AR# 4408: A1.4/F1.4 MAP - 3K mapper incorrectly reports unconnected inputs to IFDs and then trims the component.
A1.4/F1.4 MAP - 3K mapper incorrectly reports unconnected inputs to IFDs and then trims the component.
This problem only occurs when you have the "Q" pin of an input flipflop or latch directly driving the "IN" pin of an OBUFT with *no* intervening logic. The IFD will sometimes be sucked into the output IOB, the connection to the IPAD is lost, and it all gets incorrectly trimmed because there is no driver.
This issue is scheduled to be fixed in the M1.5 Performance Pack. Meanwhile, it can be worked around by inserting a buffer between the IFD and OBUFT. The buffer prevents the error, but is eventually optimised away, so there is no performance loss.