PWM (Fan)

Warning

Always POWER OFF the system before plug/unplug the fan. Failed to do so could damage the controlling GPIO due to power surge.

PWM Introduction

PWM, or pulse width modulation is a technique which allows us to adjust the average value of the voltage that’s going to the electronic device by varying duty cycle of the power at a fast rate.

The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on. When a digital signal is on half of the time and off the other half of the time, the digital signal has a duty cycle of 50% and resembles a "square" wave. When a digital signal spends more time in the on state than the off state, it has a duty cycle of >50%. When a digital signal spends more time in the off state than the on state, it has a duty cycle of <50%. Here is a pictorial that illustrates these three scenarios:

PWM duty cycle

PWM Fan Implementation

Type A

Type A Curve

Type B

Type B Curve

Type C

Type C Curve

Helios4 Fan Control Schematic

Helios4 Fan control

Description Connector J10 J17 Remarks
PWM pin gpio41 gpio55 3.3V pull up fan ONLY! Early generation of 4-wire pwm fan may use 5V pull-up
SENSE pin gpio43 gpio48 SENSE pin is not used yet
PWM Frequency 25 kHz 25 kHz defined in device tree

Bundled Fan

Fan Connector

Connector Pinout

Pin Function Wire Color
1 GND Black
2 12V Red
3 Sense Yellow
4 Control Blue

Old Fan (Batch 1)

Old Fan

Fan Specification

Parameter Value Unit Remarks
Maximum Speed 4200 RPM @ duty cycle 98%
Minimum Speed 1200 RPM @ duty cycle 24%
Shut off No Not Supported
Implementation Type A

Old Fan Speed Graph

Info

Duty cycle data is converted from Linux PWM

New Fan (Batch 2)

New Fan

Fan Specification

Parameter Value Unit Remarks
Maximum Speed 4200 RPM @ duty cycle 98%
Minimum Speed 400 RPM @ duty cycle 10%
Shut off Yes duty cycle <= 5.5% and restart @ duty cycle > 9%
Implementation Type C

New Fan Speed Graph

Info

Duty cycle data is converted from Linux PWM

Fan Speed Comparison

Fan Speed Graph

Helios4 Temperature Sensors

CPU Thermal Sensor

Armada 388 incorporates a Thermal Management engine for monitoring die temperature. It includes an on-die analog-to-digital thermal sensor, that is used to determine when the maximum specified processor junction temperature has been reached.

Ethernet PHY Thermal Sensor

Helios4's 10/100/1000 BASE-T PHY Tranceiver (Marvell 88E1512 Datasheet) features an internal temperature sensor. The sensor reports the die temperature and is updated approximately once per second.

Board Temp Sensor

Helios4 has a Digital Temperature Sensor with 2‐wire Interface (NCT75 Datasheet), located on bottom side of the board. It is used to read ambient temperature.

PWM Fan Control under Linux

Linux use 8-bit integer to represent duty cycle. PWM value 0 represent 0% duty cycle and PWM value 255 represent 100% duty cycle.

Duty Cycle Formula

Below graphs are bundled fan speed vs pwm value instead of duty cycle.

Old Fan Speed Graph

New Fan Speed Graph

Patch requirement

Currently Linux gpio-mvebu driver does not allow more than 1 PWM under the same gpio bank. Helios4 uses 2 PWM under same bank therefore this patch needs to be applied to kernel source to remove the restriction.

