OK here is the version for Uno or Mega. For the Mega it can control 2 fans. To set the speeds, call setTransistorFanSpeed or setDiodeFanSpeed. The range is 0 to maxFanSpeed, however in practice the minimum fan speed is about 30%.
// Definition of Arduino type
#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
#define IS_MEGA (1)
#define IS_UNO (0)
#else
#define IS_MEGA (0)
#define IS_UNO (1)
#endif
// Analog output (i.e PWM) pins. These must be chosen so that we can change the PWM frequency without affecting the millis()
// function or the MsTimer2 library. So don't use timer/counter 1 or 2. See comment in setup() function.
// THESE PIN NUMBERS MUST NOT BE CHANGED UNLESS THE CODE IN setup(), setTransistorFanSpeed() AND setDiodeFanSpeed() IS CHANGED TO MATCH!
#if IS_UNO
// On the Uno we can only use the OC1B pin, so these pin numbers are both 10
const int transistorFanPin = 10; // OC1B
const int diodeFanPin = 10; // OC1B
#else
// On the Mega we use OC1B and OC1C
const int transistorFanPin = 12; // OC1B
const int diodeFanPin = 13; // OC1C
#endif
// Definitions for PWM fan control
const unsigned char maxFanSpeed = 80; // this is calculated as 16MHz divided by 8 (prescaler), divided by 25KHz (target PWM frequency from Intel specification)
void setup()
{
// Set up the PWM pins for a PWM frequency close to the recommended 25KHz for the Intel fan spec.
// We can't get this frequency using the default TOP count of 255, so we have to use a custom TOP value.
#if IS_UNO
// Only timer/counter 1 is free because TC0 is used for system timekeeping (i.e. millis() function),
// and TC2 is used for our 1-millisecond tick. TC1 controls the PWM on Arduino pins 9 and 10.
// However, we can only get PWM on pin 10 (controlled by OCR1B) because we are using OCR1A to define the TOP value.
// Using a prescaler of 8 and a TOP value of 80 gives us a frequency of 16000/(8 * 80) = 25KHz exactly.
TCCR1A = (1 << COM1B1) | (1 << COM1B0) | (1 << WGM11) | (1 << WGM10); // OC1A (pin 9) disconnected, OC1B (pin 10) = inverted fast PWM
#ifdef FAN_AUDIO_TEST
// test code to get 440Hz output (= concert A) to test the logic
OCR1AH = 0;
OCR1BL = 71; // 50% duty cycle
TCCR1B = (1 << WGM13) | (1 << WGM12) | (1 << CS12); // TOP = OCRA, prescaler = 256
OCR1AL = 141; // TOP = 141, 16000000 / (256 * 142) = 440.014
OCR1BH = 0;
#else
OCR1AH = 0;
OCR1AL = 79; // TOP = 79
TCCR1B = (1 << WGM13) | (1 << WGM12) | (1 << CS11); // TOP = OCR0A, prescaler = 8
OCR1BH = 0;
OCR1BL = maxFanSpeed; // max fan speed (i.e. pin 5 initially low all the time)
#endif
TCNT1H = 0;
TCNT1L = 0;
#else
// On the Mega we use TC1 and OCR1B, OCR1C
TCCR1A = (1 << COM1B1) | (1 << COM1B0) | (1 << COM1C1) | (1 << COM1C1) | (1 << WGM11) | (1 << WGM10); // OC1A disconnected, OC1B = OC1C inverted fast PWM
TCCR1B = (1 << WGM13) | (1 << WGM12) | (1 << CS11); // TOP = OCR1A, prescaler = 8
TCCR1C = 0;
OCR1AH = 0;
OCR1AL = 79; // TOP = 79
OCR1BH = 0;
OCR1BL = maxFanSpeed;
OCR1CH = 0;
OCR1CL = maxFanSpeed;
TCNT1H = 0;
TCNT1L = 0;
#endif
// We have to enable the ports as outputs before PWM will work.
pinMode(transistorFanPin, OUTPUT);
pinMode(diodeFanPin, OUTPUT);
}
// Set the transistor fan speed, where 0 <= fanSpeed <= maxFanSpeed
void setTransistorFanSpeed(unsigned char fanSpeed)
{
OCR1BH = 0;
OCR1BL = fanSpeed;
}
// Set the diode fan speed, where 0 <= fanSpeed <= maxFanSpeed
void setDiodeFanSpeed(unsigned char fanSpeed)
{
#if IS_UNO
OCR1BH = 0;
OCR1BL = fanSpeed;
#else
OCR1CH = 0;
OCR1CL = fanSpeed;
#endif
}