161 lines
6.4 KiB
Markdown
161 lines
6.4 KiB
Markdown
## PWM Introduction
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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.
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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 Fan Implementation
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### Type-A
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### Type-B
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### Type-C
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## PWM Fan Schematic
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**Remarks**
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| Description | Header P7 | Header P6 | Remarks |
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|-----------|---------|-----------|---------|
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| PWM pin | PWM0 | PWM1 | 5V tolerant |
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| SENSE pin | GPIO4_C5 | GPIO4_C7 | no kernel module/userspace apps that make use of this pin yet |
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| PWM Frequency | 25 kHz | 25 kHz | defined in device tree |
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## PWM Fan Connector
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Connector Pinout
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| Pin | Function | Wire Color |
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|-----|----------|------------|
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| 1 | GND | Black |
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| 2 | 12V | Red |
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| 3 | Sense | Yellow |
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| 4 | Control | Blue |
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## Helios64 Temperature Sensors
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### SoC Temperature Sensor
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RK3399 has internal temperature sensor (TS-ADC) for monitoring die temperature. The system has two Temperature Sensors, channel 0 is for CPU, channel 1 is for GPU.
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The sensor has -40~125°C temperature range and 5°C temperature resolution.
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### Board Temp Sensor
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Helios64 has a **Digital Temperature Sensor with 2‐wire Interface** ([NCT75 Datasheet](https://www.onsemi.com/pub/Collateral/NCT75-D.PDF)), located on top side of the board near RTC battery. It is used to read ambient temperature.
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## PWM Fan Control under Linux
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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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### Using SYSFS interface
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Linux export the fan control mechanism to SYSFS under hwmon class.
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List of devices can be checked under /sys/class/hwmon
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```
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ls -l /sys/class/hwmon/
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total 0
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lrwxrwxrwx 1 root root 0 Oct 16 08:38 hwmon0 -> ../../devices/virtual/thermal/thermal_zone0/hwmon0
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lrwxrwxrwx 1 root root 0 Oct 16 08:38 hwmon1 -> ../../devices/virtual/thermal/thermal_zone1/hwmon1
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lrwxrwxrwx 1 root root 0 Oct 16 08:38 hwmon2 -> ../../devices/platform/ff120000.i2c/i2c-2/2-004c/hwmon/hwmon2
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lrwxrwxrwx 1 root root 0 Oct 16 09:29 hwmon3 -> ../../devices/platform/gpio-charger/power_supply/gpio-charger/hwmon3
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lrwxrwxrwx 1 root root 0 Oct 16 09:29 hwmon4 -> ../../devices/platform/p7-fan/hwmon/hwmon4
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lrwxrwxrwx 1 root root 0 Oct 16 09:29 hwmon5 -> ../../devices/platform/p6-fan/hwmon/hwmon5
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lrwxrwxrwx 1 root root 0 Oct 16 09:29 hwmon6 -> ../../devices/platform/ff3d0000.i2c/i2c-4/4-0022/power_supply/tcpm-source-psy-4-0022/hwmon6
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```
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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 *p6-fan* and *p7-fan*. On above example
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```
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hwmon4 -> ../../devices/platform/p7-fan/hwmon/hwmon4
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hwmon5 -> ../../devices/platform/p6-fan/hwmon/hwmon5
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```
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To read current PWM
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```
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cat /sys/class/hwmon4/pwm1
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cat /sys/class/hwmon5/pwm1
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```
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To set PWM
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```
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echo NEW_PWM_VALUE > /sys/class/hwmon4/pwm1
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echo NEW_PWM_VALUE > /sys/class/hwmon5/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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```
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sudo apt-get install fancontrol
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```
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fancontrol includes *pwmconfig* script to create automatically a configuration file but it can not be used for Helios64.
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#### UDEV rules
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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/helios64/90-helios64-hwmon.rules) (mainline) or [here](https://raw.githubusercontent.com/armbian/build/master/packages/bsp/helios64/90-helios64-hwmon-legacy.rules) (legacy kernel 4.4) and to be copied to **/etc/udev/rules.d/**
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*/dev/fan-p6, /dev/fan-p7, /dev/thermal-cpu* and */dev/thermal-board* are symlinks generated by the udev rules.
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#### Configuration File
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fancontrol uses **/etc/fancontrol** as configuration file. Below is an example configuration to control fan speed on Helios64.
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```
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# Helios64 PWM Fan Control Configuration
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# Temp source : /dev/thermal-cpu
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INTERVAL=10
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FCTEMPS=/dev/fan-p6/pwm1=/dev/thermal-cpu/temp1_input /dev/fan-p7/pwm1=/dev/thermal-cpu/temp1_input
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MINTEMP=/dev/fan-p6/pwm1=40 /dev/fan-p7/pwm1=40
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MAXTEMP=/dev/fan-p6/pwm1=80 /dev/fan-p7/pwm1=80
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MINSTART=/dev/fan-p6/pwm1=60 /dev/fan-p7/pwm1=60
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MINSTOP=/dev/fan-p6/pwm1=29 /dev/fan-p7/pwm1=29
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MINPWM=20
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```
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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
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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.
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Fans (**fan-p6** & **fan-p7**) are controlled based on CPU thermal sensor (**thermal-cpu**) reading.
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-----
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*Following settings can be adjusted by user to tweak further.*
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MINTEMP
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The temperature below which the fan gets switched to minimum speed.
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Fans (fan-p6 & fan-p7) runs in minimum speed if the CPU temperature below **40** degree C.
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MAXTEMP
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The temperature over which the fan gets switched to maximum speed.
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Fans (fan-p6 & fan-p7) runs in maximum speed if the CPU temperature above **80** degree C.
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