Metal Configurable Gate Array ECO
Metal configurable gate array cells are specially developed for Metal Only ECO. There are two types of gate array cells used in different backend stages. The first type is gate array spare cells which are normal filler or decap cells in the original flow. In the backend P&R stage, gate array spare cells, like GFILL/GDCAP, are added and scattered over the design. The second type is gate array functional cells. In post-mask ECO, gate array spare cells are swapped out by gate array functional cells, like GAN2, GND2, GXOR2.
Gate Array Cell Base Tile
The base unit of gate array cell is a tile. Every gate array cell consists of one or more tiles. Use one 5nm standard library as example:
|Tile Numbers||Spare Cells||Functional Cells|
|1||GFILL1||GTIE GINVD1 GND2D1 GNR2D1|
|2||GFILL2||GBUFD1 GAN2D1 GOR2D1 GAOI21D1 GDN3D1|
|3||GFILL3||GAO21D1 GAN4D1 GOR4D1|
|5||GFILL5||GMUX2D1 GXOR2D1 GXNOR2D1|
|6||GFILL6||GBUFD8 GSDFFRQD1 GSDFFSQD1|
Table: Tile Numbers in Gate Array Spare Cells and Functional Cells
Gate array cells are larger than the normal standard cells. For example, GFILL1 is four times larger than FILL1 and GND2D1 is 25% larger than ND2D1. The power consumption and timing are similar.
Each gate array spare cell has the location defined by DEF file. As shown in Figure 1, one GFILL8 spare cell has location defined as (Xg, Yg). The tile width is the width of GFILL1.
The tiles on the GFILL8 can be regrouped and rewired in metal layers to form different functional cells. For example, GBUFD1 takes two tiles and implements buffer function and GAN4D1 takes 3 tiles to form 4 inputs AND function.
Figure 1: Gate Array Spare Cell GFILL8 Regrouped Tiles to Form Functional Cells
When GOF generates a patch, it synthesizes the patch to gate array functional cell types only. Then the gate array functional cells are mapped to the optimal close by gate array spare cells with minimum wire connection costs.
Figure 2: Gate Array Spare Cells Mapping to Functional Cells
After mapping and swapping, as shown in Figure 2, some gate array spare cells have portion of tiles being used by several functional cells. When saving ECO results, these gate array spare cells should have type changed, for instance, gate array A should have type changed from GFILL8 to GFILL4. Those used up gate array spare cells should be deleted, for instance gate array B has type GFILL4 and all four tiles are used.
The mapped gate array functional cells should be moved to the locations of the corresponding gate array spare cells, and horizontal location X should be adjusted according to the starting tile location. As shown in Figure 1, GINVD1 instance should be moved to (Xg+TW, Yg) and GBUFFD1 instance should be moved to (Xg+TW*6, Yg).
GOF writes out ECO verilog file and Backend tools ECO scripts. In Verilog file, the newly added gate array functional cells have location written in comments. GOF supports Synopsys ICC script and Cadence Encounter script. Both scripts have cell location placement support.
For example, saving the result in ICC TCL script, the cells in Figure 1 have the following commands:
Encounter script format:
Note:Tile size assumed to be 0.20 X 0.22; GFILL8 location (Xg, Yg)=(253.82, 413.28)
Gate Array Spare Cells ECO vs Standard Spare Cells Case
The experimental design has both gate array spare cells and standard spare cells inserted. In the placement shown in the Figure 3, the gate array spare cells are highlighted in green and the standard spare cells are highlighted in red. It shows, there are much more gate array spare cells distribution than standard spare cells. Also one gate array cell can be used as any functional cell as long as it has the same or more number of tiles than the gate array functional cell. So gate array spare cells ECO is much more easier to accomplish than standard spare cells.
Figure 3: Gate Array Spare Cells vs Standard Spare Cells Distribution
The gate array spare cells ECO has ~70 gate array functional cells added. The wire connections are well concentrating in one location as shown in the Figure 4.
Figure 4: Gate Array ECO Gates Connections Concentrating in One Area
The standard spare cells ECO has ~90 spare gates used. The utilized spare gates scatter over a large area. The Figure 5 has comparison of the two ECO placement and wiring. The gate array ECO has very concentrated ECO area as indicated by red color. But the standard spare cells ECO spreads over a large area as shown in blue in the Figure.
Figure 5: Placement and Wiring Comparison of Gate Array Cells ECO and Standard Spare Cells ECO
Check Metal Configurable Gate Array Cells ECO for detail