How important is the selection of scan_in / scan_out pairs?

[risccpu]

scan chain help

Hi,
When I do dft, I just assign test_scan_in port and test_scan_out port based upon the scan ports given by frontend. Could you tell me what impact by the selections of scan_in and scan_out pairs or the orders of scan_in and scan_out pairs? Or need I do other what work such as connecting the chains instead of the automation of tools?
Another question, why did the test coverage decrease by 7% when I reduced the pairs of scan_in port and scan_out port from 30 to 20? The length of chains have increased, but the test coverage decreased a lot. I want to know why this happened.

Best regards,
risccpu

 

[Garbielhijo]

Re: scan chain help

You can assign an IO port in a shared function (Scan_in and data_in or Scan_out and data_out) just indicating the start and end point of a chain. If you don't do this the DFT compiler will create the specific pins for Scan_in and Scan_out. Also if you have a constraint for max length your chain will be cut and another pin pair will be added.

Second. If you have a different clock domains (i.e. clk and /clk) or an improper use of the reset/preset of the flops and you mix the scan chains you will lose coverage because between the domains you will have a data lost. Try to avoid mixing the domains.

 

[jackson_peng]

scan chain help

the second problem may be solved by inlarge the FTPG pattern number in Tetramax generation.