VAR-SOM-MX91 GPIO

GPIO state

The current state of the system's GPIOs can be obtained in user-mode, as shown in the following example:

Each GPIO is defined as in or out and the state is shown as lo or hi.
For example pin 44 is the SD card card-detect. When an SD card is plugged in, the state will be:

gpio-44  (                    |cd                  ) in  lo IRQ

When the SD card is removed, the state will be:

gpio-44  (                    |cd                  ) in  hi IRQ

Manipulating GPIO using libgpiod

The Linux GPIO sysfs interface is being deprecated. Moving forward, user space should use the character device /dev/gpiochip* instead. libgpiod provides bindings and utilities for for manipulating GPIO via user space.

libgpiod via command line

libgpiod provides command line utilities for GPIO:

gpiodetect	List all gpiochips present on the system, their names, labels and number of GPIO lines
gpioinfo	List all lines of specified gpiochips, their names, consumers, direction, active state and additional flags
gpioget	Read values of specified GPIO lines
gpioset	Set values of specified GPIO lines, potentially keep the lines exported and wait until timeout, user input or signal
gpiofind	Find the gpiochip name and line offset given the line name
gpiomon	Wait for events on GPIO lines, specify which events to watch, how many events to process before exiting or if the events should be reported to the console

i.MX GPIOs are organized in banks of 32 pins. Each bank corresponds to a character device /dev/gpiochip<bank index>. The gpiodetect utility can be used to inspect the available gpiochip character devices:

# gpiodetect
gpiochip0 [30200000.gpio] (32 lines)
gpiochip1 [30210000.gpio] (32 lines)
...

The gpioinfo utility can be used to inspect the lines for a given gpiochip:

# gpioinfo -c gpiochip0
gpiochip0 - 32 lines:
        line   0:      unnamed    "spi_imx"  output  active-high [used]
        line   1:      unnamed       unused   input  active-high
        line   2:      unnamed       unused   input  active-high
        ...

The gpioset and gpioget utilities can be used to manipulate GPIO from the command line.

For example, assuming GPIO4_21 is configured as a GPIO in your device tree:

Set GPIO4_21 high:

gpioset -t 0 -c gpiochip3 21=1

Set GPIO4_21 low:

gpioset -t 0 -c gpiochip3 21=0

Read GPIO4_21:

gpioget -c gpiochip3 21

libgpiod C++ Application

libgpiod provides bindings for C++, Python and Rust applications. C++ examples are available in the libgpiod /tree/bindings/cxx/examples directory.

Below is a simple C application demonstrating how to use the bindings with GPIO0_IO05:

Makefile:

LDFLAGS = -lgpiodcxx -lgpiod

# Nome do executável e arquivos fonte
TARGET = main
SRCS = main.cpp

all: $(TARGET)

$(TARGET): $(SRCS)
	$(CXX) $(SRCS) $(LDFLAGS) -o $(TARGET)

clean:
	rm -f $(TARGET)

main.cpp

// SPDX-License-Identifier: GPL-2.0-or-later
// SPDX-FileCopyrightText: 2023 Kent Gibson <warthog618@gmail.com>

/* Minimal example of reading a single line. */

#include <cstdlib>
#include <filesystem>
#include <gpiod.hpp>
#include <iostream>

namespace {

/* Example configuration - customize to suit your situation */
const ::std::filesystem::path chip_path("/dev/gpiochip0");
const ::gpiod::line::offset line_offset = 5;

} /* namespace */

int main()
{
	auto request = ::gpiod::chip(chip_path)
			       .prepare_request()
			       .set_consumer("get-line-value")
			       .add_line_settings(
				       line_offset,
				       ::gpiod::line_settings().set_direction(
					       ::gpiod::line::direction::INPUT))
			       .do_request();

	::std::cout << line_offset << "="
		    << (request.get_value(line_offset) ==
					::gpiod::line::value::ACTIVE ?
				"Active" :
				"Inactive")
		    << ::std::endl;

	return EXIT_SUCCESS;
}

libgpiod Python Application

libgpiod provides bindings for python applications:

# pip3 install gpiod

Python examples are available in the libgpiod /tree/bindings/python/examples directory.

Kernel Device Tree GPIO configuration

Device Tree GPIO files

Pin Func files

Adding only the one with the GPIO4_IO2 suffix (function) to your dts file will let you use the pin as GPIO.

Define a pin as GPIO in the kernel Device Tree

You need to add the relevant definitions to your device tree, as explained in the Pin Func files section above.

Please consult Variscite's blog post i.MX Device Tree Pinmux Settings Guide for further information.

Device Tree GPIO attribute

If you look at the pin control definitions in arch/arm64/boot/dts/ in the Linux kernel source tree, the number to the right of the pin mux macro can be used for additional attributes like pull-up, slew rate, open drain, drive strength, etc. This value is written to the IOMUXC_SW_PAD_CTRL_ register of the relevant pin.

Please consult the SOC reference manual for details about the relevant register.

MX8M GPIO

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