• Gate Array
• Standard Cell
• Macro Cell
• Full Custom: Block/Cell and transistor aspect ratios, shape (need not be rectangular), floorplanning can be controlled by the designer to achieve a high degree of optimization —hand tuned designs
• Field Programmable Gate Array (FPGA)

Gate Array Design Style

Gate Array Design Style (contd)

Gate Array Design Style (contd)

Gate Array Design Style (contd)

Constrainedwidth routing
Gate Array Design Style (contd)

Standard Cell Design Style

Standard Cells (contd)

Standard Cells (contd)

• More flexibility than gate arrays---cells have the same height but varying widths, and thus a wide range of simple to medium-complexity functions can be designed as cells
• Only horizontal channels
(can be varying widths) are available for routing
• Feed through cells needed for vertical routing
• Over-the-cell (otc) routing can be done, which is just routing on another metal layer not occupied by cell interconnects
• Placement algorithm needs to take into account that nonadjacent inter-row routing space is limited (unless otc routing is allowed), so most interconnects should be between adj. rows.
• Channel routing is performed in the detailed routing phase
Feedthrough cell
Rows w/ differing lengths  wasted white-space (WS)
• Also during placement,the max row length should be minimized (given # of rows) or the std-devn in the row size should be minimized
(when # of rows is flexible) to minimize chip area

• Cells are of varying sizes (generally rectangular) and widely varying functional complexity (gates to register files to arithmetic units like adders & multipliers)
• Standard cells can be part of the design as well
• More flexibility than standard cells but the resulting placement and routing problems are more complex
• Placement: Placement for such cells is called floorplanning, and there are no pre-assigned slots to place the cells
• Routing: There are no predefined channels. Channel definition is one of the routing phases followed by global and detailed routing; the latter is channel routing + switchbox routing .

• Both logic and routing are programmable
• Least flexibilty in routing: needs to be done along prefab’ed routing tracks going along hor. & vert. channels
• Programmable switchboxes at the intersection of routing channels for interconnecting hor. & vert. tracks i-to-i & i-to-(i+3) mode 4 sw-box connections

One “switch” of a switchbox for simple i-to-I connections
FFs storing routing configuration data

Direct fast interconnects between adjacent cells

FPGA Design Style (contd)
The H Func. Gen. can take the 2 4-func. i/ps
F(a,b,c,d), G(a,b,c,d) [note same 4 i/p vars in this case), and w/ 1 extra var e, can produce a
5-variable function H(a,b,c,d,e,) based on
Shannon’s expansion:
H(a,b,c,d,e) = e*H(a,b,c,d,1) + e’*H(a,b,c,d,0), where G(a,b,c,d) = H(a,b,c,d,0) and
F(a,b,c,d) = H(a,b,c,d,1).
Note that the H Func. Gen. is being used here as a 3-i/p LUT (i/ps: e, G, F).

• Q: How should technology mapping be done for an FPGA have cells (CLBs) of the type shown for the Xilinx X4000E? In other words, what are the criteria for covering subcircuits by
FPGA cells?

— Logic synthesis & Tech. mapping

(library constr.)
- Gate arrays: much less expensive than std. cell design
(mass produced base arrays)
- Gate arrays: cells are small  more delay (on interconns), restricted cell lib. and routing
Slow and consumes high power (e.g., LUT
access for each tech-mapped “cell”)