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:
When the SD card is removed, the state will be:
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:
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:
Set GPIO4_21 low:
Read GPIO4_21:
Note: To verify the output value of a GPIO pin after using the gpioset command, you can physically connect (jumper) it to a separate GPIO configured as an input. Then, use the gpioget command on the input pin to read the output state.
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 <[email protected]>
/* 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:
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.