Linux and I2C (using LM73 Temperature Sensor as Slave)


This article shows how you use Linux to communicate with I2C devices. We use …

  • Rasperry Pi because everything’s easy there. This article’s principles hold unmodified for other devices that run Linux (more handwork might be needed though).

  • The Hardware Monitoring interface in /sys/class/hwmon

  • The Texas Instruments LM73 temperature sensor.

Configuring I2C Master

The Raspberry has two I2C host controllers (“masters”) built-in. One of those, i2c-0, is dedicated to display and touch handling and is of no relevance here.

The other, i2c-1, is for maker’s use. This is what this section is about.

Enable I2C, Load BCM2835 Platform Driver

By default, on Raspberry Pi OS everything’s off. To turn something on, you configure the bootloader to turn it on. The bootloader will then pass the relevant information to the kernel which will react accordingly - load the appropriate drivers, for example.

In /boot/config.txt insert the following line 1


The effect of enabling I2C is that

  • pins GPIO2 and GPIO3 are not GPIO pins anymore, but rather their alternative configurations as data and clock, respectively, are enabled. See the pinout diagram.

  • the platform I2C driver is loaded.

Reboot, and check:

I2C platform driver
$ lsmod |grep i2c
i2c_bcm2835            16384  0

We can see a userspace representation of the bus in sysfs,

I2C bus #1 visible in sysfs
$ ls -l /sys/bus/i2c/devices/i2c-1
lrwxrwxrwx 1 root root 0 Oct  4 12:43 /sys/bus/i2c/devices/i2c-1 -> ../../../devices/platform/soc/fe804000.i2c/i2c-1


You can use the raspi-config tool to do the same in a more comfortable manner. Here in this article, we do not use any of those decadent tools. These are Raspberry specific and not available on any other Linux device 2.

See here for decadent documentation of raspi-config.

Enable Userspace I2C

We will be using a LM73 temperature sensor, for which a kernel driver exists. This means it is not strictly necessary to enable the I2C userspace interface - it is helpful though if you want to do diagnostics such as scanning an I2C bus for available devices.

Load the driver, i2c_dev, manually first to see what is going on. This will create a character device /dev/i2c-1 which represents the I2C bus #1.

# modprobe i2c-dev
$ ls -l /dev/i2c-1
crw-rw---- 1 root i2c 89, 1 Sep 29 14:27 /dev/i2c-1

We do not want to load i2c-dev manually everytime the Raspberry has booted, so we write the module name in /etc/modules,



The same is accomplished by creating a dedicated file, say /etc/modules-load.d/i2c, with i2c-dev in it.

Reboot, and check if /etc/i2c-1 is still there. We will later see how to detect devices on it using the i2cdetect tool.

Slave Device: LM73

LM73 comes in a SOT-23 package which means that it is rather small. Power can be supplied in a range between 2.7V and 5.5V. This is practical since the Pi’s I2C operating voltage is 3.3V; we use the 3.3V rail to power the chip.

During a larger project, I had to write a larger software package and, among other tasks like PCI communication, talk to LM73. This is where I decided to isolate the chip for easy testing, and came up with a PCB to carry only the LM73 and a capacitor.


The breakout board has a 10-pin IDC header with the following pin assignments:




2 (GND)


3 (VDD)





../../../../../../_images/2x5-IDC-Pin-Numbering.png ../../../../../../_images/lm73-pinout.png

Optional: Address Selection

The LM73 lets you choose between three different addresses, via pin 1.

Left unconnected (floating)


Connected to GND


Connected to VDD


The breakout board takes this into account: a 3 pin header lets you connect pins with a jumper.

Connecting LM73 to Raspberry Pi

Given the above IDC header pinout, we can now connect to the Raspberry Pi as follows:

Raspberry Pi Header


1 (3V3)


6 (GND)

6 (GND)

3 (SDA)


5 (SCL)


i2c-tools/i2cdetect: Diagnostics, Detecting Devices

Now we use the i2cdetect program from the i2c-tools package to check if everything’s connected correctly. I omitted the address jumper, so LM73 pin 1 is left floating - the chip should appear on address 0x48.

