Using a design which includes both Prohibits and MGTs can lead to confusing DRC errors in PlanAhead.
For example, relates to a xc6vlx240tff1759 part where errors from PlanAhead DRC checks for a design containing 45 Prohibits and 86 MGT pins:
"IOCNT #1 Fatal Design has more IOs (683) than placeable pins on the package (675)"
MAP reports the following:
"Number of bonded IOBs: 597 out of 720 82%"
The reports show conflicting information which is confusing.
PlanAhead DRC for IOBs takes into account Prohibits when calculating available I/Os on the device. As a result, there might be a difference between the available I/Os reported in PlanAhead and the available I/Os reported in MAP and PAR.
In the example, there are 45 Prohibits in the design (i.e., 720 pins - 45 Prohibits = 675 Pins available; the amount reported by PlanAhead). This is expected behavior in the PlanAhead software and was added in 11.1.
Another confusing part of the above DRC error shows PlanAhead reporting 683 I/Os in the design, but MAP reports 597. The MAP number is indeed correct while the DRC in PlanAhead takes into account MGT pins and treats them as I/O. In the example, there are 86 MGT pins (597 + 86 = 683). This is a known issue in 11.5 and is scheduled to be fixed in 12.1.