cm4iofan

cm4iofan

Simple utility to get/set the PWM duty cycle and to measure the RPM for a fan connected to the 4-pin header on the CM4IO.

Requirements

• Enabled i2c_vc overlay (dtparam=i2c_vc=on in /boot/config.txt)
• Loaded i2c-dev module (e.g. modprobe i2c-dev or using /etc/modules)
• A user in the i2c group (or root)

Using the Go Module

package main

import "github.com/tmsmr/cm4iofan"

ctrl, err := cm4iofan.New()
if err != nil {
	panic(err)
}
err = ctrl.SetDutyCycle(50)
if err != nil {
	panic(err)
}

Using fanctl

• Build with cd fanctl && go build.
• For every tagged version of the Go Module, the fanctl utility is built (Available on the Releases page).

$ fanctl set 50
$ fanctl get
50
$ fanctl rpm
2623

Comments

Direct Setting mode vs. Fan Speed Control mode

The EMC2301 has a built-in closed loop Fan Speed Control algorithm (Besides many awesome features), that allows the user to select a target RPM which is controlled by a PID. I opted for the Direct Setting mode, because it feels more natural to me (Yes, i know that duty cycle/RPM is far from linear...).

Should i use this for my 24/7-running project?

If you want a reliable/automatic cooling solution, you should probably go with something like https://github.com/neg2led/cm4io-fan.

There are scenarios where this Module might be handy (Well, that's why i built it...). But keep in mind, all you do with it, is setting the fan's PWM signal:

• Some fans won't stop with 0% duty cycle.
• Most fans will stall with a low duty cycle.
• ...

TACH to RPM conversion

Unfortunately it is not possible to detect RPM's lower than 500 using the EMC2301. That's why the RPM measurement returns a RPMResult:

type RPMResult struct {
    Rpm int
    Stopped bool
    Undef bool
}

• When the PWM duty cycle is set to 0%, Stopped is true. This is only true for fans that are able to stop completely!
• When the calculated RPM is below 500 (smallest value could be 480), it is not possible to determine the real RPM. In this case Undef is true.

On poles, edges and ranges...

The EMC2301 may be used to control a wide variety of PWM fans. TACH measurement works different based on the fan's design type (emc2301.pdf: 4.4 Tachometer Measurement).

Noctua's PWM specification states that (regardless of the actual motor design) the tachometer always pulses two times per revolution (Emulating a fan with 2 poles). I think this is true for other regular, modern PWM fans. That's why i settled with the following values for the RPM calculation:

• poles = 2
• n = 5

Since i wanted to measure RPM's as low as possible, RANGE[1:0] is set to 500 RPM. This means the TACH multiplier is m = 1.

Using these values, a simplified formula for TACH conversion can be used (emc2301.pdf: EQUATION 4-3).