Some progress:
I'm able to fake the CPU fans to be detected by using an Arduino and writing a 50% duty cycle 25Hz PWM signal to the TACH pin (pin 3) on the motherboard CPU fan connector.
The code for doing this is simply:
-code-
int pwmPin = 3; // e.g.
void setup()
{
setPwmFrequency(3, 1250); // 31250/1250 = 25Hz
}
void loop()
{
analogWrite(pwmPin, 127); // 127/255 ≈ 50% duty cycle, which is a duty cycle tested to work.
}
/**
* Divides a given PWM pin frequency by a divisor.
*
* The resulting frequency is equal to the base frequency divided by
* the given divisor:
* - Base frequencies:
* o The base frequency for pins 3, 9, 10, and 11 is 31250 Hz.
* o The base frequency for pins 5 and 6 is 62500 Hz.
* - Divisors:
* o The divisors available on pins 5, 6, 9 and 10 are: 1, 8, 64,
* 256, and 1024.
* o The divisors available on pins 3 and 11 are: 1, 8, 32, 64,
* 128, 256, and 1024.
*
* PWM frequencies are tied together in pairs of pins. If one in a
* pair is changed, the other is also changed to match:
* - Pins 5 and 6 are paired on timer0
* - Pins 9 and 10 are paired on timer1
* - Pins 3 and 11 are paired on timer2
*
* Note that this function will have side effects on anything else
* that uses timers:
* - Changes on pins 3, 5, 6, or 11 may cause the delay() and
* millis() functions to stop working. Other timing-related
* functions may also be affected.
* - Changes on pins 9 or 10 will cause the Servo library to function
* incorrectly.
*
* Thanks to macegr of the Arduino forums for his documentation of the
* PWM frequency divisors. His post can be viewed at:
*
http://forum.arduino.cc/index.php?topic=16612#msg121031
*/
void setPwmFrequency(int pin, int divisor) {
byte mode;
if(pin == 5 || pin == 6 || pin == 9 || pin == 10) {
switch(divisor) {
case 1: mode = 0x01; break;
case 8: mode = 0x02; break;
case 64: mode = 0x03; break;
case 256: mode = 0x04; break;
case 1024: mode = 0x05; break;
default: return;
}
if(pin == 5 || pin == 6) {
TCCR0B = TCCR0B & 0b11111000 | mode;
} else {
TCCR1B = TCCR1B & 0b11111000 | mode;
}
} else if(pin == 3 || pin == 11) {
switch(divisor) {
case 1: mode = 0x01; break;
case 8: mode = 0x02; break;
case 32: mode = 0x03; break;
case 64: mode = 0x04; break;
case 128: mode = 0x05; break;
case 256: mode = 0x06; break;
case 1024: mode = 0x07; break;
default: return;
}
TCCR2B = TCCR2B & 0b11111000 | mode;
}
}
-code-
The same logic doesn't seem to work on the memory fan though (I might try to make better measurements). I've tried different duty cycles, but the memory fan connector doesn't seem to be able to be tricked or then the values I'm giving it are still wrong.