"USB - Mass Storage Software for STK-05 & STK-10"
http://www.atmel.com/dyn/resou...-1_0_6.zip

config.h

#define  FOSC       16000


usb_drv.c

//! Usb_configure_clock
//!
//! This function configure the PLL to generate the 48MHz clock required by
//! the USB controller, considering the FOSC defined in the "config.h" file.
//!
//! @warning Code:xx bytes (function code length)
//!
//! @param none
//! The possible value for FOSC are :
//!              3000   ( 3MHz)
//!              6000   ( 6MHz)
//!              8000   ( 8MHz)
//!             12000   (12MHz)
//!             16000   (16MHz)
//!             18000   (18MHz)
//!             20000   (20MHz)
//!             24000   (24MHz)
//!             32000   (32MHz)
//!             40000   (40MHz)
//!             48000   (48MHz)
//!              0000   (frequency auto-dectection)
//!
//! @return none
//!