Control the speed of a PC fan by PWM

1int icr, frecuency, duty_cycle;
2void setup() {
3  Serial.begin(9600);
4  pinMode(11, OUTPUT);
5  
6  // Timer/Counter 1 initialization
7  TCCR1A = 0xA2;
8  TCCR1B = 0x19;
9  TCNT1H = 0x00;
10  TCNT1L = 0x00;
11
12  duty_cycle = 20; // Change this value to the desired duty cycle (0-100 %)
13  icr = 639; // icr and frecuency values are set to generate a 25 kHZ signal
14  frecuency = 16000000 / (1 + icr);
15
16  ICR1H = icr >> 8;
17  ICR1L = icr & 0x00ff;
18  // Sets the duty cycle with the value entered in line 12 
19  OCR1A = icr * (duty_cycle / 100.0);
20}
21
22void loop() {
23  // Prints the current duty cycle on the serial monitor every second
24  Serial.print(" Duty cycle (%) = "); Serial.println(duty_cycle);
25  delay(1000);
26}