Hi Again Yann,

Check out the following datasheet on the Atmel Website (it's a pdf file).

http://www.atmel.com/dyn/resou...oc4254.pdf

This is the data sheet for the AT89C51SND1 USB Bootloader. I still don't know for sure if your particular chip will do both UART and USB ISP, or just one or the other. That you're going to have to ask Atmel about.