HX711 for Jetson Nano (libgpiod)

This library is based on HX711 for Rasberry Pi using libgpiod instead of RPi.GPIO.

Motivation for libgpiod on Jetson Nano

HX711 has a sevior timing requirement while reading data from DOUT. We have only 50us between 1 and 0 of PD_SCK pulse. Original hx711py library uses legacy/slow /sys/class/gpio interface with RPi.GPIO so that Jetson Nano violates 50us timing.

libgpiod is the solution using GPIO character device interface, it's faster than /sys/class/gpio interface. Let's use it!

Instructions

Check example.py to see how it works.

Prerequirements

libgpiod

You need to install libgpiod <=v1.6.x because newer version (v2.0) requires Linux kernel 5.5, but Jetson Nano's kernel version is 4.9.

sudo apt install autoconf-archive
git clone git://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git
cd libgpiod
git checkout v1.6.3 -b v1.6.3
./autogen.sh --enable-tools=yes --prefix=/usr/local --enable-bindings-python
make
sudo make install

You have to configure something to process libgpiod in user space.

Add /usr/local/lib line to /etc/ld.so.conf.d/libgpiod.conf
Rebuild /etc/ld.so.cache as the below.

sudo ldconfig

Add the line to /etc/udev/rules.d/99-gpio.rules as same as RPi.GPIO setting as the below.

SUBSYSTEM=="gpio", KERNEL=="gpiochip*", GROUP="gpio", MODE="0660"

Reload udev.

sudo udevadm control --reload-rules && sudo udevadm trigger

Confirm the group permission for gpiochips.

$ ll /dev/gpio*
crw-rw---- 1 root gpio 254, 0  Mar 24 15:53 /dev/gpiochip0
crw-rw---- 1 root gpio 254, 1  Mar 24 15:53 /dev/gpiochip1

Installation for Jetson Nano

Clone or download and unpack this repository
In the repository directory, run

python setup.py install

Using a 2-channel HX711 module

Channel A has selectable gain of 128 or 64. Using set_gain(128) or set_gain(64) selects channel A with the specified gain.

Using set_gain(32) selects channel B at the fixed gain of 32. The tare_B(), get_value_B() and get_weight_B() functions do this for you.

This info was obtained from an HX711 datasheet located at https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf

kempei / hx711py-jetsonnano