kobol-wiki/docs/pwm.md

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!!! 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 thats 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:
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![PWM duty cycle](/img/pwm/pwm_duty_cycle_graph.png)
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## PWM Fan Implementation
### Type-A
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![Type A Curve](/img/pwm/fan_type_a_curve.jpg)
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### Type-B
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![Type B Curve](/img/pwm/fan_type_b_curve.jpg)
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### Type-C
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![Type C Curve](/img/pwm/fan_type_c_curve.jpg)
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## Helios4 Fan Control Schematic
### Board Rev 1.1
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![Helios4 Fan control Rev1.1](/img/pwm/fan_control_schematic_rev1_1.png)
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**Remarks**
| Description | Header J10 | Header J17 | Remarks |
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|-----------|---------|-----------|---------|
| 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 implemented yet |
| PWM Frequency | 25 kHz | 25 kHz | defined in device tree |
### Board Rev 1.2
![Helios4 Fan control Rev 1.2](/img/pwm/fan_control_schematic_rev1_2.png)
**Remarks**
| Description | Header J10 | Header J17 | Remarks |
|-----------|---------|-----------|---------|
| PWM pin | gpio41 | gpio55 | 5V tolerant |
| SENSE pin | gpio43 | gpio48 | SENSE pin is not implemented yet |
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| PWM Frequency | 25 kHz | 25 kHz | defined in device tree |
## Bundled Fan
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![Fan Connector](/img/pwm/fan_connector.png)
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Connector Pinout
| Pin | Function | Wire Color |
|-----|----------|------------|
| 1 | GND | Black |
| 2 | 12V | Red |
| 3 | Sense | Yellow |
| 4 | Control | Blue |
### Type-A Fan (Batch 1 & 3)
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![Type-A Fan](/img/pwm/fan_type_a_photo.jpg)
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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 | | |
![Type-A Fan Speed Graph](/img/pwm/fan_speed_graph_type_a_fan.png)
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!!! info
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Duty cycle data is converted from Linux PWM
### Type-C Fan (Batch 2)
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![Type-C Fan](/img/pwm/fan_type_c_photo.jpg)
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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 | | |
![Type-C Speed Graph](/img/pwm/fan_speed_graph_type_c_fan.png)
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!!! info
Duty cycle data is converted from Linux PWM
### Fan Speed Comparison
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![Fan Speed Graph](/img/pwm/fan_speed_comparison.png)
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## 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](http://www.marvell.com/documents/eoxwrbluvwybgxvagkkf/)) 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 2wire Interface** ([NCT75 Datasheet](https://www.onsemi.com/pub/Collateral/NCT75-D.PDF)), 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.
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![Duty Cycle Formula](/img/pwm/fan_duty_cycle_formula.png)
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Below graphs are bundled fan speed vs pwm value instead of duty cycle.
![Type-A Fan Speed Graph](/img/pwm/fan_speed_graph_type_a_fan_linux.png)
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![Type-C Fan Speed Graph](/img/pwm/fan_speed_graph_type_c_fan_linux.png)
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### 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](https://raw.githubusercontent.com/armbian/build/master/patch/kernel/mvebu-next/92-mvebu-gpio-remove-hardcoded-timer-assignment.patch) needs to be applied to kernel source to remove the restriction.
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### 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 Jul 26 07:39 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 Jul 26 07:39 hwmon1 -> ../../devices/virtual/thermal/thermal_zone0/hwmon1
lrwxrwxrwx 1 root root 0 Jul 26 07:39 hwmon2 -> ../../devices/platform/soc/soc:internal-regs/f1011000.i2c/i2c-0/0-004c/hwmon/hwmon2
lrwxrwxrwx 1 root root 0 Jul 26 07:39 hwmon3 -> ../../devices/platform/j10-pwm/hwmon/hwmon3
lrwxrwxrwx 1 root root 0 Jul 26 07:39 hwmon4 -> ../../devices/platform/j17-pwm/hwmon/hwmon4
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```
!!! info
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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](#fancontrol-automated-software-based-fan-speed-control)
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To identify which hwmon belong to fan, look for *j10-pwm* and *j17-pwm*. On above example
```
hwmon3 -> ../../devices/platform/j10-pwm/hwmon/hwmon3
hwmon4 -> ../../devices/platform/j17-pwm/hwmon/hwmon4
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```
To read current PWM
```
cat /sys/class/hwmon3/pwm1
cat /sys/class/hwmon4/pwm1
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```
To set PWM
```
echo NEW_PWM_VALUE > /sys/class/hwmon3/pwm1
echo NEW_PWM_VALUE > /sys/class/hwmon4/pwm1
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```
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### Fancontrol - automated software based fan speed control
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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.
