AVR-Chat

Hello Everyone, 
 Thats my first time writing to this group and I have a question
concerned with USB interface. I hope any of you can help me.

I am developing a board that uses the a new ATMEL microcontroller
(AT90PWM). I need this board to have a interface that will let me plug
16 of these in a PC. I am considering USB (with one ore two USB hubs)
but my microcontroller does not have this interface. I have a USART
and a SPI though. Are there such a thing like a USART <> USB chips
available? I dont what that part of my project to take a great deal of
time. It does not have to be a very cheap solution because it
represents only a small share of the project.

Thanks In Advance
Hugo Vieira de Oliveira