Key points mapping determines the quality of netlist LEC and ECO

The process of key points mapping, also known as compare points matching, plays a crucial role in netlist LEC and ECO. When examining the flops in Figure 1, it is important to determine whether the flop DFF1 in the Reference netlist should be mapped to DFF1 or DFF1_1 in the implementation netlist.

The aforementioned scenario is an actual output from Cadence Conformal ECO when there is a change in the flop DFF1 from a resettable type to a settable type. In the ECO result, Conformal ECO preserves the original state of the flop DFF1 and makes it only drive the original scan chain. This is likely done to maintain the functionality of the Design For Testing (DFT), although it is not necessary, as explained later in the paragraph.

However, a new settable type flop, DFF1_1, is created with its data pins shared with the original flop DFF1. This situation poses a problem because Conformal LEC is unable to perform a proper key point mapping. By default, Conformal LEC maps the flop DFF1 in the Reference netlist to the flop DFF1 in the implementation netlist using a naming priority method. As a result, the new flop DFF1_1 remains unmapped. This mapping failure leads to an LEC failure because the flop DFF1 in the implementation netlist does not drive any functional logic.

Interestingly, the compare points matching in another LEC tool, Synopsys Formality, also fails in the given scenario. Similar to Conformal ECO, Formality maps the flop DFF1 in the Reference netlist to the flop DFF1 in the implementation netlist, resulting in a failed formal verification outcome.

The correct approach in this key points mapping issue would be to disregard the flop DFF1 in the implementation netlist since it does not drive any functional logic. By utilizing topology and signature analysis, the flop DFF1 in the reference netlist should be mapped to the flop DFF1_1 in the implementation netlist. GOF employs this specific approach to ensure a successful key points mapping and achieve accurate logic equivalence checking results.

Returning to the ECO case that led to the aforementioned situation, it is worth noting that the flop DFF1 can be directly changed from a resettable type to a settable type during the ECO process. This modification does not disrupt the Design For Testing (DFT) functionality. In fact, it aligns with the concept of DFT Friendly ECO, as discussed in this article.

Visit us nandigits.com for more details on GOF ECO product.

#nandigits #netlisteco #DFT