Linux and I2C (using LM73 Temperature Sensor as Slave)¶
Overview¶
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]
…
dtparam=i2c_arm=on
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:
$ lsmod |grep i2c
i2c_bcm2835 16384 0
We can see a userspace representation of the bus 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
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
,
i2c-dev
Note
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:
|
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 |
IDC/LM73 |
---|---|
1 (3V3) |
1 |
6 (GND) |
6 (GND) |
3 (SDA) |
3 |
5 (SCL) |
5 |
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.
#!/usr/bin/python
import sys, os, fcntl, struct
I2C_SLAVE = 0x0703 # from <linux/i2c-dev.h>
fd = os.open('/dev/i2c-1', os.O_RDWR)
fcntl.ioctl(fd, I2C_SLAVE, 0x49)
os.write(fd, '\x00')
msb_lsb = os.read(fd, 2)
msb, lsb = struct.unpack('BB', msb_lsb)
print float((msb<<24)|(lsb<<16)) / 65535 / 128
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <iostream>
#include <linux/i2c-dev.h>
using namespace std;
int main(int argc, char** argv)
{
int err, dev_fd;
if (argc != 2) {
cerr << argv[0] << " i2c-device" << endl;
return 1;
}
if ((dev_fd = open(argv[1], O_RDWR)) == -1) {
perror("open");
return 1;
}
if ((err = ioctl(dev_fd, I2C_SLAVE, 0x49)) == -1) {
perror("set address");
return 1;
}
/* set register address for next read operation */
uint8_t byte = 0x0;
if ((err = write(dev_fd, &byte, 1)) != 1) {
cerr << err << endl;
perror("write");
return 1;
}
/* read temperature */
uint8_t msb_lsb[2];
if ((err = read(dev_fd, msb_lsb, 2)) != 2) {
perror("read");
return 1;
}
close(dev_fd);
cout << (double)((msb_lsb[0]<<24)|(msb_lsb[1]<<16)) / 65535 / 128 << endl;
return 0;
}
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 https://github.com/raspberrypi/linux
$ 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,
# 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
22000
Note
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>
.
Footnotes