Code:

/* -----------------------------------------------------------------------------*/
static void calibrateOscillator(void)
{
uchar       step = 128;
uchar       trialValue = 0, optimumValue;
int         x, optimumDev, targetValue = (unsigned)(1499 * (double)F_CPU / 10.5e6 + 0.5);
 
    /* do a binary search: */
    do{
        OSCCAL = trialValue + step;
        x = usbMeasureFrameLength();    // proportional to current real frequency
        if(x < targetValue)             // frequency still too low
            trialValue += step;
        step >>= 1;
    }while(step > 0);
    /* We have a precision of +/- 1 for optimum OSCCAL here */
    /* now do a neighborhood search for optimum value */
    optimumValue = trialValue;
    optimumDev = x; // this is certainly far away from optimum
    for(OSCCAL = trialValue - 1; OSCCAL <= trialValue + 1; OSCCAL++){
        x = usbMeasureFrameLength() - targetValue;
        if(x < 0)
            x = -x;
        if(x < optimumDev){
            optimumDev = x;
            optimumValue = OSCCAL;
        }
    }
    OSCCAL = optimumValue;
}
 
/* ------------------------------------------------------------------------- */

int main(void)
{
uchar   i;
uchar   calibrationValue;
    
   wdt_enable(WDTO_1S);
    


    calibrationValue = eeprom_read_byte(0); /* calibration value from last time */
    if(calibrationValue != 0xff){
        OSCCAL = calibrationValue;
    }
   
   /* Even if you don't use the watchdog, turn it off here. On newer devices,
     * the status of the watchdog (on/off, period) is PRESERVED OVER RESET!
     */
    //DBG1(0x00, 0, 0);       /* debug output: main starts */
    /* RESET status: all port bits are inputs without pull-up.
     * That's the way we need D+ and D-. Therefore we don't need any
     * additional hardware initialization.
     */
    //odDebugInit();
    usbInit();
    usbDeviceDisconnect();  /* enforce re-enumeration, do this while interrupts are disabled! */
    i = 250;
    while(--i){             /* fake USB disconnect for > 250 ms */
        wdt_reset();
        _delay_ms(1);
    }
   usbDeviceConnect();
    sei();
    //DBG1(0x01, 0, 0);       /* debug output: main loop starts */
    for(;;){                /* main event loop */
      //  DBG1(0x02, 0, 0);   /* debug output: main loop iterates */
        wdt_reset();
        usbPoll();
    }
    return 0;
}

/* ------------------------------------------------------------------------- */

Code:

#if USB_CFG_CLOCK_KHZ==16500
#define USB_CFG_HAVE_MEASURE_FRAME_LENGTH   1
#include "osccal.h"
#else
#define USB_CFG_HAVE_MEASURE_FRAME_LENGTH   0
#endif
/* define this macro to 1 if you want the function usbMeasureFrameLength()
 * compiled in. This function can be used to calibrate the AVR's RC oscillator.
 */

Code:

General Description:
This module contains a function which calibrates the AVR's internal RC
oscillator so that the CPU runs at F_CPU (F_CPU is a macro which must be
defined when the module is compiled, best passed in the compiler command
line). The time reference is the USB frame clock of 1 kHz available
immediately after a USB RESET condition. Timing is done by counting CPU
cycles, so all interrupts must be disabled while the calibration runs. For
low level timing measurements, usbMeasureFrameLength() is called. This
function must be enabled in usbconfig.h by defining
USB_CFG_HAVE_MEASURE_FRAME_LENGTH to 1. It is also recommended to call
calibrateOscillator() from the reset hook in usbconfig.h:
*/
#ifndef __ASSEMBLER__
#include <avr/interrupt.h>  /* for sei() */
extern void calibrateOscillator(void);
#define USB_RESET_HOOK(resetStarts)  if(!resetStarts){cli(); calibrateOscillator(); sei();}
/*
This routine is an alternative to the continuous synchronization described
in osctune.h.