Am 19.09.2016 um 10:42 schrieb Colin Law:
> On 4 September 2016 at 19:59, Jan Kandziora <j...@gmx.de> wrote:
>> Am 04.09.2016 um 19:08 schrieb Colin Law:
>>> ...
>>> To summarize then, for parasitic mode I should not need an external
>>> strong pullup and should be able to use just a 1.5k pullup and to
>>> specify
>>> dtoverlay=w1-gpio,gpiopin=4
>>> and
>>> sudo modprobe w1-gpio pullup=1
>>>
>>> With in addition the 3.3 to 5V level shifter as in your first post if
>>> any devices on the bus need 5V
>>>
>> Yes.
>
> Sorry, I need yet further clarification on this. I have been trying
> to get to grips with device tree and (on pi with raspbian jessie) in
> /boot/overlays/README it says
>
> "Device Tree makes it possible
> to support many hardware configurations with a single kernel and without the
> need to explicitly load or blacklist kernel modules."
>
> which I think means that it should not be necessary to use modprobe
> w1-gpio or to put the module in /etc/modules.
>
This is a misunderstanding.
If you don't modprobe the module, someone else has to do. It depends on
your udev setup whether udev does it for you. The kernel doesn't load
modules automatically just because a device is listed in the loaded
device tree.
Before device trees, hardware layout has been hardcoded in the driver
modules or have to be specified by module options. That's what is no
longer necessary.
> Also for w1-gpio it has
>
> "Name: w1-gpio
> Info: Configures the w1-gpio Onewire interface module.
> Use this overlay if you *don't* need a GPIO to drive an external
> pullup.
> Load: dtoverlay=w1-gpio,<param>=<val>
> Params: gpiopin GPIO for I/O (default "4")
> pullup Non-zero, "on", or "y" to enable the parasitic
> power (2-wire, power-on-data) feature
>
> Does this mean I can (should?) use
> dtoverlay=w1-gpio,gpiopin=4,pullup=y
>
Haven't checked yet. Technically it is possible to use the internal push
transistor of the GPIO port to act as a strong pullup.
HOWEVER, you should use a series resistor outside the GPIO port then.
Because, overcurrent from shorting the Onewire too long may kill the
GPIO4 push transistor inside the BCM2835 SoC.
> and remove the modprobe command?
>
Check with lsmod whether the module gets loaded automatically by udev.
Kind regards
Jan
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