/*
Disclaimer:
Use it as it is, completely and entirely at YOUR OWN RISK.
YOU CAN DAMAGE YOUR HARDWARE WITHOUT PROPER COOLING, LEAVE THE THERMAL SHUTDOWN ON!

Generic CPU fan tachometer signal emulator for 4 wire pwm fans.

Tuned for Delta AFB1212SH, 0 - 3400 RPM (found in HP Microserver Gen 8)

The fan will send 2 pulses per revolution, with a duration of 1/4 rev.

->  1 RPM  -> 1 rotation / 60 seconds -> 1 Hz
2 pulses per rotation -> 2 Hz signal freq. -> 500 miliseconds per pulse ( 250ms UP and 250ms DOWN )

-> 3400 RPN -> 3400 rotations / 60 seconds -> ~56.7 Hz
2 pulses per rotation -> 2x56.7 -> ~113.4 Hz -> ~8.8 miliseconds per pulse (4.4ms UP and 4.4ms DOWN )


-> 1000 RPM -> 1000 rotations / 60 seconds -> 16.7 Hz
4 pulses per rotation -> 4x16.7 -> ~66.7 Hz -> ~15 milliseconds per pulse(7.5 UP and 7.5 DOWN)

-> 14000RPM -> 14000 rotations / 60 seconds -> ~233.3 Hz
4 pulses per rotation -> 4x233.3 -> ~933.2 Hz -> 1.1 milliseconds per pulse( 0.55 UP and 0.55 DOWN )

After computing the above ( min, max rpm ) put them in the voltage to pulse duration mapping in the loop() function

pFreq = map(Voltage, 0, 1023, 500.0, 8.8235);

Volatage = voltage read from the motherboard pwm driving pin by the ADC
(10 bit resolution, means 0-5V will return 0-1023)

Voltage range - 0, 1023
Ms pulse range - 500.0 - 8.8235

Hardware:
Tested on Arduino Pro Mini 5V/16Mhz - ( ~10$)
Programmer - FTDI Basic Breakout - 5V - ( ~15$)

Software:
Adruino IDE 1.0.5-r2
Arduino Timer Library - http://playground.arduino.cc/Code/Timer#.UxdUPfS1a7M

Pin A0 connected to the motherboard PWM pin
Pin 3 connected to the tach. signal on the motherboard
GND to GNDs (see fan connector gnd loop)




*/


// Timer lib
#include "Timer.h";

// Timer
Timer t;
// Pulse output pin
int pPin = 3;
// Pulse freq.
float pFreq;
// Voltage measure pin
int iPin = A0;
// Voltage
int Voltage;
// Led pin - alive blink
int ledPin = 13;


void setup()
{
  //Initial config

  //IO Pins
  pinMode(ledPin, OUTPUT);
  pinMode(pPin, OUTPUT);
  digitalWrite(pPin, LOW);
  pinMode(iPin, INPUT);

  //Timers
  t.every(pFreq, sendPulse);
  t.every(100, readIV);
  t.oscillate(ledPin,1500,HIGH);
}

void loop()
{
  // Map V to Pulse Freq
  //pFreq = map(Voltage, 0, 1023, 500.0, 8.8235);
  pFreq = map(Voltage, 0, 1023, 16.7, 1.1);
  // Update timers
  t.update();
}

void sendPulse()
{
    // Half period on
    digitalWrite(pPin, HIGH);
    delay(pFreq/2);

    // Half period off
    digitalWrite(pPin, LOW);
    delay(pFreq/2);
}

void readIV()
{
  //Analog voltage read - little bit imprecise but works
  Voltage =  analogRead(iPin);
}