VAR-SOM-MX6 GPIO

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VAR-SOM-MX6 - GPIO

1 GPIO state

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

root@var-som-mx6:~# cat /sys/kernel/debug/gpio 
GPIOs 0-31, platform/209c000.gpio, 209c000.gpio:
 gpio-25  (phy-reset           ) out lo    

GPIOs 32-63, platform/20a0000.gpio, 20a0000.gpio:

GPIOs 64-95, platform/20a4000.gpio, 20a4000.gpio:
 gpio-77  (ov5640_mipi_pwdn    ) out lo    
 gpio-86  (usb_otg_vbus        ) out lo    

GPIOs 96-127, platform/20a8000.gpio, 20a8000.gpio:
 gpio-101 (tlv320aic3x reset   ) out lo    
 gpio-106 (ov5640_mipi_reset   ) out lo    
 gpio-110 (2194000.usdhc cd    ) in  hi    
 gpio-111 (2194000.usdhc ro    ) in  hi    
 gpio-120 (spi_imx             ) out lo    
 gpio-121 (ads7846_pendown     ) in  hi    

GPIOs 128-159, platform/20ac000.gpio, 20ac000.gpio:
 gpio-141 (PCIe reset          ) out lo    

GPIOs 160-191, platform/20b0000.gpio, 20b0000.gpio:
 gpio-178 (sysfs               ) out lo    

GPIOs 192-223, platform/20b4000.gpio, 20b4000.gpio:
 gpio-200 (wlan-en-regulator   ) out lo

Each GPIO is defined as in or out and the state is shown as lo or hi.
For example pin 110 is the SD card card-detect. When an SD card is plugged in, the state will be: 

 gpio-110 (2194000.usdhc cd    ) in  lo

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

 gpio-110 (2194000.usdhc cd    ) in  hi

2 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.

2.1 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 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 gpiochip3 21=1

Set GPIO4_21 low: 

gpioset gpiochip3 21=0

Read GPIO4_21: 

gpioget gpiochip3 21

2.2 libgpiod C Application

libgpiod provides bindings for C/C++ 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 GPIO4_IO21:

Makefile: 

all: main.cpp
	$(CC) $(CCFLAGS) -Og -lgpiod main.c -g -o hello.bin
clean:
	rm -f hello.bin

main.c 

#include <gpiod.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define    CONSUMER    "Variscite Demo"

int main(int argc, char **argv)
{
    unsigned int i, ret, val;
    struct gpiod_chip *chip;
    struct gpiod_line *line;
    const char * chipname = "gpiochip3";
    const unsigned int line_num = 21;

    chip = gpiod_chip_open_by_name(chipname);
    if (!chip) {
        perror("Open chip failed\n");
        goto end;
    }

    line = gpiod_chip_get_line(chip, line_num);
    if (!line) {
        perror("Get line failed\n");
        goto close_chip;
    }

    ret = gpiod_line_request_output(line, CONSUMER, 0);
    if (ret < 0) {
        perror("Request line as output failed\n");
        goto release_line;
    }

    /* Blink 5 times */
    val = 0;
    for (i = 0; i < 5; i++) {
        ret = gpiod_line_set_value(line, val);
        if (ret < 0) {
            perror("Set line output failed\n");
            goto release_line;
        }
        printf("Output %u on line #%u\n", val, line_num);
        sleep(1);
        val = !val;
    }

release_line:
    gpiod_line_release(line);
close_chip:
    gpiod_chip_close(chip);
end:
    return 0;
}

2.3 libgpiod Python Application

libgpiod provides bindings for python applications: 

# pip3 install gpiod

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


3 Manipulating a single GPIO via /sys/class/gpio

Note: The /sys/class/gpio ABI is being deprecated in the Linux kernel and is being replaced by the GPIO character device and libgpiod. Please refer to the kernel Documentation for the latest information.

3.1 Using a command line or a script

GPIOs in i.MX are grouped in groups of 32 pins.
For example, GPIO1_3 belong to the first group, pin 3. Its absolute number will be 3.
GPIO7_4 will be (7-1)*32+4=196.
Assuming this GPIO is defined in your device tree, the following is an example of how to use it from userspace.

To export the GPIO for userspace use: 

# echo 196 > /sys/class/gpio/export


To configure as output: 

# echo out > /sys/class/gpio/gpio196/direction

Set GPIO high: 

# echo 1 > /sys/class/gpio/gpio196/value

Set GPIO low: 

# echo 0 > /sys/class/gpio/gpio196/value


To configure as input: 

# echo in > /sys/class/gpio/gpio196/direction

Read the current value: 

# cat /sys/class/gpio/gpio196/value


To free the GPIO after you're done using it: 

# echo 196 > /sys/class/gpio/unexport

3.2 Manage GPIO from a C application

All of the command line operations above can be translated to C code:
Reserve (export) the GPIO:

#define IMX_GPIO_NR(port, index)    ((((port)-1)*32)+((index)&31))

int fd;
char buf[MAX_BUF]; 
int gpio = IMX_GPIO_NR(7, 4); /* Just an example */

fd = open("/sys/class/gpio/export", O_WRONLY);

sprintf(buf, "%d", gpio); 

write(fd, buf, strlen(buf));

close(fd);

Set the GPIO direction:

sprintf(buf, "/sys/class/gpio/gpio%d/direction", gpio);

fd = open(buf, O_WRONLY);

/* Set out direction */
write(fd, "out", 3); 
/* Set in direction */
write(fd, "in", 2); 

close(fd);

In case of out direction set the GPIO value: 

sprintf(buf, "/sys/class/gpio/gpio%d/value", gpio);

fd = open(buf, O_WRONLY);

