DFT_MODE is set to zero to force DFT logic in inactive mode. During functional ECO, some nets are equivalent but some are good for DFT, the others would break DFT. For example, as shown in Figure 1 an ECO needs to pick one signal from two equivalent signals mux_in or mux_out to drive a flop's reset pin. Cadence Conformal ECO picks mux_in which breaks DFT. GOF can detect the presence of the MUX and pick mux_out as the right signal to drive the flop's reset pin.
Figure 1: Pick the right signal to maintain DFT logic
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