I'm brand new to the Arduino and I have an UNO. I'm trying to control a 4-pin CPU fan using PWM at 25khz and then read back the fan speed using a 10kohm pullup resistor. However, the fan speed I'm reading back is not exactly what I'd expect based on the fan specs.
At 20% duty cycle the fan is supposed to run at 2,600rpm and at 100% duty cycle the fan is supposed to run at 11,500rpm. At 20% duty cycle I'm reading about 3360rpm and at 100% duty cycle I'm reading about 11,900rpm.
I'm trying to decide if the difference between the rated speeds and the speeds I'm reading back is reasonable and can be attributed to variations from fan to fan and/or somewhat unstable PWM control using the PWM.h library or if my code has some problems. Since this is my first project with the arduino I was hoping someone could take a look at my code and give me some feedback, any help would be greatly appreciated.
//fan speed sensor wire attached to digital pin 2 with a 10kohm pullup resistor
//fan PWM control wire attached directly to digital pin 9
#include <PWM.h> //include PWM library http://forum.arduino.cc/index.php?topic=117425.0
volatile int half_revolutions; //allow half_revolutioins to be accesed in intterupt
int rpm; //set rpm as an integer
void setup()
{
InitTimersSafe(); //not sure what this is for, but I think i need it for PWM control?
bool success = SetPinFrequencySafe(9, 25000); //set frequency to 25kHz
pwmWrite(9, 51); // 51=20% duty cycle, 255=100% duty cycle
pinMode(2,INPUT); //set RPM pin to digital input
half_revolutions = 0;
rpm = 0;
Serial.begin(9600);
}
void loop()
{
sei(); //enable intterupts
attachInterrupt(digitalPinToInterrupt(2), fan_rpm, RISING); //record pulses as they rise
delay(10000);
detachInterrupt(digitalPinToInterrupt(2));
cli(); //disable intterupts
rpm = (half_revolutions/2)*6;
Serial.print("SPEED: ");
Serial.print(rpm);
Serial.println("rpm");
Serial.println();
rpm = 0;
half_revolutions = 0;
}
void fan_rpm()
{
++half_revolutions; //increment before returning value
}
The following image is basically how I've connected everything, with the addition of a brown PWM control wire coming out of the fan and going straight into digital pin 9.
