Code:

#include <avr/io.h>
#include <avr/wdt.h>
#include <avr/eeprom.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <util/delay.h>
#include <stdlib.h>

#include "usbdrv.h"
#include "oddebug.h"

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

static uchar    reportBuffer[2];    /* buffer for HID reports */
static uchar    idleRate;           /* in 4 ms units */
static uchar    reportCount;      /* current report */
static unsigned int   TimerDelay;      /* counter for delay period */

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

PROGMEM char usbHidReportDescriptor[USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH] = { /* USB report descriptor */
    0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)
    0x09, 0x06,                    // USAGE (Keyboard)
    0xa1, 0x01,                    // COLLECTION (Application)
    0x05, 0x07,                    //   USAGE_PAGE (Keyboard)
    0x19, 0xe0,                    //   USAGE_MINIMUM (Keyboard LeftControl)
    0x29, 0xe7,                    //   USAGE_MAXIMUM (Keyboard Right GUI)
    0x15, 0x00,                    //   LOGICAL_MINIMUM (0)
    0x25, 0x01,                    //   LOGICAL_MAXIMUM (1)
    0x75, 0x01,                    //   REPORT_SIZE (1)
    0x95, 0x08,                    //   REPORT_COUNT (8)
    0x81, 0x02,                    //   INPUT (Data,Var,Abs)
    0x95, 0x01,                    //   REPORT_COUNT (1)
    0x75, 0x08,                    //   REPORT_SIZE (8)
    0x25, 0x65,                    //   LOGICAL_MAXIMUM (101)
    0x19, 0x00,                    //   USAGE_MINIMUM (Reserved (no event indicated))
    0x29, 0x65,                    //   USAGE_MAXIMUM (Keyboard Application)
    0x81, 0x00,                    //   INPUT (Data,Ary,Abs)
    0xc0                           // END_COLLECTION
};

static void buildReport(void)
{
uchar   key = 0;


    if(reportCount == 0){
        key = 0x39;
    }

   reportCount++;
    reportBuffer[0] = 0;    /* no modifiers */
    reportBuffer[1] = key;
}

static void timerPoll(void)
{
   static unsigned int timerCnt;

    if(TIFR & (1 << TOV1)){
        TIFR = (1 << TOV1); /* clear overflow */
        if(++timerCnt >= TimerDelay){       /* check for end of pseudorandom delay */
            
         TimerDelay = 2835 + rand();      /* 1/63s * 63 * 30 + 0...32767 */
         timerCnt = 0;
         reportCount = 0; /* start report */
        }
    }
}

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

static void timerInit(void)
{
    TCCR1 = 0x0b;           /* select clock: 16.5M/1k -> overflow rate = 16.5M/256k = 62.94 Hz */
}


/* ------------------------------------------------------------------------- */
/* ------------------------ interface to USB driver ------------------------ */
/* ------------------------------------------------------------------------- */

uchar   usbFunctionSetup(uchar data[8])
{
usbRequest_t    *rq = (void *)data;

    usbMsgPtr = reportBuffer;
    if((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS){    /* class request type */
        if(rq->bRequest == USBRQ_HID_GET_REPORT){  /* wValue: ReportType (highbyte), ReportID (lowbyte) */
            /* we only have one report type, so don't look at wValue */
            buildReport();
            return sizeof(reportBuffer);
        }else if(rq->bRequest == USBRQ_HID_GET_IDLE){
            usbMsgPtr = &idleRate;
            return 1;
        }else if(rq->bRequest == USBRQ_HID_SET_IDLE){
            idleRate = rq->wValue.bytes[1];
        }
    }else{
        /* no vendor specific requests implemented */
    }
   return 0;
}

/* ------------------------------------------------------------------------- */
/* --------------------------------- main ---------------------------------- */
/* ------------------------------------------------------------------------- */

int main(void)
{
uchar   i;

    odDebugInit();
    usbDeviceDisconnect();
    for(i=0;i<20;i++){  /* 300 ms disconnect */
        _delay_ms(15);
    }
    usbDeviceConnect();

    wdt_enable(WDTO_1S);
    timerInit();
   TimerDelay = 630; /* initial 10 second delay */

    usbInit();
    sei();
    for(;;){    /* main event loop */
        wdt_reset();
        usbPoll();

