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AR# 8324

FPGA Compiler 2, version 3.3.1: Bidirectional pads may cause multiple driver error in Translate (NgdHelpers:336)


Keywords: FPGA, Express, Compiler, translate, multiple driver, express, 3.3.1

Urgency: Standard

General Description:
In version 3.3.1 of FPGA Compiler II, a bug has been identified where a multiple driver error
may occur in Translate (NGDBUILD). When using a bidirectional pad, the EDIF may be
generated incorrectly; the netlist will contain both an IOPAD and IPAD on the same net.
The following error will occur:

ERROR:NgdHelpers:336 - logical net "bidir" has both active and tristate drivers
ERROR:NgdHelpers:346 - input pad net "bidir" has an illegal connection

This error should only occur when the bidirectional signal is used to drive clock pins and
therefore has had a BUFG instantiated on the input side of that port.



One resolution is to disable the global buffer insertion in the Express Constraints Editor. Under
the Ports tab, set the DEFAULT Global Buffer setting to DONT USE. Due to an issue
documented in (Xilinx Solution 4791), you must change this setting on the top line (default),
not the bidirectional port itself.

Any global buffers you do wish to have inserted must be specified individually in this column
under their respective port listing, or instantiated in the HDL code.

Note that this solution will not work if you do wish to have the BUFG inserted on the input of
the bidirectional port in question.


Another resolution is to instantiate all the components of the bidirectional I/O. This
means instantiating the IOPAD (pin name is IOPAD), BUFG, and OBUFT (or variation
thereof) in your HDL source. You must also remove that port from the top level port
declaration so Express does not try to insert I/O components. See (Xilinx Solution 6085)
for more details about instantiating pads.
AR# 8324
Date 04/25/2007
Status Archive
Type General Article