attiny13 adc to pwm

Good morning I'm beginner and needs YOUR assistance.
I have downloaded the program from below gist.github.com/adnbr/9289235 adc to pwm,and I want to convert the duration of pwm as the picture which presented YOUR,


.....................9,2m ...................0to500us
__.....................................................__
.. |________________________________| |______

here is the c code;
/* ---------------------------------------------------------------------
* PWM LED Brightness control for ATtiny13.
* Datasheet for ATtiny13: http://www.atmel.com/images/doc2535.pdf
*
* Pin configuration -
* PB1/OC0B: LED output (Pin 6)
* PB2/ADC1: Potentiometer input (Pin 7)
*
* ~100 bytes.
*
* Find out more: http://bit.ly/1eBhqHc
* -------------------------------------------------------------------*/

// 9.6 MHz, built in resonator
#define F_CPU 9600000
#define LED PB1


#include <avr/io.h>

void adc_setup (void)
{
// Set the ADC input to PB2/ADC1
ADMUX |= (1 << MUX0);
ADMUX |= (1 << ADLAR);

// Set the prescaler to clock/128 & enable ADC
// At 9.6 MHz this is 75 kHz.
// See ATtiny13 datasheet, Table 14.4.
ADCSRA |= (1 << ADPS1) | (1 << ADPS0) | (1 << ADEN);
}

int adc_read (void)
{
// Start the conversion
ADCSRA |= (1 << ADSC);

// Wait for it to finish
while (ADCSRA & (1 << ADSC));

return ADCH;
}

void pwm_setup (void)
{
// Set Timer 0 prescaler to clock/8.
// At 9.6 MHz this is 1.2 MHz.
// See ATtiny13 datasheet, Table 11.9.
TCCR0B |= (1 << CS01);

// Set to 'Fast PWM' mode
TCCR0A |= (1 << WGM01) | (1 << WGM00);

// Clear OC0B output on compare match, upwards counting.
TCCR0A |= (1 << COM0B1);
}

void pwm_write (int val)
{
OCR0B = val;
}

int main (void)
{
int adc_in;

// LED is an output.
DDRB |= (1 << LED);

adc_setup();
pwm_setup();

while (1) {
// Get the ADC value
adc_in = adc_read();
// Now write it to the PWM counter
pwm_write(adc_in);
}
}


very please to help me,use avr studio 5.0 mkll