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I am working a PIC µC project. I am using 12VDC PC fan (Coolermaster) for my application. A part of the project is to return the RPM from the third wire of the fan to an LCD.
First of all, How can I read the signal? Is it a sort of pulses every rotation? Also, Do I need to use an ADC pin or just a Digital one would suffice? I found lots of diagrams telling to connect a pull up resistor to +5V. Why?
A sample code of how this works will be helpful.
Thanks.
Reading the signal
The main point to notice is that the tach output from the 3 wire fan switches from 0 to 12V. If connected directly to the input of the PIC it would exceed the voltage rating of the device. Connecting the tach output to a 5V supply through a resistor would not prevent it from exceeding 5V as that resistor would simply form a potential divider with R1 and leave the tach output sitting at a voltage between 12 and 5 volts. A quick check with an oscilloscope or voltmeter would verify this voltage.
To reduce the output to a 0 - 5V pulse suitable for the PIC input you could:
(i) Use a resistor (R2) across the output (a slightly smaller value than R1) to give a maximum 5V at the output (quick and dirty solution) ( R2 should be about 0.7 R1 to give a 5V output)
(ii) Connect a small signal npn transistor - base to the tach output. Connect a pull up resistor from its collector to 5V. Emitter to ground. Take the new tach (inverted) output from the collector to the PIC input line.
(iii) Connect an opto isolator with a pull up resistor to the 5V line (pin 2, 4 to ground (0V) , pull up resistor from 5V to pin 5. Take output from pin 5. Pin 6 is left unconnected) This circuit will invert the pulse signal but as you are counting pulses it does not matter. It also protects the PIC input from exceeding 5V.
The safest way is to force the fan tacho output to behave only as open-collector by adding a diode and then using your own pullup:
simulate this circuit – Schematic created using CircuitLab
That way it doesn't matter if the fan has a pullup resistor or what its supply voltage is, it cannot pull your MCU pin too high. You can use the "weak pullup" functionality on your MCU pins if it supports them, or add an external resistor as pictured.
Computer fans usually produce 2 pulses per revolution but there are variations. Even a fairly fast fan (6000RPM) is only 100Hz, which means pulses at 200Hz. The usual way to measure speed is to use an "input capture" feature on a timer, i.e. leave a free-running timer going fairly fast (tens of kHz) configured to capture the timer value on the falling edge of the input, then subtract consecutive captured values in order to measure the time between pulses. Don't forget you also need to catch the timer-overflow interrupt and figure that into your period calculations.
Here's a specification for 4-wire (PWM controlled) fans from formfactors.org "brought to you by Intel": http://www.formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdf
Tachometer Output Signal Fan shall provide tachometer output signal with the following characteristics:
- Two pulses per revolution
- Open-collector or open-drain type output
- Motherboard will have a pull up to 12V, maximum 12.6V
Note that the pull up is supposed to be on the motherboard, i.e. not on the fan.
Typically, the third (tach) wire is grounded "n" times per revolution, where "n" varies between fan models, but typically 2 or 4 times per revolution. (The data sheet fro the fan should specify this.)
You will need to add a pull-up resistor to give you a square wave signal which you can feed into a digital input. You will need to measure the period of the square wave or count transitions per second to compute the RPM.
For example, assume 6000RPM and 4 pulses per revolution, you will see 400 pulses per second and the average period of a pulse (leading-edge to leading-edge) would be 2.5ms (1/400).
As for actually reading the pin, the easiest way would be to set up an interrupt on a transition (either up or down) and record the value of a high resolution timer in a buffer, which you can read in your main line to compute average period over several cycles. Alternatively, you can loop pooling the pin looking for changes and record the timer values.
Edit: Jim Deardan correctly notes that on most fans, the tach wire is already pulled up to +12V and the voltage needs to be reduced before connecting to an input on the PIC. Please read his comment.
Elaborating on DoxyLover's answer. You can use the Capture/Compare/PWM (CCP) module for this. Basically the capture mode does exactly what DoxyLover described, it records the value of a timer based on a programmable event on a pin (in this case, you probably want it to record the falling edge). So you can have the associated timer run, have the module record the timer value on the falling edge, and then read the value of that capture register at your leisure. This way you don't have to have an interrupt for every time the fan contact is pulsed, and also the result is not dependent on things such as interrupt latency.
