I wasted some hours yesterday trying to understand why Pin0.0 on a LPC1549 was behaving as an output though it was configured as input and not assigned to any function described in the manual through PINASSIGNx.
After I could rule out a HW defect or obvious configuration problem, I checked all involved registers in the peripheral view of LPCXpresso while debugging. I noticed that LPCXPresso shows four fields for PINASSIGN6 while the manual describes only three and the last missing one (31..24) is just named "reserved". However, in contrary to PINASSIGN15, writing a 0 (or not writing 0xff) to bit 31..24 of PINASSIGN6 actually assigns Pin0.0 to some output function.
According to LPCXpresso this function is USB_CONNECT - which is mentioned only twice in the manual, but not in a way that would make clear how to assign it to a pin.
So I figure the description of PINASSIGN6 is missing the field for USB_CONNECT. Shouldn't this be corrected in the manual? I'm using Rev. 1.1 (March 2014) which seems to be the newest one...
As a side note: the PINASSIGNx registers all have a reset value of 0xff defined for their configuration fields. My yesterday's investigation seems to show that this is not really true. Maybe it's a power on reset initialization, but the values there seem to have survived several warm resets and reprogramming with a simple pin test software which didn't write to the PINASSIGNx registers.