AR# 4963


FPGA Express 3.x: How to access special IOB components for Virtex/E


Keywords: Virtex, Express, IOB, IBUF, OBUF, IFD, OFD, voltage, standard

Urgency: Standard

General Description:
The Virtex/E and Spartan-II architectures have special Select I/O components that allow users to
specify the voltage standards each pin must have. This solution explains how to access these
components when using FPGA Express for synthesis.


These special IOB components exist in the FPGA Express synthesis library but must be instantiated
in your HDL code. The components are IBUF, IBUFG, IOBUF and OBUF, and the names are
followed by an underscore and then the voltage standard. For example:


A complete list of components understood by FPGA Express can be found in the \lib\virtex
directory under the FPGA Express tree (%XILINX%\synth for Foundation users). FPGA Express
will understand these components and will not attempt to place any I/O logic on these ports:

Warning: Existing pad cell '/ver1-Optimized/U1' is connected to the port 'clk' - no pads cells
inserted at this port. (FPGA-PADMAP-1)

FPGA Express does not merge flip flops into IOBs for Virtex families. Therefore, you have two
options if you wish to have this done.
1. Use the map -pr switch to globally (for inputs, outputs, or both) merge flops into IOBs, or
2. Instantiate library primitives (FDCE, FDPE) and attach the IOB=TRUE attribute in your HDL
code. See (Xilinx Solution 4392) for more information about attribute passing in FPGA Express.

FPGA Express 3.4 will add the new standards unique to Virtex-E. Also included in that release will
be the ability to set these voltage standards from within the Constraints Editor.
AR# 4963
Date 08/31/2001
Status Archive
Type General Article
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