Answers Database

PPR: Design not routing (unroutes) because placement is to tightly packed.


Record #1963

Problem Title:
PPR: Design not routing (unroutes) because placement is to tightly
packed.



Problem Description:
Keywords:

Urgency: standard

General Description:
PPR will do an optimized placement on a design by placing
different design components very close together to minimize
routing delays. Though sometimes it places everything
to close together, resulting in a lack of routing resources in
certain areas, which causes PPR to end with unroutes.


Solution 1:

The best resolution to this problem is to floorplan certain
critical components in the design laying the structure for the
rest of the automatic placement to follow.



Solution 2:

Putting a prohibit placement constraint on a few CLBs in the
high density areas of the device may help the placer spread things out a little
more.
The constraint file syntax for PPR (XC4000, XC5200 series only) prohibiting plac
ement in a CLB is:
notplace instance *: clb_r1c5;
This constraint would not allow anything to be placed in the
CLB at row 1, column 5.



Solution 3:

There is an undocumented option in PPR which will may also
help to spread things out.
The option goes in the xactinit.dat file, the syntax is:
/ppr/realcut_type_fit_factor=100

This option has not been fully tested, and although it may help
the design to route, it will usually effect timing.



End of Record #1963