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AR# 8406

2.1i Virtex Constraints Editor: Drive strength constraints is ignored. (FPGA Express only)


Keywords: Constraints Editor, Slew Rate, Drive strength, UCF, Drive

Urgency: Standard

Problem Description:

After specifying Drive Strength or Slew Rate from the pull down menu's for a specific
outout in Constraints Editor, the constraint is ignored after implementation.


This is a know issue with Synopsis FPGA Express design. After specifying a constraint
in the constraint editor, the contraint is written into the constraint file.

For example, the syntax for a drive strength of 4 is:

NET <output portname> DRIVE=4;

fpga_express writes out the edif file with OBUF_S_12 by default, instead
of just an OBUF. After you set NET netname DRIVE=4 attribute in the
constraint file, what happens in the ngd file after running Ngdbuild is that
the OBUF instance still uses DRIVE=12, but the OPAD instance uses DRIVE=4ma;
MAP only picks up the constraint on the OBUF,not the OPAD.

The workaround is to place the constraints on the OBUF instance. In the
.UCF file, place:

INST C_<pad name> DRIVE = 4;

The C_ is the OBUF instance name for nets synthesized in Express.

This problem will be fixed in the next major release.
AR# 8406
Date Created 01/12/2000
Last Updated 12/10/2001
Status Archive
Type General Article