The CP2102 - CP2104 will do 250k; the CP2101 will not. Also, the CP2102 and CP2103 have a fixed table of rates (one of which is 250k) while the CP2104 can do arbitrary rates (within reason, see section 6.1 for details: http://www.silabs.com/Support%20Documents/TechnicalDocs/cp21...).