EBC Exercise 31 Dallas 1-Wire

Embedded Linux Class by Mark A. Yoder

The DS18B20 is an interesting temperature sensor that uses Dallas Semiconductor's 1-wire based interface. The data communication requires only one wire! (However you still need wires for ground and 3.3V.) You can wire it to any GPIO port.

Here we show you how to wire a DS18B20 to a Beagle Bone Black and a MAX31820 to a PocketBeagle.

DS18B20 on a Black

SparkFun sells a DS18B20 that's in a waterproof probe. You'll need it and maybe a 4.7kΩ pull up resistor.

Attach the leads a follows.

DS18B20 Lead	Attach to
Red	3.3V
Black	ground
White	P9_12

You may also need to attach the 4.7kΩ resistor between P9_12 and 3.3V.

Software Setup

The DS18B20 can be attached to any GPIO pin, but there's a device tree already created to attach it to P9_12.

Finding the device tree

/lib/firmware contains many device trees. Let's see which ones work with one-wire interface

bone$ ls /lib/firmware/*W1*
/lib/firmware/BB-W1-P9.12-00A0.dtbo

Looks like there is one setup for P9_12. Let's check the source code. The Bone should already have the source files loaded.

bone$ cd /opt/source/bb.org-overlays
bone$ git pull
bone$ ls
COPYING  dtc-overlay.sh  include     jenkins_build.sh  Makefile          readme.md  src
debian   examples        install.sh  Jenkinsfile       readme-legacy.md  scripts    tools

If the cd fails you will have to clone the repository.

bone$ git clone https://github.com/beagleboard/bb.org-overlays
bone$ cd bb.org-overlays

Either way

bone$ cd src/arm
bone$ ls *W1*
BB-W1-P9.12-00A0.dts
bone$ less *W1*

Page down a ways to see

       fragment@3 {
               target-path="/";
               __overlay__ {

                       onewire {
                               status = "okay";
                               pinctrl-names = "default";
                               pinctrl-0 = <&dallas_w1_pins>;

                               compatible = "w1-gpio";
                               gpios = <&gpio1 28 GPIO_ACTIVE_HIGH>;
                       };
               };
       };
};

gpio0, pin 28 is P9_12.

Installing

Now edit /boot/uEnv.txt and find the line:

#uboot_overlay_addr4=/lib/firmware/<file0>.dtbo

and change it to

uboot_overlay_addr4=/lib/firmware/BB-W1-P9.12-00A0.dtbo

Note the # is missing from the beginning of the line.

Reboot

bone$ sudo reboot

Reading the DS18B20

bone$ cd /sys/class/hwmon/
hwmon0  hwmon1

Oh, we have two devices here. Let's see which is which

bone$ cat */name
w1_slave_temp
tmp101

So one is our one-wire temp sensor and the other is a tmp101 sensor. Let's read ours.

bone$ cd hwmon0
bone$ ls
device  name  power  subsystem  temp1_input  uevent
bone$ cat temp1_input
20812

The 20812 is the temperature in C times 1000, that is, divide this number by 1000 to get the temp in C.

Warm up the probe and see what happens to the temp.

Using a Different GPIO Pin

You can use pins other than the P9_12. Follow the unconfiguring instructions for the GPIO pin of your choice. Then

bone$ cd /opt/source/bb.org-overlays/sr/arm
bone$ cp BB-W1-P9.12-00A0.dts BB-W1-P9.14-00A0.dts

Substitute your pin number for P9.14. Then edit your newly created file and switch all the occurrences of P9_12 and P9.12 to the new pin number. Also look for the lines near the end:

compatible = "w1-gpio";
gpios = <&gpio1 28 GPIO_ACTIVE_HIGH>;

Change the gpio port number and pin number to match your new pin. Then compile and install.

bone$ cd /opt/source/bb.org-overlays
bone$ make install

Now edit the line in /boot/uEnv.txt to point to your new device.

Wire your DS18B20 to the new pin and test it.

Maxim MAX31820 on a PocketBeagle

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Embedded Linux Class by Mark A. Yoder