Electronic Design Automation

7.3.1.1. Unknown source blocking

There are many unknown (X) sources in a CUT or BIST-ready core. Any unknown (X) source in the BIST-ready core, which is capable of propagating its unknown (X) value to the ORA directly or indirectly, must be blocked and fixed with a DFT repair approach often called X-bounding or X-blocking. Figure 7.12 shows a few of the more typically used X-bounding methods for blocking an unknown (X) source: The 0-control point forces an X source to 0; the 1-control point controls the X source to 1; the bypass logic allows the output of the X source to receive both 0 and 1 from a primary input (PI) or an internal node; the control-only scan point drives both 0 and 1 through a storage element, such as D flip-flop; and finally, ...