Monday, April 15, 2024

[fedora-arm] Re: Mapping GPIO Pins from Wandboard to Fedora/libpiod

Hi Peter,

On Mon, April 15, 2024 1:47 pm, Peter Robinson wrote:

>> >> I'm using
>> >>
>> >> as a reference for the board layout. Specifically, on page 27, it
>> shows
>> >> me that the JP4 header connects to GPIO3_12, GPIO3_27, GPIO6_31,
>> >> CPIO1_24,
>> >> GPIO7_8, GPIO3_26, GPIO_18, and GPIO_19.
>> >>

>> Being only ancillarily associated with Arm/Embedded HW -- what does it
>> mean for a GPIO to be "used for other things"? And more importantly,
>> why
>> would it be wired to a header if it's being used for something else?
> So in the case I mention below the GPIO pin is used for i2c and it's
> on that header so you could add say a i2c based temp sensor or other
> i2c device.
> Also board designers may use a GPIO to hook up a mSD card detect pin,
> or a WiFi interface reset pin, or something else on the board layout.

I guess I don't understand why they would expose GPIO-x on a header and
ALSO use it elsewhere on the board. In my case, I just need to find 4
open "button" inputs.

> You can see the default pin allocation here from line 152-195:
> And the GPIOs mapped to i2c here on lines 103-104 and again 113-114,
> and then as a camera enable/reset at 139-140:

Thanks. I see the duplication of scl-gpois and sda-gpios names in those
two sections, but in the first section it uses gpio3 21/28 and in the
second section is used gpio4 13/14.

I also don't see the specific bindings for the pins on the JP4 header
(e.g. I don't see gpio3 12 being specified here).

>> > A quick look at the dtsi for the wandboards some of the GPIOs re used
>> > for SCL/SDA pins on two of the i2c buses. The i2c1 seems to not have
>> > anything attached so I guess in on a pin header for end user use, and
>> > i2c12 has a audio codec and for the camera connector.
>> How exactly is this done? Is the pin wired to two places on the PCB?
> It depends, for example on a RPi header you can use a DT overlay to
> change the default use of a PIN, by default is might be a standard
> GPIO but you apply an overlay that remaps it so it routes a i2s audio
> interface so you can use a DAC to output sound. So it's generally more
> about being able to use the reduced amounts of external pins for
> different usecases, someone might want it in a robot, someone else
> might want it to output audio.

How would an end-user do that without building a custom kernel? Like I
said, all I need is to read from four input GPIOs for a water-detection
system, so instead of using a 'sleep' after opening the relay, it waits
for the appropriate gpio to turn on based on a water detector connected to
it (so it will turn off the relay as soon as it detects the water tank is

So really I just need to choose 4 pins that I can use, and map those to an
event manager to wait for the pin to go on. JP4 seems to be the only
layout with GPIO labeled, so I just need to figure out which pins to use
and how to read those inputs in this brave new world.



Derek Atkins 617-623-3745
Computer and Internet Security Consultant
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