Using SYSFS interface

Linux export the fan control mechanism to SYSFS under hwmon class. List of devices can be checked under /sys/class/hwmon

ls -l /sys/class/hwmon/
total 0
lrwxrwxrwx 1 root root 0 Nov  7 07:23 hwmon0 -> ../../devices/platform/soc/soc:internal-regs/f1072004.mdio/mdio_bus/f1072004.mdio-mii/f1072004.mdio-mii:00/hwmon/hwmon0
lrwxrwxrwx 1 root root 0 Nov  7 07:23 hwmon1 -> ../../devices/virtual/hwmon/hwmon1
lrwxrwxrwx 1 root root 0 Nov  7 07:23 hwmon2 -> ../../devices/platform/j10-pwm/hwmon/hwmon2
lrwxrwxrwx 1 root root 0 Nov  7 07:23 hwmon3 -> ../../devices/platform/j17-pwm/hwmon/hwmon3
lrwxrwxrwx 1 root root 0 Nov  7 07:23 hwmon4 -> ../../devices/platform/soc/soc:internal-regs/f1011000.i2c/i2c-0/0-004c/hwmon/hwmon4

Info

The numbering may different from above example output. It depends on whether the driver built as kernel module or built-in, device initialization order. Take this as consideration when using fancontrol

To identify which hwmon belong to fan, look for j10-pwm and j17-pwm. On above example

hwmon2 -> ../../devices/platform/j10-pwm/hwmon/hwmon2
hwmon3 -> ../../devices/platform/j17-pwm/hwmon/hwmon3

To read current PWM

cat /sys/class/hwmon2/pwm1
cat /sys/class/hwmon3/pwm1

To set PWM

echo NEW_PWM_VALUE > /sys/class/hwmon2/pwm1
echo NEW_PWM_VALUE > /sys/class/hwmon3/pwm1

Fancontrol - automated software based fan speed control

fancontrol is a shell script for use with lm_sensors. It reads its configuration from a file, then calculates fan speeds from temperatures and sets the corresponding PWM outputs to the computed values.

sudo apt-get install fancontrol

fancontrol includes pwmconfig script to create automatically a configuration file but it can not be used for Helios4.

UDEV rules

Since hwmon order can be changed between kernel version or even between reboot, on Armbian we use udev rules as workaround. The rules can be found from here and to be copy to /etc/udev/rules.d/

Configuration File

fancontrol uses /etc/fancontrol as configuration file. Below is an example configuration to control fan speed on Helios4.

# Helios4 PWM Fan Control Configuration
# Temp source : armada_thermal sensor
INTERVAL=10
DEVPATH=hwmon2=devices/platform/j10-pwm hwmon3=devices/platform/j17-pwm hwmon4=devices/platform/soc/soc:internal-regs/f1011000.i2c/i2c-0/0-004c
DEVNAME=hwmon1=armada_thermal
FCTEMPS=hwmon2/pwm1=hwmon1/temp1_input hwmon3/pwm1=hwmon1/temp1_input
MINTEMP=hwmon2/pwm1=60 hwmon3/pwm1=60
MAXTEMP=hwmon2/pwm1=80 hwmon3/pwm1=80
MINSTART=hwmon2/pwm1=20 hwmon3/pwm1=20
MINSTOP=hwmon2/pwm1=29 hwmon3/pwm1=29
MINPWM=20

INTERVAL

This variable defines at which interval in seconds the main loop of fancontrol will be executed.

DEVPATH

Maps hwmon class devices to physical devices. This lets fancontrol check that the configuration file is still up-to-date.

It expect the hwmonN as symlink to devices/*

DEVNAME

Records hwmon class device names. This lets fancontrol check that the configuration file is still up-to-date.

Since armada_thermal does not create symlink, use DEVNAME instead of DEVPATH

FCTEMPS

Maps PWM outputs to temperature sensors so fancontrol knows which temperature sensors should be used for calculation of new values for the corresponding PWM outputs.

Fans (hwmon2 & hwmon3) are controlled based on CPU thermal sensor (hwmon1) reading.

MINSTART

Sets the minimum speed at which the fan begins spinning. You should use a safe value to be sure it works, even when the fan gets old.

New bundled fan restart at 15, added 5 for safety (in case of aging fan) give us 20. The value does not affect old bundle fan.

MINSTOP

The minimum speed at which the fan still spins. Use a safe value here, too.

New bundled fan stopped at 24, added 5 for safety (in case of aging fan) give us 29. The value does not affect old bundle fan.


Following settings can be adjusted by user to tweak further.

