EBC Exercise 31 Dallas 1-Wire

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.

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
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  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>; };               };        }; };

gpio1, pin 28 is P9_12.

Installing
Now edit  and find the line: 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.
 * 1) uboot_overlay_addr4=/lib/firmware/ .dtbo

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  to point to your new device.

Wire your DS18B20 to the new pin and test it.

Maxim MAX31820 on a PocketBeagle Grove Kit
The MAX31820 uses the same interface as the 18D20. The only difference is editing the device tree file for the PocketBeagle. The device is wired as:



I attached the data line (DQ) to the UART4-TX line on the Grove cape. The table in shows that UART4-TX is attached to   on the Pocket.

The PocketBeagle Header Table shows that  is   and also appears at   on the Black. With this information you can now copy and edit the device tree.

bone$ cd /opt/source/bb.org-overlays/src/arm/ bone$ cp BB-W1-P9.12-00A0.dts BB-W1-P2.07-00A0.dts

Edit  and change the three appearances of   to.

Note: On line 50 BONE_2_07 isn't defined so we use the BeagleBone Black pin name instead.

Finally note, line 66 uses GPIO 0_31.

Do like before to make and install

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

Then edit  to include the new device tree. Reboot and test.