Dont Care vs Dont Touch on Scan Chain

When ECO involves scan chain, the scan chain path should be in "Dont Care" mode or "Dont Touch" mode. There is different ECO performance in the two modes. The following paragraph explains the detail difference by one ECO example.

ECO Involving Scan Chain

During functional ECO, DFT related pins are configured to set DFT circuit in ignore mode. However, in order to achieve the best functional ECO result, DFT logic should be treated as "Dont Care" instead of "Dont Touch". There is difference between "Dont Care" and "Dont Touch". "Dont Care" mode can tune up the DFT logic and "Dont Touch" mode forbids any logic change in the DFT logic.

For example, Figure 1 shows an ECO to change the flop reg1 from reset type flop to set type flop. The best solution is to convert the flop reg1 to a set type flop which means the DFT logic is treated as "Dont Care", since the scan chain fanout reg2 is now driven by different type of flop.

GOF treats DFT logic as "Dont Care" in this ECO case. And the ECO change is only one flop type change.

Figure 1: Reset flop changed to set flop in ECO

Conformal ECO treats DFT logic as absolutely "Dont Touch". So after ECO, a new set type flop reg1_1 is created to drive the original functional circuit and the old flop reg1 still drives the scan chain fanout reg2. It is not optimal solution and also it causes a new problem. Conformal LEC treats the new flop reg1_1 as not mapped key point. Ant it results in lots of non-equivalent points in equivalence report after ECO.

Figure 2: Conformal uses inefficient solution to add new set type flop in ECO

Conclusion

In order to achieve optimal ECO result, DFT logic should be treated as "Dont Care" mode instead of "Dont Touch" mode.

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