02-24-2014 09:00 AM
It's not negative. It's just a hyphen to separate the part number from the speed grade so you don't get confused. For example XC3S200-3FT256 clearlay sparates the base number "3S200" from the speed grade "3" where if you wrote XC3S2003FT256 it would look like the base part number is "3S2003". Then somehow the hyphen remained with the speed grade when they use it by itself. By the way, the part marking always separates the base number and package from the speed grade and temperature grade. The last two are marked separately (after test) on the last line of the part marking.
02-25-2014 05:58 AM
Yes It's just a hyphen. No need to bother about that.
All part markin related information can be found in Chapter-1 of the below link from page 12 onwards http://www.xilinx.com/support/documentation/user_guides/ug112.pdf