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 we're attaching it to P9_12. The default device tree has P9_12 configured as a GPIO. We need to remove that configuration so it can be used with the DS18B20.

Unconfiguring P9_12
First see if it is configured. bone$ config-pin -q P9_12 P9_12 Mode: default Direction: in Value: 1 If you get the response above, you need to unconfigure it. First find which device tree you are using. bone$ export SLOTS=/sys/devices/platform/bone_capemgr/slots bone$ cat $SLOTS 0: PF -1 1: PF -1 2: PF -1 3: PF -1 4: P-O-L-  0 Override Board Name,00A0,Override Manuf,univ-emmc The last part of the last line tells you which device tree you are using. In my case it univ-emmc. Next: bone$ cd /opt/source/bb.org-overlays bone$ ls src/arm Look for the file that begins with the name you found above and ends in .dts. It's univ-emmc-00A0.dts in my case. Now edit it. bone$ vi src/arm/univ-emmc-00A0.dts Comment out all the lines containing P9.12 (1) and P9_12 (16). bone$ make bone$ make install bone$ reboot bone$ config-pin -q P9_12 P9_12 pinmux file not found! cape-universala overlay not found run "config-pin overlay cape-universala" to load the cape

Configuring P9_12
Good, the P9_12 pin is not configured. Let's configure it. bone$ export SLOTS=/sys/devices/platform/bone_capemgr/slots bone$ echo BB-W1-P9.12 > $SLOTS bone$ dmesg -H | tail [Oct26 10:04] bone_capemgr bone_capemgr: part_number 'BB-W1-P9.12', version 'N/A' [ +0.000046] bone_capemgr bone_capemgr: slot #5: override [ +0.000024] bone_capemgr bone_capemgr: Using override eeprom data at slot 5 [ +0.000024] bone_capemgr bone_capemgr: slot #5: 'Override Board Name,00A0,Override Manuf,BB-W1-P9.12' [ +0.020178] bone_capemgr bone_capemgr: slot #5: dtbo 'BB-W1-P9.12-00A0.dtbo' loaded; overlay id #1 [ +0.039562] Driver for 1-wire Dallas network protocol. Good, it looks like it's configured.

Reading the DS18B20
bone$ cd /sys/bus/w1/devices bone$ ls 28-00000829ed85 w1_bus_master1 You should see two directories, the first will have a different number than mine. The number is the serial number of your DS18B20, which is unique to each device. bone$ cd 28-00000829ed85 bone$ ls 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.

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.