Fan Control — Lectron Jetson Autopilot

Hardware Overview

The system fan is driven by a PWM signal routed through the power board connector. The relevant net on the schematic is FAN_PWM, which maps to:

Jetson Xavier NX pin: 230
GPIO function: GPIO14 / GP_PWM6
PWM controller base address: 0x32d0000 (PWM instance 5, nvidia,hw-instance-id = <0x05>)
sysfs chip: pwmchip3 → /sys/devices/pwm-fan/

The tachometer feedback signal (FAN_TACH) is handled by a separate controller at 0x39c0000.tachometer (pwmchip5).

Device Tree Configuration

PWM Controller Node

pwm@32d0000 {
    compatible = "nvidia,tegra194-pwm";
    reg = <0x00 0x32d0000 0x00 0x10000>;
    nvidia,hw-instance-id = <0x05>;
    clocks = <0x04 0x6e>;
    clock-names = "pwm";
    #pwm-cells = <0x02>;
    resets = <0x05 0x49>;
    reset-names = "pwm";
    status = "okay";
    linux,phandle = <0xcd>;
    phandle = <0xcd>;
};

Tachometer Nodes

tachometer@39c0000 {
    compatible = "nvidia,pwm-tegra194-tachometer";
    reg = <0x00 0x39c0000 0x00 0x10>;
    #pwm-cells = <0x02>;
    clocks = <0x04 0x98>;
    clock-names = "tach";
    resets = <0x05 0x5f>;
    reset-names = "tach";
    pulse-per-rev = <0x02>;       /* 2 pulses per revolution */
    capture-window-length = <0x02>;
    disable-clk-gate;
    status = "okay";
    linux,phandle = <0xb2>;
    phandle = <0xb2>;
};

generic_pwm_tachometer {
    compatible = "generic-pwm-tachometer";
    pwms = <0xb2 0x00 0xf4240>;   /* phandle, channel 0, period 1 000 000 ns (1 Hz reference) */
    status = "okay";
};

The generic-pwm-tachometer driver wraps the hardware tachometer and exposes RPM through the hwmon subsystem.

pwm-fan Node

The PWM signal on this board is hardware-inverted relative to the standard Jetson carrier reference. The active_pwm profiles are therefore inverted: a value of 0xff (255) corresponds to the fan off state, and lower values increase fan speed.

pwm-fan {
    compatible = "pwm-fan";
    status = "okay";
    #pwm-cells = <0x01>;
    pwms = <0xcd 0x00 0xb116>;   /* phandle, channel 0, period 45334 ns (~22 kHz) */
    shared_data = <0xce>;
    vdd-fan-supply = <0xcf>;

    profiles {
        default = "quiet";

        quiet {
            state_cap = <0x04>;
            /* PWM values for states 0-9; inverted: 0xff = off, lower = faster */
            active_pwm = <0xff 0x82 0x5f 0x37 0x00 0x00 0x00 0x00 0x00 0x00>;
        };

        cool {
            state_cap = <0x04>;
            active_pwm = <0xff 0x73 0x55 0x37 0x00 0x00 0x00 0x00 0x00 0x00>;
        };
    };
};

Period note: 0xb116 = 45334 ns → approximately 22.06 kHz.

At runtime the kernel confirms this configuration:

platform/32d0000.pwm, 1 PWM device
 pwm-0   (pwm-fan): requested enabled  period: 45334 ns  duty: 23020 ns  polarity: normal

sysfs Layout

/sys/class/pwm/
├── pwmchip0  →  3280000.pwm
├── pwmchip1  →  c340000.pwm
├── pwmchip2  →  32c0000.pwm
├── pwmchip3  →  32d0000.pwm   ← fan PWM (owned by pwm-fan driver)
├── pwmchip4  →  32f0000.pwm
└── pwmchip5  →  39c0000.tachometer

pwmchip3 is claimed by the pwm-fan kernel driver. Attempting to export channel 0 directly will fail:

$ echo 0 > /sys/class/pwm/pwmchip3/export
-bash: echo: write error: Device or resource busy

Controlling the Fan

Because the pwm-fan driver owns the hardware channel, fan speed must be set through its own sysfs interface.

Manual Speed Control

target_pwm accepts values from 0 (minimum duty / hardware inverted → full speed) to 255 (maximum duty / hardware inverted → fan off). Adjust according to the inverted mapping in your profile tables.

# Full speed (duty cycle minimum after inversion)
echo 0 | sudo tee /sys/devices/pwm-fan/target_pwm

# ~50 % speed
echo 128 | sudo tee /sys/devices/pwm-fan/target_pwm

# Fan off (duty cycle maximum after inversion)
echo 255 | sudo tee /sys/devices/pwm-fan/target_pwm

Switching Thermal Profiles

The active thermal profile is set in the device tree (default = "quiet"). There is no runtime sysfs interface for switching profiles on this build. Use target_pwm for direct manual override, or modify the device tree profile and rebuild if a different automatic curve is needed.

# Read back the current duty value being sent to the hardware
cat /sys/devices/pwm-fan/cur_pwm

Reading Fan Tachometer

The generic_pwm_tachometer driver registers an hwmon device. Read RPM with:

sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm

The fan takes several seconds to spin up to its steady-state speed after target_pwm is changed. Example session showing spin-up from rest to full speed:

$ echo 255 | sudo tee /sys/devices/pwm-fan/target_pwm   # fan off
$ sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm
0
$ echo 0 | sudo tee /sys/devices/pwm-fan/target_pwm     # full speed
$ sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm
1533    ← still accelerating
$ sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm
8493
$ sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm
13950
$ sudo cat /sys/devices/generic_pwm_tachometer/hwmon/hwmon1/rpm
14035   ← near steady state

Note that readings can fluctuate during spin-up and wind-down; allow at least 3-5 seconds after a target_pwm change before treating the RPM value as stable.

PWM Signal Inversion — Summary Table

`active_pwm` byte	Duty cycle sent	Fan behaviour
`0xff` (255)	~100 %	Off
`0x82` (130)	~51 %	Low speed
`0x55` (85)	~33 %	Medium speed
`0x37` (55)	~22 %	High speed
`0x00` (0)	0 %	Full speed

The inversion arises from the FET-based drive stage (Q4) on the power board: a high PWM output from the Jetson turns the FET off, reducing current to the fan.