Raspberry Pi Weather Station. 3 – 1-wire again

Following on from the weather station wiringPi page

So I’ve had a thought on the one-wire temperature sensor…. 1-Wire is a well defined system and works well on the Pi, however wiringPi has a “pin” based system, so how about adding it into wiringPi? Since I’d have to write the C code to parse the data file to present back to a program, then using that code in wiringPi as an analog input pin ought to be easy – right?

So from the “how hard can it be” department, I’ll embark on making the popular DS18B20 temperature sensors a wiringPi device…

A quick recap – when working, they appear in the /sys/bus/w1/devices directory. This is a virtual filesystem that dynamically changes depending on the 1-wire sensors/devices attached. Files are identified by a family-code prefix which identifys the device type and a serial number. This is a unique 64-bit number for every 1-wire device.

Checking my setup:

$ cat /sys/bus/w1/devices/28-0000053af458/w1_slave 
f6 00 4b 46 7f ff 0a 10 d6 : crc=d6 YES
f6 00 4b 46 7f ff 0a 10 d6 t=15375

So if I write a program to open that file, then read it in, all I need to do is scan for the “YES” which indicates a good read, then scan for the t= part to extract the temperature.

And done. Testing as usual by updating wpiExtensions.c and the Makefile, then running the gpio command and comparing with the direct file output:

$ gpio -x ds18b20:100:0000053af458 aread 100
$ cat /sys/bus/w1/devices/28-0000053af458/w1_slave 
09 01 4b 46 7f ff 07 10 bf : crc=bf YES
09 01 4b 46 7f ff 07 10 bf t=16562

And they both read just fine and my rounding seems OK too.

Back to earlier – “How hard can it be” – it was actually quite easy to implement this in wiringPi in C – afterall, the hard work has already been done in the OWFS (One Wire File System), so all I needed to do here was parse it. wiringPi could maintain 1000’s of these devices if required too (the limit is the number of open file handles – normally set at 1024, but can be trivially increased, if required)

Next on the list is tackling the analog to digital converter chip and seeing if we can make sense of the air quality sensor and wind direction indicator…

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