Guided PAR

An optional guide design file can be fed into PAR. The guide file is an NCD file which is used as a template for placing and routing the input design. This is useful if minor incremental changes have been made to create a new design. To increase productivity, you can use your last design iteration as a guide design for the next design iteration, that is, your output NCD file becomes the guide design file for your next iteration of the design (see the following figure).

Figure 10.4 Guided PAR

Two command line options control guided PAR. The -gf option specifies the NCD guide file, and the -gm option determines whether exact mode or leveraged mode is used to guide PAR.

The guide design is used as follows.

• If a component in the new design has the same name as a component in the guide, it is placed where it was in the guide design.
  
  
• If an unnamed component in the new design is of the same type as an unnamed component in the guide design, and the two components have identical signals attached to them, the component is placed where the matching component was placed in the guide design.
  
  
• If the signals attached to a component in the new design match the signals attached to the component in the guide design, the pins are swapped to match the guide design, if possible.
  
  
• If the signal names in the input design match the guide, and have the same sources and loads, the routing information from the guide design is copied to the new design.
  
  

When PAR runs using a guide design as input, PAR first places and routes any components and signals that fulfill the matching criteria described above. Then PAR places and routes the remainder of the logic.

To place and route the remainder of the logic, PAR does the following.

• If you have selected exact guided PAR (by entering the -gm exact option on the PAR command line), the placement and routing of the matching logic are locked. Neither placement nor routing can be changed to accommodate the additional logic.
  
  
• If you have selected leveraged guided PAR (by entering the -gm leverage option on the PAR command line), PAR tries to maintain the placement and routing of the matching logic, but changes placement or routing if it is necessary in order to place and route to completion and achieve your timing constraints (if possible).
  
  Some cases where the leveraged mode is necessary are
  
  
  • You have added logic that makes it impossible to meet your timing constraints without changing the placement and routing in the guide design.
    
    
  • You have added logic that demands a certain site or certain routing resource, and that site or routing resource is already being used in the guide design.
    As one example of this condition, in XC4000EX devices TBUFs must be routed along long lines. If you add TBUFs to an XC4000EX design but your guide design uses too many of the required long lines, you are not able to route this design to completion unless you use the leveraged option.
    
    

If you enter a -gm (guide mode) option but do not specify a guide file with the -gf option, PAR is guided by the placement and routing information in the input NCD file. Depending on whether you specify exact mode or leveraged mode, PAR locks the input NCD's existing placement and routing (exact mode), or tries to maintain the placement and routing, but modifies them in an effort to place and route to completion and achieve your timing constraints (leveraged mode).

NOTE

For Verilog or VHDL netlist input designs, re-synthesizing modules typically cause signal and instance names in the resulting netlist to be significantly different from the netlist obtained in earlier synthesis runs. This occurs even if the source level Verilog or VHDL code only contains a small change. Because guided PAR depends on signal and component names, synthesis designs often have a low "match rate" when guided. Therefore, guided PAR is not recommended for most synthesis-based designs, although there may be cases where it could be a successful alternative technique.