuit

Abstract

Operating a scan chain of a test circuit of an integrated circuit to have either a single fanout or multiple fanout to a compressor. The test circuit receives a fanout control signal for configuring the fanout of the scan chain. If the fanout control signal indicates configuring of the scan chain with a single fanout, the output of the scan chain is sent to one input of a compressor. If the fanout control signal indicates configuring of the scan chain with multiple fanout, the output of the scan chain is sent to multiple inputs of the compressor.

What is claimed is: 
     
         1 . A method for operating a test circuit of an integrated circuit, comprising:
 receiving, by the test circuit, a mode control signal for configuring the test circuit in one of a plurality of modes, each of the plurality of modes setting a different combinations of first scan chains with single fanout and second scan chains with multiple fanout;   selecting, based on the received mode control signal, a fanout control signal for configuring a fanout of a scan chain;   selecting, based on the received mode control signal, a mask control signal for configuring a masking of the scan chain;   configuring the fanout of the scan chain based on the selected fanout control signal; and   masking an output of the scan chain based on the selected mask control signal.   
     
     
         2 . The method of  claim 1 , further comprising:
 generating fanout control signals for other scan chains corresponding to the received mode control signal to configure fanout of the other scan chains, at least one of the other scan chains set to have a single fanout by the fanout control signals; and   generating mask control signals for the other scan chains corresponding to the received mode control signal, a mask control signal dedicated to each of the at least one of the other scan chains set to have the single fanout.   
     
     
         3 . The method of  claim 1 , further comprising:
 generating fanout control signals for other scan chains corresponding to the received mode control signal to configure fanout of the other scan chains, at least one of the other scan chains set to have a single fanout by the fanout control signals; and   generating mask control signals for the other scan chains corresponding to the received mode control signal, a mask control signal shared by a subset of scan chains that are separated by a predetermined number of the scan chains.   
     
     
         4 . The method of  claim 1 , further comprising:
 generating fanout control signals for other scan chains corresponding to the received mode control signal to configure fanout of the other scan chains, at least one of the other scan chains set to have a single fanout by the fanout control signals; and   generating mask control signals for the other scan chains corresponding to the received mode control signal, a mask control signal shared by a predetermined number of adjacent scan chains.   
     
     
         5 . The method of  claim 1 , further comprising:
 responsive to the fanout control signal indicating configuring of the scan chain as a single fanout, sending an output of the scan chain to one input of a compressor for compression; and   responsive to the fanout control signal indicating configuring of the scan chain as a multiple fanout, sending the output of the scan chain to three or more inputs of the compressor for compression.   
     
     
         6 . The method of  claim 5 , wherein sending the output of the scan chain to one input of the compressor comprises disabling all but one output of the scan chain responsive to the fanout control signal indicating configuring of the scan chain as the single fanout. 
     
     
         7 . The method of  claim 1 , further comprising:
 receiving a mask enable signal configured to enable or disable masking of an output of the scan chain to the compressor for each shift of a test pattern fed to the scan chain; and   masking the output of the scan chain responsive to the mask enable signal being active and based on the mask control signal and the mask enable signal.   
     
     
         8 . The method of  claim 7 , wherein the mask enable signal is received via a dedicated pin of the integrated circuit during testing of the integrated circuit. 
     
     
         9 . The method of  claim 1 , further comprising storing a value representing the single fanout or the multiple fanout in a fanout control register associated with the scan chain, the fanout of the scan chain set by reading the value from the fanout control register. 
     
     
         10 . A test circuit in an integrated circuit comprising:
 a plurality of scan chains, each scan chain configured to generate scan chain outputs representing test responses of a subset of sub-circuits of the integrated circuit;   a compressor configured to compress the scan chain outputs of the plurality of scan chains;   a fanout circuit between at least one of the plurality of the scan chains and the compressor, the fanout circuit configured to set a fanout of the scan chain based on a fanout control signal;   a masking circuit coupled to the output of the scan chain for masking the output of the scan chain based on the selected mask control signal;   a fanout multiplexer coupled to the fanout circuit for selecting the fanout control signal based on a mode control signal; and   a multiplexer coupled to the masking circuit for selecting a mask control signal based on the mode control signal.   
     
     
         11 . The test circuit of  claim 10  wherein:
 the mask control signal is dedicated to the at least one scan chain responsive to the scan chain having a single fanout; and 
 the mask control signal is shared between the at least one scan chain and other scan chains responsive to the at least one scan chain having multiple fanout. 
 
     
     
         12 . The test circuit of  claim 10  wherein:
 the fanout circuit is configured to send a scan chain output of the at least one scan chain to one input of the compressor for compression responsive to the fanout control signal indicating configuring of the at least one scan chain as a single fanout, and send the scan chain output of the at least one scan chain to three or more inputs of the compressor for compression responsive to the fanout control signal indicating configuring of the scan chain as a multiple fanout. 
 
     
     
         13 . The test circuit of  claim 12  wherein the fanout circuit comprises an AND gate for disabling all but one output of the scan chain responsive to the fanout control signal indicating configuring of the scan chain as the single fanout. 
     
     
         14 . The test circuit of  claim 12  further comprising:
 a multiplexer coupled to the fanout circuit for selecting the fanout control signal based on a mode control signal. 
 
     
     
         15 . The test circuit of  claim 12  further comprising:
 a multiplexer coupled to the fanout circuit for selecting a mask control signal based on a mode control signal; 
 a masking circuit coupled to the output of the scan chain for masking the output of the scan chain based on the selected mask control signal. 
 
     
     
         16 . The test circuit of  claim 15  wherein:
 the mask control signal dedicated to the at least one scan chain responsive to the scan chain having a single fanout; and 
 the mask control signal shared between the at least one scan chain and other scan chains responsive to the at least one scan chain having multiple fanout. 
 
     
     
         17 . The test circuit of  claim 12 , wherein the fanout circuit comprises fanout control registers, each fanout control register configured to store a value representing the single fanout or the multiple fanout of an associated scan chain. 
     
     
         18 . A non-transitory computer readable medium storing data representing a test circuit in an integrated circuit, the test circuit comprising:
 a plurality of scan chains, each scan chain configured to generate scan chain outputs representing test responses of a subset of sub-circuits of the integrated circuit;   a compressor configured to compress the scan chain outputs of the plurality of scan chains;   a fanout circuit between at least one of the plurality of the scan chains and the compressor, the fanout circuit configured to set a fanout of the scan chain based on a fanout control signal;   a masking circuit coupled to the output of the scan chain for masking the output of the scan chain based on the selected mask control signal;   a fanout multiplexer coupled to the fanout circuit for selecting the fanout control signal based on a mode control signal; and   a multiplexer coupled to the masking circuit for selecting a mask control signal based on the mode control signal.   
     
     
         19 . The non-transitory computer readable medium of  claim 18 , wherein the test circuit further comprises:
 the mask control signal is dedicated to the at least one scan chain responsive to the scan chain having a single fanout; and   the mask control signal is shared between the at least one scan chain and other scan chains responsive to the at least one scan chain having multiple fanout.   
     
     
         20 . The non-transitory computer readable medium of  claim 18 , wherein:
 the fanout circuit is configured to send a scan chain output of the at least one scan chain to one input of the compressor for compression responsive to the fanout control signal indicating configuring of the at least one scan chain as a single fanout, and send the scan chain output of the at least one scan chain to three or more inputs of the compressor for compression responsive to the fanout control signal indicating configuring of the scan chain as a multiple fanout.