TLDR⌗

Here’s the code you probably want. Modify it as you see fit.

loadcell_en_reg: fixedregulator@3 {
  compatible = "regulator-fixed";
  regulator-name = "loadcell-en-regulator";
  regulator-min-microvolt = <3300000>;
  regulator-max-microvolt = <3300000>;

  /* ADC_PWR_EN */
  gpio = <&gpio1 13 0>;
  enable-active-high;
};

hx711: hx711 {
  compatible = "avia,hx711";
  sck-gpios = <&gpio1 14 GPIO_ACTIVE_HIGH>;
  dout-gpios = <&gpio1 15 GPIO_ACTIVE_HIGH>;
  avdd-supply = <&loadcell_en_reg>;
};

$ cat /sys/bus/iio/devices/iio:device2/in_voltage0_raw
6818404

Using HX711 from Linux⌗

The HX711 is a really common device, most commonly used to measure weight via a load cell. It uses a simple 2 wire interface with a clock and signal. It’s not a standard protocol, but simple anyway.

You can find devboards at SparkFun. You will also probably need a load cell, which can also be found at SparkFun.

There are a ton of implementations out there for using this particular device:

But for embedded Linux, none of these are particulary suitable. Luckily, there is a Linux Kernel Module available for that use case. Source

Unfortunately, as is common with these sorts of devices in Linux, there is almost no documentation for it besides the Kernel patch’s original submission and a small blurb in the Device Tree.

By the way, there’s a typo in the above document. Don’t spend an entire work day figuring that out like me.

Enable the Kernel Module⌗

To use this device with Linux, the first thing you will need to do is enable the kernel module. The name of the module is CONFIG_HX711. You can enable it in the make menuconfig menu. Use / to search for it.

Configure the Device Tree⌗

This is the hardest part for me to getting this device working was setting up the Device tree. The first section has the code required, but I’ll walk thru the parts that held me up.

  avdd-supply = <&loadcell_en_reg>;

This was probably the single most time consuming part of the entire project. In my particular case, the regulator is external too my system regulator.

loadcell_en_reg: fixedregulator@3 {
  compatible = "regulator-fixed";
  regulator-name = "loadcell-en-regulator";
  regulator-min-microvolt = <3300000>;
  regulator-max-microvolt = <3300000>;

  /* ADC_PWR_EN */
  gpio = <&gpio1 13 0>;
  enable-active-high;
};

This looks pretty simple now that I know what everything does. The trick was finding out I needed a regulator-fixed device. I spent a lot of time trying to get a different node functioning: regulator-gpio. My thought process being

I have a regulator, it’s enabled by gpio, therefor I must want gpio regulator.

This turned out to be false, and regulator-fixed itself actually supports using gpio to enable it.

Getting Data from the HX711⌗

The module uses the iio subsystem. That document was a little overwealming to me, so here’s the cheat codes:

/sys/bus/iio/devices/iio:device2/in_voltage0_raw

Calibration⌗

Read that file, it will give you the voltage from the device. This value will need to be calibrated in userspace as documented by the original author of the driver. This varries depending on which load cell you have and how your device is positioned in the real world. I may update this post in the future with how i calibrate the device, for now you probably want to consult the Datasheet.