Install the i2c-tools package,

# apt install i2c-tools

i2c-tools brings a set of low-level programs to manipulate I2C device registers. i2cdetect is a tool to “probe” a bus for devices. Lets proble I2C bus 1 (i.e. /dev/i2c-1),

$ i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Voila, everything there - one device at address 0x48.

Using the Device: Talking I2C from Userspace

Reading the datasheet thoroughly 4, one can implement the device’s protocol in userspace. On the bus device /dev/i2c-1, you kind of connect to the device’s address (0x48), and send bytes back and forth.


import sys, os, fcntl, struct

I2C_SLAVE = 0x0703 # from <linux/i2c-dev.h>

fd ='/dev/i2c-1', os.O_RDWR)
fcntl.ioctl(fd, I2C_SLAVE, 0x49)

os.write(fd, '\x00')
msb_lsb =, 2)

msb, lsb = struct.unpack('BB', msb_lsb)
print float((msb<<24)|(lsb<<16)) / 65535 / 128

Implementing the device’s protocol in userspace is always possible for I2C devices. See the kernel documentation for detailed information - we are scratching only the surface here.

But this is rarely necessary because most devices are supported by Linux out of the box, and LM73 is no exception.

Using the Device: hwmon - Hardware Monitoring

The Linux kernel comes with a driver for LM73 (documentation). Sadly, Raspberry Pi OS does not package that driver, so you have to build your own kernel for this.

Building the Kernel, Enabling LM73

This is relatively easy; follow the kernel build documentation. In short:

Install prerequisites (as root),

# apt install git bc bison flex libssl-dev make libncurses-dev

Clone the kernel,

$ git clone
$ git branch
* rpi-5.10.y

Massage the configuration,

$ cd linux
$ make bcm2711_defconfig
$ make menuconfig

Apply your changes in the following places:

  • General Setup / Local version - append to kernel release: add something to differentiate your kernel from the prebuilt kernel. My choice is -v7l-jfasch.

  • Device Drivers / Hardware Monitoring support / National Semiconductor LM73: build as module (”M”)

Next, build the kernel. Time for coffee,

$ make -j4 zImage modules dtbs

Install the kernel, and reboot (as root),

# make modules_install
# cp arch/arm/boot/dts/*.dtb /boot/
# cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
# cp arch/arm/boot/dts/overlays/README /boot/overlays/
# cp arch/arm/boot/zImage /boot/kernel7l.img
# reboot

Loading the Driver

I2C is a simple protocol. PCI, at the other end of the protocol complexity scale, supports automatic device identification via vendor and device IDs, so device drivers can be automatically loaded - hotplugged.

With I2C, we don’t have such luck: we know what type of device sits on each address, and we have to supply that information to the kernel - triggering a kind of a “fake hotplug”. Knowing that the driver name is lm73, and the chip is on address 0x48, as root 3 do the following,

Announce LM73 on address 0x48
# echo lm73 0x48 > /sys/bus/i2c/devices/i2c-1/new_device

Check that the driver has been loaded. (If you haven’t compiled the kernel, or made any other mistake during the installation of it, then the driver simply isn’t there and will silently not be loaded.)

$ lsmod | grep lm73
lm73                   16384  0

Device up and running. Consequentially, the new device is represented as a directory in sysfs,

$ ls -l /sys/bus/i2c/devices/1-0048/
total 0
-r--r--r-- 1 root root 4096 Oct  4 12:54 modalias
-r--r--r-- 1 root root 4096 Oct  4 12:54 name
drwxr-xr-x 2 root root    0 Oct  4 12:54 power
lrwxrwxrwx 1 root root    0 Oct  4 12:54 subsystem -> ../../../../../../bus/i2c
-rw-r--r-- 1 root root 4096 Oct  4 12:53 uevent

Hardware Monitoring (hwmon) Devices

/sys/bus/i2c/devices/1-0048/ represents the device as a generic I2C device. A different aspect of LM73 is that it is a temperature sensor. There is an entire framework inside the kernel, hwmon, to cover such devices - no matter if they are Onewire or I2C (or …) devices, or if they are reachable via a CPU internal bus.

