Monday, April 15, 2024

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

Hi again,
I just went and cracked open the case.
According the chip ID, it's an IMX6 Dual. Although cpuinfo only reports
cpu 0.
The main board says revision A1; the baseboard says revision A0. I guess
this is one of my older units.


On Mon, April 15, 2024 2:12 pm, Derek Atkins wrote:
> 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.
>>> >>
> [snip]
>>> 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
> full).
> 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.
> Thanks,
> -derek
> --
> Derek Atkins 617-623-3745
> Computer and Internet Security Consultant

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