MINTEMP

The temperature below which the fan gets switched to minimum speed.

Fans (hwmon2 & hwmon3) runs in minimum speed if the CPU temperature below 70 degree C.

MAXTEMP

The temperature over which the fan gets switched to maximum speed.

Fans (hwmon2 & hwmon3) runs in maximum speed if the CPU temperature above 90 degree C.

MINPWM

The PWM value to use when the temperature is below MINTEMP. Typically, this will be either 0 if it is OK for the fan to plain stop, or the same value as MINSTOP if you don't want the fan to ever stop. If this value isn't defined, it defaults to 0 (stopped fan).

Set minimum PWM value to 0. On new bundled fan, it would stopped the fan while on old bundled fan it would run in minimal speed.

Note

The Helios4 fancontrol configuration file can be found here.

Thermal Zone on Device Tree

As an alternative to userspace tool like fancontrol, Linux Kernel provides Thermal Framework to do thermal management.

Below is an example of device tree nodes that can be added to Helios4 device tree to make use of Linux Thermal Framework.

Note

Currently armada_thermal driver (CPU Thermal Sensor) does not support thermal-zone binding in device tree, therefore it can not be used as thermal-sensor yet.

/ {
   ...

    fan1: j10-pwm {
        compatible = "pwm-fan";
        pwms = <&gpio1 9 40000>;    /* Target freq:25 kHz */
        cooling-min-state = <0>;
        cooling-max-state = <3>;
        #cooling-cells = <2>;
        cooling-levels = <0 25 128 255>;
    };

    fan2: j17-pwm {
        compatible = "pwm-fan";
        pwms = <&gpio1 23 40000>;   /* Target freq:25 kHz */
        cooling-min-state = <0>;
        cooling-max-state = <3>;
        #cooling-cells = <2>;
        cooling-levels = <0 25 128 255>;
    };

    thermal-zones {
        microsom_thermal: microsom-thermal {
            thermal-sensors = <&thermal>;
            polling-delay-passive = <250>; /* milliseconds */
            polling-delay = <500>; /* milliseconds */
            trips {
                cpu_active: cpu_active {
                    /* millicelsius */
                    temperature = <70000>;
                    hysteresis = <2000>;
                    type = "active";
                };

                cpu_alert: cpu_alert {
                    /* millicelsius */
                    temperature = <90000>;
                    hysteresis = <2000>;
                    type = "hot";
                };

                cpu_crit: cpu-crit {
                    /* millicelsius */
                    temperature = <115000>;
                    hysteresis = <5000>;
                    type = "critical";
                };
            };
        };

        board_thermal: board-thermal {
            thermal-sensors = <&temp_sensor>;
            polling-delay-passive = <0>; /* milliseconds */
            polling-delay = <1500>; /* milliseconds */
            trips {
                board_active: board-active {
                    /* millicelsius */
                    temperature = <40000>;
                    hysteresis = <2000>;
                    type = "active";
                };

                board_alert: board-alert {
                    /* millicelsius */
                    temperature = <60000>;
                    hysteresis = <2000>;
                    type = "hot";
                };

                board_critical: board-critical {
                    /* millicelsius */
                    temperature = <70000>;
                    hysteresis = <2000>;
                    type = "critical";
                };
            };

            cooling-maps {
                map0 {
                    trip = <&board_active>;
                    cooling-device = <&fan1 THERMAL_NO_LIMIT 2>,
                             <&fan2 THERMAL_NO_LIMIT 2>;
                };
                map1 {
                    trip = <&board_alert>;
                    cooling-device = <&fan1 2 THERMAL_NO_LIMIT>,
                             <&fan2 2 THERMAL_NO_LIMIT>;
                };
            };
        };
    };
    ...
};

&temp_sensor {
    #thermal-sensor-cells = <0>;
};

References

Pulse-width modulation

4-Wire Pulse Width Modulation (PWM) Controlled Fans Specification rev. 1.3

fancontrol man page

Linux Thermal Framework Device Tree descriptor