AR# 6588


Synopsys FPGA Compiler 1998.08/1999.05: Slew rate specifications on Virtex IOBs are ignored during synthesis.


Keywords : virtex, slew, rate, obuf, fast, slow, attribute, high, low, obuf_f, fpga compiler.

Urgency: Standard

Problem Description:

When using set_pad_type -slewrate LOW all_outputs() to specify FAST slew rate for
LVTTL outputs in Virtex, the slewrate attribute is ignored by the FPGA Compiler synthesis tool.

Other inaccuracies may arise when trying to infer specific slew rates or drive strengths.



This problem is fixed in the latest 2.1i Service Pack available at:


Without the service pack, three workarounds exist:

1. Instantiate the specific output buffer directly in your HDL code.

2. Use the -exact switch in your Compiler run script when declaring pads:

set_pat_type -exact OBUF_F_12 "my_port"

3. Use the following UCF constraint on the output nets requiring a fast slew rate :

NET "my_port" FAST;

Note: Use the type of buffer along with the slew rate, drive strength, and voltage standard
when defining the port (i.e. OBUF_F_12, OBUF_SSTL3_II).
AR# 6588
Date 04/25/2007
Status Archive
Type General Article
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