Re: Single gpio shared by LED and switch?

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Jacek,

On Wed, Sep 9, 2015 at 9:25 AM, Jacek Anaszewski
<j.anaszewski@samsung.com> wrote:
> On 09/08/2015 04:37 PM, Linus Walleij wrote:
>> On Tue, Sep 8, 2015 at 12:51 PM, Jacek Anaszewski
>> <j.anaszewski@samsung.com> wrote:
>>> On 09/07/2015 03:22 PM, Geert Uytterhoeven wrote:
>>>> Something like this?
>>>>
>>>>                        VCC         VCC
>>>>                         |           |
>>>>                         R           R
>>>>                         |           |
>>>>                 __|__   |           |
>>>>             +---o   o---+          _|_
>>>>             |           |          \ /
>>>>             |           |          ---
>>>>            GND          R           |
>>>>                         |           |
>>>>                         |         |-+
>>>>                         |         |<+
>>>> GPIO -----------------+---------|-|
>>>>                                     |
>>>>                                     |
>>>>                                    GND
>>>
>>> You would need also resistor to avoid short-circuit
>>> when the switch is on.
>>
>> There is a resistor AFAICT.
>
> Indeed. For some reason my mind filtered out these 'R'
> characters.

Sorry for the bad drawing, which is a simplified extract from the real
schematic diagram.

>> But consider the GPIO side which is opaque
>> here. When used as input it is often high impedance
>> but when used to drive it is not.
>>
>> Doesn't this mean the output actually has to be
>> open collector (open drain)?

I think the GPIO is just a standard push-pull output in output mode.

> I think that GPIO control register would have to be
> altered to switch between particular modes.
>
> Let's analyze possible modes, assuming that the transistor
> is a bipolar PNP:

Actually it's an N-channel MOSFET.

>>> We'd have to ask Linus if he welcomes drivers for this type
>>> of home-made circuits in mainline. If yes, then probably someone would
>>> have to build the circuit, develop and test the driver on it, and
>>> finally come up with a detailed documentation.
>>> Are you the volunteer? :)
>>
>> Of course he's considering it or he wouldn't write :)

This is not a home-made circuit.
Let's see how it goes when the hardware arrives...

Thanks, now I know no driver exists yet.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds