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.

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>; };               };        }; };

gpio3, 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.