The physical measure of the quality of placement of macros can be as follows:
1.wire length
2.data flow
3.port accessibility
4.timing
Placement of the macro should not lead to segmented floor plan.In a segmented floor plan , standard cells area is not continuous.There is excessive interconnections between the standard cells located at the bottom of the die to those located at the top.This leads to increase in wire length.
To avoid segmentation , macro should be placed such that standard cell area is continuous.Macros could be kept along the ASIC core area.
Segmented Floor Plan
There can be increase in wire length due to the orientation of macro placement and pin locations, to nets being connected may be of different length.Macros should be placed accordingly so that their ports are facing the core area or to the standard cells, orientation should match available routing layers.
Floor plan with Macros Facing Standard Cells Region.
Placement of macro relative to standard cell placement and macro port accessibility has an impact on chip's final routing.The global router gives a statistical and graphical report on the analysis of routing congestion.
Routing congestion is caused by not enough space to provide routing channels between the macros for I/O connection and macro;routing is prohibited and standard cells trap pockets at the edges of the macros or within the corners of the floor plan.
Standard cell trap pockets are long, thin channels between macros.If there are more cells in the channel then it may cause routing congestion.Therefore,these channels need to be kept free for most standard cells and should be available for repeater or buffer insertion
Floor plan with Standard Cells Trap Pocket
Blockage layers:
Most Physical designs require keep-out regions or buffer only which is defined by blockage layer ,which refrain placer from moving the standard cells into this region.
Blockage layers avoid routing congestion.Suppose a macro blocks routing layers and then wire try to detour along the corners of the macros causing congestion at the corners.Blockage layer can be placed at the corners of the macros to reserve resources for the router.These are placed over pre-placed macros covering the power and ground rings.
Naturally, the wires that are used in keep-out regions have a tendency to be long. By allowing buffer insertion in those areas by using a buffer-only region (or blockage), the placer will taper these long nets and thus avoid the long transition times associated with them.
Fly lines:
After macro placement and standard cells are placed , connectivity analysis is performed.This is done by the fly lines.This is to study the connections between i/o pads,macros, standard cells.One can identify seeing the fly lines that whether moving or rotating the macros can reduce wire length which improve routability during floor planning stage.
