Metal-only spare-cell ECO

Metal-only spare-cell ECO. Once the base (transistor) layers are fabricated, a late change can touch only the metal and via layers: no new transistors. Designs are seeded with spare cells, unused gates of fixed types scattered around, precisely so late fixes have something to wire up. A real metal-only ECO is a batch of small fixes competing for one shared pool of spares.

The block is already placed and wired. Most of the cells (the grey hatched ones: flip-flops, a multiplexer, an AND-OR-Invert, and the rest) are functional logic in use. Their transistors are committed, so they cannot be re-wired. Only the idle spare cells scattered among them can be claimed.

Three changes here share that spare pool. Site 1 needs a 2-input AND, built as a spare NAND2 (Not-AND) feeding a spare inverter. Site 2 needs a buffer, one spare cell. Site 3 needs a 2-input OR, built as a spare NOR2 (Not-OR) feeding a spare inverter. Because only metal wire is added, the cost is the length of that wire, and a spare can serve only one change, so the two inverters must be different cells.

Click a change to make it active, then click spare cells to wire them into it. Clicking a functional cell explains why it is off limits. Get every change built from the right types and keep the total added wire at or under budget by claiming spares near each site. Then Submit ECO.