      /* A USB keypress cycle is defined as a scancode being present in a report, and
      then absent from a later report. To press and release the Caps Lock key, instead of
      holding it down, we need to send the report with the Caps Lock scancode and
      then an empty report. */

        if(usbInterruptIsReady() && reportCount < 2){ /* we can send another key */
            buildReport();
            usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
           }
        timerPoll();
    }
    return 0;
}

Code:

    TCCR1 = 0x0b;           /* select clock: 16.5M/1k -> overflow rate = 16.5M/256k = 62.94 Hz */

Code:

static void timerInit(void)
{
    TCCR1 = 0x0b;           /* select clock: 16.5M/1k -> overflow rate = 16.5M/256k = 62.94 Hz */
}

/* ------------------------ interface to USB driver ------------------------ */

uchar   usbFunctionSetup(uchar data[8])
{
usbRequest_t    *rq = (void *)data;

    usbMsgPtr = reportBuffer;
    if((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS){    /* class request type */
        if(rq->bRequest == USBRQ_HID_GET_REPORT){  /* wValue: ReportType (highbyte), ReportID (lowbyte) */
            /* we only have one report type, so don't look at wValue */
            buildReport();
            return sizeof(reportBuffer);
        }else if(rq->bRequest == USBRQ_HID_GET_IDLE){
            usbMsgPtr = &idleRate;
            return 1;
        }else if(rq->bRequest == USBRQ_HID_SET_IDLE){
            idleRate = rq->wValue.bytes[1];
        }
    }else{
        /* no vendor specific requests implemented */
    }
   return 0;
}

/* ------------------------ Oscillator Calibration ------------------------- */

/* Calibrate the RC oscillator to 8.25 MHz. The core clock of 16.5 MHz is
 * derived from the 66 MHz peripheral clock by dividing. Our timing reference
 * is the Start Of Frame signal (a single SE0 bit) available immediately after
 * a USB RESET. We first do a binary search for the OSCCAL value and then
 * optimize this value with a neighboorhod search.
 * This algorithm may also be used to calibrate the RC oscillator directly to
 * 12 MHz (no PLL involved, can therefore be used on almost ALL AVRs), but this
 * is wide outside the spec for the OSCCAL value and the required precision for
 * the 12 MHz clock! Use the RC oscillator calibrated to 12 MHz for
 * experimental purposes only!
 */
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;
}
/*
Note: This calibration algorithm may try OSCCAL values of up to 192 even if
the optimum value is far below 192. It may therefore exceed the allowed clock
frequency of the CPU in low voltage designs!
You may replace this search algorithm with any other algorithm you like if
you have additional constraints such as a maximum CPU clock.
For version 5.x RC oscillators (those with a split range of 2x128 steps, e.g.
ATTiny25, ATTiny45, ATTiny85), it may be useful to search for the optimum in
both regions.
*/

void    usbEventResetReady(void)
{
    calibrateOscillator();
    eeprom_write_byte(0, OSCCAL);   /* store the calibrated value in EEPROM */
}

/* ------------------------------------------------------------------------- */
/* --------------------------------- main ---------------------------------- */
/* ------------------------------------------------------------------------- */

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

    calibrationValue = eeprom_read_byte(0); /* calibration value from last time */
    if(calibrationValue != 0xff){
        OSCCAL = calibrationValue;
    }
    odDebugInit();
    usbDeviceDisconnect();
    for(i=0;i<20;i++){  /* 300 ms disconnect */
        _delay_ms(15);
    }
    usbDeviceConnect();

    wdt_enable(WDTO_1S);
    timerInit();
   TimerDelay = 630; /* initial 10 second delay */

    usbInit();
    sei();
    for(;;){    /* main event loop */
        wdt_reset();
        usbPoll();

      /* A USB keypress cycle is defined as a scancode being present in a report, and
      then absent from a later report. To press and release the Caps Lock key, instead of
      holding it down, we need to send the report with the Caps Lock scancode and
      then an empty report. */

        if(usbInterruptIsReady() && reportCount < 2){ /* we can send another key */
            buildReport();
            usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
           }
        timerPoll();
    }
    return 0;
}