As such - a temperature sensor - the device appears under an alternative location under /sysfs/class/hwmon/, among other.

Prior to loading the driver, on the Raspberry there are two such hwmon devices preinstalled; these apparently represent temperature sensors that are built-in to the CPU, and which are enabled as part of Linux’s Raspberry board support.

$ ls -l /sys/class/hwmon/
total 0
lrwxrwxrwx 1 root root 0 Nov 12 07:14 hwmon0 -> ../../devices/virtual/thermal/thermal_zone0/hwmon0
lrwxrwxrwx 1 root root 0 Nov 12 07:14 hwmon1 -> ../../devices/platform/soc/soc:firmware/raspberrypi-hwmon/hwmon/hwmon1

After we load the driver (remember, the echo lm73 0x48 > ... above), another symlink appears in /sys/class/hwmon/.

$ ls -l /sys/class/hwmon/
total 0
lrwxrwxrwx 1 root root 0 Nov 12 07:14 hwmon0 -> ../../devices/virtual/thermal/thermal_zone0/hwmon0
lrwxrwxrwx 1 root root 0 Nov 12 07:14 hwmon1 -> ../../devices/platform/soc/soc:firmware/raspberrypi-hwmon/hwmon/hwmon1
lrwxrwxrwx 1 root root 0 Nov 12 07:36 hwmon2 -> ../../devices/platform/soc/fe804000.i2c/i2c-1/1-0048/hwmon/hwmon2

All these ``/sys/class/hwmon/hwmon*`` symlinks refer to directories
in a different location in ``sysfs`` where the fun stuff is. Lets
look at our sensor,
$ ls -l /sys/class/hwmon/hwmon2/
total 0
lrwxrwxrwx 1 root root    0 Oct  5 08:57 device -> ../../../1-0048
-r--r--r-- 1 root root 4096 Oct  5 08:57 name
drwxr-xr-x 2 root root    0 Oct  5 08:57 power
lrwxrwxrwx 1 root root    0 Oct  5 08:57 subsystem -> ../../../../../../../../class/hwmon
-r--r--r-- 1 root root 4096 Oct  5 08:57 temp1_input
-rw-r--r-- 1 root root 4096 Oct  5 08:57 temp1_max
-r--r--r-- 1 root root 4096 Oct  5 08:57 temp1_max_alarm
-rw-r--r-- 1 root root 4096 Oct  5 08:57 temp1_min
-r--r--r-- 1 root root 4096 Oct  5 08:57 temp1_min_alarm
-rw-r--r-- 1 root root 4096 Oct  5 08:56 uevent
-rw-r--r-- 1 root root 4096 Oct  5 08:57 update_interval

temp1_input is what contains information for us (the temperature in milli-celsius):

$ cat /sys/class/hwmon/hwmon2/temp1_input


  • Question: how do I know that it’s my sensor in hwmon2? hwmon2 seems like a randomly/sequentially chosen name, and I assume the order is not always the same across boots.

  • Answer: correct. You can identify your sensor, though, by looking at the device symlink,

    $ ls -l /sys/class/hwmon/hwmon2/device
    lrwxrwxrwx 1 root root 0 Oct  5 08:57 /sys/class/hwmon/hwmon2/device -> ../../../1-0048

    Apparently, the nomenclature is <bus>-<address>.



The line is already there, you only have to uncomment it.


The functionality that the Raspberry bootloader (via /boot/config.txt) brings is already decadent enough.


While members of group i2c are permitted to talk to I2C devices, adding devices is considered an administrative task.


Beware, data sheet authors have a strange kind of humor!