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```
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](https://raw.githubusercontent.com/armbian/build/master/packages/bsp/helios4/90-helios4-hwmon.rules) and to be copy to **/etc/udev/rules.d/**
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*/dev/fan-j10, /dev/fan-j17, /dev/thermal-cpu, /dev/thermal-board,* and */dev/thermal-eth* are symlinks generated by the udev rules.
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#### 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 : /dev/thermal-cpu
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INTERVAL=10
FCTEMPS=/dev/fan-j10/pwm1=/dev/thermal-cpu/temp1_input /dev/fan-j17/pwm1=/dev/thermal-cpu/temp1_input
MINTEMP=/dev/fan-j10/pwm1=40 /dev/fan-j17/pwm1=40
MAXTEMP=/dev/fan-j10/pwm1=80 /dev/fan-j17/pwm1=80
MINSTART=/dev/fan-j10/pwm1=20 /dev/fan-j17/pwm1=20
MINSTOP=/dev/fan-j10/pwm1=29 /dev/fan-j17/pwm1=29
MINPWM=20
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```
INTERVAL
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This variable defines at which interval in seconds the main loop of fancontrol will be executed.
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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 (**fan-j10** & **fan-j17**) are controlled based on CPU thermal sensor (**thermal-cpu**) reading.
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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.
Type-C fan restart at 15, added 5 for safety (in case of aging fan) give us **20**. The value does not affect Type-A fan.
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MINSTOP
The minimum speed at which the fan still spins. Use a safe value here, too.
Type-C fan stopped at 24, added 5 for safety (in case of aging fan) give us **29**. The value does not affect Type-A fan.
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-----
*Following settings can be adjusted by user to tweak further.*
MINTEMP
The temperature below which the fan gets switched to minimum speed.
Fans (fan-j10 & fan-j17) runs in minimum speed if the CPU temperature below **40** degree C.
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MAXTEMP
The temperature over which the fan gets switched to maximum speed.
Fans (fan-j10 & fan-j17) runs in maximum speed if the CPU temperature above **80** degree C.
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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 Type-C fan, it would stopped the fan while on Type-A fan it would run in minimal speed.
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!!! note
The Helios4 fancontrol configuration file can be found [here](https://raw.githubusercontent.com/armbian/build/master/packages/bsp/helios4/fancontrol_pwm-fan.conf).
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### Thermal Zone on Device Tree
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As an alternative to userspace tool like [fancontrol](#fancontrol-automated-software-based-fan-speed-control), Linux Kernel provides Thermal Framework to do thermal management.
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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](#cpu-thermal-sensor)) does not support thermal-zone binding in device tree, therefore it can not be used as thermal-sensor yet.
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```
/ {
...
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 = <40000>;
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hysteresis = <2000>;
type = "active";
};
cpu_alert: cpu_alert {
/* millicelsius */
temperature = <80000>;
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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](https://en.wikipedia.org/wiki/Pulse-width_modulation)
[4-Wire Pulse Width Modulation (PWM) Controlled Fans Specification rev. 1.3](/files/fan/4_Wire_PWM_Spec.pdf)
[fancontrol man page](https://linux.die.net/man/8/fancontrol)
[Linux Thermal Framework Device Tree descriptor](https://www.kernel.org/doc/Documentation/devicetree/bindings/thermal/thermal.txt)