Many thanks for your answers !!
I think it would help if you write what you mean with "USB key
I'm sorry 0815Achim, (for me too), english is not my native language...
Best words for "USB Key" are probably "USB Flash Drive"
my project is initialy build around ATMega128 and I hope to keep this MCU if it's possible
I will search now to find one OTG interface from SPI or from I2C
but do you think that all USB OTG can drive this type of device ??
Best regards. MEE