Pre-placement Activities in Physical Design

 In a broader sense, PnR (Place and Route) stage in physical design is basically Placement and Routing of all the instances present in the netlist in a defined core area in such a way that it should meet design rules and timing requirements. But before starting the actual automatic placement of instances by the PnR tool, there are certain activities which must be done prior to placement and those are called pre-placement activities. In this article, we will discuss some important pre-placement activities. 

Pre-placement activities in PnR

Major pre-placement activities:

• Pin placement
• Macro placement 
• Halo and routing blockage 
• Power plan
• Boundary cell/End cap cell placement
• Well tap cell placement
• Partial placement blockage /Density screen creation

Here we will discuss these activities in details in order as they needed to be performed. 

Pin placement:

In block-level PnR, input-output pins location are generally decided by the full-chip owner and the pin def is given to block owners. But some times pin location are not fixed at the top level and meanwhile block owner need to place them as per their convenience.

PnR tools provide a pin editing utility in their tools through which large numbers of pins can be placed easily. For innovus we can open the pin editor as Edit --> Pin Editor

Basically, we need to provide the following inputs to pin editor and corresponding image is shown a  typical pin placement.

 

 Pin list 

Metal layer 

Pin width

Pin depth

Side/edge

Spread / Distance between two pins 

 

Pin placement in PnR

Pins location could be either on edge of core or inside the core also. In case of pin def is available, we just need to defIn the pin.def file. In Innovus we can defIn the pin def file as bellow.

defIn <pin.def> 

Once all pins are placed, we can check that. In innovus we have a command. 

checkPinAssignment

The above command will give the total number of pins, the number of legal/illegal pins, the number of placed/unplaced pins.

Sometimes some i/o pins might have short with the PG structure, We can verify those shorts using following innovus command.

verify_PG_short -no_routing_blkg -no_cell_blkg

In case there are some shorts, we can fix those using following innovus command.

editPin -pin <pin name> -fixOverlap

Once all the pins are placed, we can defOut pins in a file for future use.

selectPin *    ;  Or   selectPin [dbGet top.terms]

defOut -selected <file_name>

Macro Placement:

Macro placement is a major step of the floorplan and the QoR (quality of result) of PnR is strongly dependent on the macro placement. A good macro placement requires thorough analysis of data flow in the block.
A bad floorplan could result in congestion and bad internal timings. There are some steps which must be followed especially in a macro dominating block. A detail discussion on macro placement strategy is explained in this article (will be linked soon).

Halo and Routing blockage:

Macros having high pins count near the edges generally and if the standard cell placement is high there, it could lead congestion. To avaoid this congestion we neet to put halo around the macro. (Halo is explained here - will be linked soon). The macro design needs more metal layers than normal standard cell and its pins are available in higher metal layers than the standard cells. So we need to put routing blockage for the layers which are used inside the macro. The power rails are blocked over the macros and power is delivered to the macros directly from power stripes.

Power Plan:

A power plan is a very robust power grid structure to deliver power to all macros and standard cells available in the design without much IR drop in the power grid. power grid takes power from bumps on the top metal layer and it delivers power to the lowest maetal layer in which standard cells follow pin available. 

From bumps, power goes to power stripe and power stripe delivers power to the VDD and VSS rails. Macros get power directly from power stripe as in place of macro there are no power rails drawn. 

Boundary cell placement:

Each placement row must be terminated with a boundary cell at both ends. Why we need boundary cells and what are the function of boundary cells, has been discussed in this article.

Well tap cell placement:

To get tap the psub to VSS and the nwell to VDD in order to avoid the latch-up issue in the design we need to place well tap cells at regular intervals in the core area. A detail discussion on well tap cells and its placement has been discussed in this article.

Partial placement blockage:

To avoid the congestion, we need to place partial placement blockage especially in the region where the pin density is more. We generally place partial placement blockage in the channel regions and the regions where io pins are placed. If the cell placement density will become high in this region, it may cause congestion as these areas already has lots of pin connections. 

Apart from these major activities, there are many other things which we need to on block specific like antenna cell placement, TCD Cells, PCLAMP cell placement. In the lower node, we need to check base DRC also after the macro placement steps. 

Thanks! 

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