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. 

Scan Process

The operating procedure of the scan circuitry is as follows:

1. Enable the scan operation to allow shifting (to initialize scan cells).

2. After loading the scan cells, hold the scan clocks off and then apply stimulus to the primary inputs.

3. Measure the outputs.
4. Pulse the clock to capture new values into scan cells.
5. Enable the scan operation to unload and measure the captured values while simultaneously loading in new values via the shifting procedure (as in step 1).