Ebrahim,

I cannot understand why the usb 8051 cannot defer send/receiving information. USB packetises its data into 1mS slots so you've got around 1mS to unload your data. Is the other side (PC or usb master) so aggressive that it would give you back to back usb packets? Surely your usb micro has enough ram to buffer some data whilst you sent it out the serial port at high speed? You say your slave processor runs a user interface - does it have a graphic lcd or something that requires a lot of data. If this is the case, why not use a beefier cpu with usb and connect the user interface directly to it? My suggestion would be to use something like the FTDI usb->serial chip and use the high speed serial to a 8051 or whatever cpu. At least you don't need to write any usb drivers!

At first thought the fifos seem like an elegant solution, but I think a little lateral thought may create a easier, more cost effective solution. If I'm telling you how to suck eggs, please humour me!!