Can all the test signals be shared

In a Scan based design for testability we generally need the following test signals

  • scan_in
  • scan_out
  • scan_en
  • test_mode(optional)
If you are having only the first three test signals then you need to have scan_en dedicated and the rest two can be shared. However, it is recommended to use all the above listed signals wherein only test_mode will be dedicated and the rest can be shared. This dedicated test_mode signal can be used to control other non-scan test logic inserted into the design.