Difference between revisions of "EBC Exercise 31 Dallas 1-Wire"

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m (Software Setup: Updated for 4.19 kernel)
m (Reading the DS18B20)
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=== Reading the DS18B20 ===
 
=== Reading the DS18B20 ===
  bone$ '''cd /sys/bus/w1/devices'''
+
  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'''
 
  bone$ '''ls'''
  28-00000829ed85  w1_bus_master1
+
  device  name  power  subsystem temp1_input uevent
You should see two directories, the first will have a different number than mine.
+
  bone$ '''cat temp1_input'''
The number is the serial number of your DS18B20, which is unique to each device.
+
  20812
bone$ '''cd 28-00000829ed85'''
+
 
bone$ '''ls'''
+
The '''20812''' is the temperature in C times 1000, that is, divide this number by 1000 to get the temp in C.
driver  id name  power  subsystem  uevent w1_slave
 
  bone$ '''cat w1_slave'''  
 
  87 01 4b 46 7f ff 09 10 48 : crc=48 YES
 
87 01 4b 46 7f ff 09 10 48 t=24437
 
The '''t=24437''' 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.
 
Warm up the probe and see what happens to the temp.

Revision as of 11:55, 24 October 2019

thumb‎ 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.

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$ 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>;
                       };
               };
       };
};

gpio3, pin 28 is P9_12.

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.

bone$ cd /opt/source/bb.org-overlays
bone$ make install
bone$ echo BB-W1-P9.14 > $SLOTS

Wire your DS18B20 to the new pin and test it.



thumb‎ Embedded Linux Class by Mark A. Yoder