are 1-wire devices "discoverable"?

are 1-wire devices "discoverable"?

[Trevor Woerner]

Hi,

I just watched the amazing device tree presentation recently done by Thomas
Petazzoni (from Bootlin) for ST[1]. During the presentation, which is
amazing, by the way ;-), Thomas describes the 1-wire protocol as not being
discoverable.

I was recently playing with the DS18B20 temperature sensor, which uses the
1-wire bus.[2] In fact, I was playing with a bunch of different temperature
sensors with a RaspberryPi, and then gave a talk at a local RaspberryPi Meetup
group that I regularly attend.[3]

My only experience with the 1-wire bus is with this one device, so maybe the
details of how the DS18B20 work are specific to the DS18B20 and not the way
1-wire devices work in general? But my (humble) conclusion is that the 1-wire
bus is discoverable (or at least quasi-discoverable).

It's true that you do need a device tree overlay to tell the kernel that you
want to use the 1-wire bus, and you have to tell the kernel which GPIO pin you
want to use as the 1 wire, but after that, attaching DS18B20 devices to a
running system works quite magically.[4]

Each DS18B20 has a unique 64-bit number burned into it, the first 8 bits
specify the device type (i.e. the DS18B20), the next 48 bits are a unique
serial number, and the last 8 bits are a CRC of the previous 56 bits. Due to
the inclusion of the 8-bit device type, when I plug a DS18B20 into my board,
the kernel automatically creates a sysfs entry for it with a "temperature"
file that I can read to obtain the temperature in Celcius.

I don't know if that qualifies as "discoverable"? It's certainly a lot more
discoverable than I2C or SPI, although maybe not quite as discoverable as,
say, PCI. Specifying the 1 wire is not discoverable, but plugging 1-wire
devices into my board is maybe something that could be described as
discoverable?

Best regards,
	Trevor

[1] https://www.youtube.com/watch?v=a9CZ1Uk3OYQ
[2] https://twoerner.blogspot.com/2020/12/temperature-readings-with-ds18b20-and.html
[3] https://twoerner.blogspot.com/2021/01/sensing-temperature-with-raspberrypi.html
[4] https://twoerner.blogspot.com/2020/12/multiple-ds18b20-temperature-probes.html

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Re: are 1-wire devices "discoverable"?

[jim.cromie]

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On Sat, Feb 13, 2021 at 11:58 AM Trevor Woerner <twoerner@gmail.com> wrote:

> Hi,
>

> It's true that you do need a device tree overlay to tell the kernel that
> you
> want to use the 1-wire bus, and you have to tell the kernel which GPIO pin
> you
> want to use as the 1 wire, but after that, attaching DS18B20 devices to a
> running system works quite magically.[4]
>
> Each DS18B20 has a unique 64-bit number burned into it, the first 8 bits
> specify the device type (i.e. the DS18B20), the next 48 bits are a unique
> serial number, and the last 8 bits are a CRC of the previous 56 bits. Due
> to
> the inclusion of the 8-bit device type, when I plug a DS18B20 into my
> board,
> the kernel automatically creates a sysfs entry for it with a "temperature"
> file that I can read to obtain the temperature in Celcius.
>
>
so, this device looks discoverable on a bus,
once you know the bus is there.
thats the 1st distinction to make.
there are degrees/features.
Id also ask:
is that ID at a standard location ?
via some base transaction that all devices support ?

in my own house, I can find a switch in the dark.
I dont have Alexa listening to render help.





> I don't know if that qualifies as "discoverable"? It's certainly a lot more
> discoverable than I2C or SPI, although maybe not quite as discoverable as,
> say, PCI. Specifying the 1 wire is not discoverable, but plugging 1-wire
> devices into my board is maybe something that could be described as
> discoverable?
>
> Best regards,
>         Trevor
>
> [1] https://www.youtube.com/watch?v=a9CZ1Uk3OYQ
> [2]
> https://twoerner.blogspot.com/2020/12/temperature-readings-with-ds18b20-and.html
> [3]
> https://twoerner.blogspot.com/2021/01/sensing-temperature-with-raspberrypi.html
> [4]
> https://twoerner.blogspot.com/2020/12/multiple-ds18b20-temperature-probes.html
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>

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