|Pi IoT In Python Using Linux Drivers -1-Wire And The DS18B20|
|Written by Harry Fairhead & Mike James|
|Monday, 12 July 2021|
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Linux drivers make working with devices so easy - assuming you know how. Here's how to get a Raspberry Pi to get data from the 1-Wire Bus.
This content comes from our newly published book:
Raspberry Pi IoT In PythonUsing Linux Drivers
By Harry Fairhead & Mike James
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The 1‑Wire bus is a proprietary protocol that is very easy to use and has a lot of useful devices you can connect to it, including the iButton security devices. However, probably the most popular of all 1‑wire devices is the DS18B20 Temperature Sensor - it is small, very cheap and very easy to use. This is the device that we are going to focus on in this chapter but the techniques generalize to working with any 1-wire device you care to use.
1-wire devices are very simple and only use a single wire, hence the name, to transmit data:
The device can even be powered from the bus line by drawing sufficient current through the pull-up resistor - so called parasitic mode. Low-power devices work well in parasitic mode, but some devices have such a heavy current draw that the master has to provide a way to connect them to the power line - so called strong pull-up. In practice parasitic mode can be difficult to make work reliably for high power devices.
In normal-powered mode there are just three connections - V power (usually 3.3V for the Pi), Ground, and Data:
The pull-up resistor varies according to the device, but anything from 2.2K to 4.7kΩ works. The longer the bus the lower the pull-up resistor has to be to reduce “ringing”. There can be multiple devices on the bus and each one has a unique 64-bit lasered ROM code, which can be used as an address to select the active devices.
The GPIO Driver
There are a number of drivers that implement the 1-Wire master, but the only one supported without extra work for the Raspberry Pi is the w1-gpio driver which implements the 1-Wire protocol on any GPIO pin that you aren’t using for something else. There is also the w1-gpio-pullup variant of the driver, which is only needed if you are driving a 1-Wire device over a long wire connection.
You can enable the 1-Wire driver using:
where n is the GPIO line you want to use. The default is GPIO4, but you really can use any GPIO line.
You can add this dtoverlay line to the /config/boot.txt file or you can enable it dynamically using:
def load1w(pin): indicator = "w1-gpio" command =["sudo", "dtoverlay", "w1-gpio","gpiopin="+str(pin)] temp = subprocess.Popen(["sudo", "dtparam", "-l"], stdout = subprocess.PIPE) output = str(temp.communicate()) print(output,flush=True) if output.find(indicator)==-1: temp = subprocess.Popen(command, stdout = subprocess.PIPE) output = str(temp.communicate()) print(output,flush=True)
This just checks to see if there is an overlay currently active and if not it sets the pin you specify to be the 1-Wire bus.
As with all dynamic overlays, removing it isn’t a good idea and in the case of the 1-Wire driver, at the time of writing, it will crash the system. Once loaded, the overlay can stay active until the next reboot.
|Last Updated ( Monday, 12 July 2021 )|