/* Set GPIO high status */
write(fd, "1", 1); 
/* Set GPIO low status */
write(fd, "0", 1); 

close(fd);

In case of in direction get the current GPIO value: 

char value;

sprintf(buf, "/sys/class/gpio/gpio%d/value", gpio);

fd = open(buf, O_RDONLY);

read(fd, &value, 1);

if (value == '0') { 
     /* Current GPIO status low */
} else {
     /* Current GPIO status high */
}

close(fd);

Once finished, free (unexport) the GPIO: 

fd = open("/sys/class/gpio/unexport", O_WRONLY);

sprintf(buf, "%d", gpio);

write(fd, buf, strlen(buf));

close(fd);

Important notes:

  • Remember that after the first read operation the file pointer will move to the next position in the file, so to get a correct value for each read operation you simply have to set the file pointer at the beginning of the file before read by using the following command:
lseek(fd, 0, SEEK_SET);
  • This is only a short example. If you want to use it in your code remember add error handling to it.

4 Kernel Device Tree GPIO configuration

4.1 Device Tree GPIO files

4.1.1 Pin Func files

In the directory arch/arm/boot/dts/ of the Linux kernel source you will find the pin functions definitions files.
The relevant files are imx6dl-pinfunc.h and imx6q-pinfunc.h, depending on the platform you are using.
For example, if you edit imx6q-pinfunc.h and search for GPIO7_IO04, you will see a group of of definitions with same prefix (pad name), "MX6QDL_PAD_SD3_DAT0". 

#define MX6QDL_PAD_SD3_DAT0__SD3_DATA0              0x2c0 0x6a8 0x000 0x0 0x0
#define MX6QDL_PAD_SD3_DAT0__UART1_CTS_B            0x2c0 0x6a8 0x000 0x1 0x0
#define MX6QDL_PAD_SD3_DAT0__UART1_RTS_B            0x2c0 0x6a8 0x91c 0x1 0x2
#define MX6QDL_PAD_SD3_DAT0__FLEXCAN2_TX            0x2c0 0x6a8 0x000 0x2 0x0
#define MX6QDL_PAD_SD3_DAT0__GPIO7_IO04             0x2c0 0x6a8 0x000 0x5 0x0

Adding only the one with the GPIO7_IO04 suffix (function) to your dts file will let you use the pin as GPIO.

4.1.2 Variscite dts files

Variscite defines dts file for each platform.

Device Tree Name
 Include dtsi file
 SOM type
 Carrier Board type
 LCD Type
 Evaluation Kit name
imx6q-var-som.dts imx6qdl-var-som.dtsi VAR-SOM-MX6_V2 (Quad / Dual) VAR-MX6CustomBoard Capacitive/Resistive touch VAR-DVK-MX6_V2-PRO
VAR-STK-MX6_V2
imx6q-var-som-vsc.dts imx6qdl-var-som.dtsi VAR-SOM-MX6_V2 (Quad / Dual) VAR-SOLOCustomBoard Capacitive LVDS touch N/A
imx6dl-var-som.dts imx6qdl-var-som.dtsi VAR-SOM-MX6_V2 (DualLite/ Solo) VAR-MX6CustomBoard Capacitive/Resistive touch N/A
imx6dl-var-som-solo-vsc.dts imx6qdl-var-som.dtsi VAR-SOM-SOLO / VAR-SOM-DUAL VAR-SOLOCustomBoard Capacitive LVDS touch VAR-DVK-SOLO/DUAL VAR-STK-SOLO/DUAL
imx6dl-var-som-solo.dts imx6qdl-var-som.dtsi VAR-SOM-SOLO / VAR-SOM-DUAL VAR-MX6CustomBoard Capacitive/Resistive touch N/A
imx6q-var-dart.dts imx6qdl-var-dart.dtsi VAR-SOM-SOLO / VAR-SOM-DUAL VAR-DT6CustomBoard Capacitive LVDS touch VAR-STK-DT6.VAR-DVK-DT6

imx6q-var-som.dts starts with definitions and includindg dtsi files. 

#define VAR_SOM_MX6

#include "imx6q.dtsi"
#include "imx6qdl-var-som.dtsi"


The imx6q.dtsi define the CPU platform and which pinfunc file will be included. This feature allow the pin name to be agnostic to the CPU type (i.MX6Q vs i.MX6DL)
imx6qdl-var-som.dtsi has the major VAR-SOM-MX6 definitions.

4.2 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.
Edit arch/arm/boot/dts/imx6qdl-var-som.dtsi (or imx6qdl-var-dart.dtsi in case of DART-MX6) and add the definition for the GPIO you need in the section below.

	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_hog>;

	imx6qdl-var-som-mx6 {

		pinctrl_hog: hoggrp {
			fsl,pins = <
				/* PMIC INT */
				MX6QDL_PAD_GPIO_17__GPIO7_IO12			0x80000000
				/* Wifi Slow Clock */
				MX6QDL_PAD_ENET_RXD0__OSC32K_32K_OUT		0x000b0
				/* Audio Clock */
				MX6QDL_PAD_GPIO_0__CCM_CLKO1 			0x130b0
				/* Camera Clock */
				MX6QDL_PAD_GPIO_3__CCM_CLKO2			0x130b0
                                MX6QDL_PAD_KEY_ROW0__GPIO4_IO07                 0x0b0b1
                                MX6QDL_PAD_KEY_COL1__GPIO4_IO08                 0x0b0b1

			>;
		};

4.2.1 Device Tree GPIO attribute

If you look at Documentation/devicetree/bindings/pinctrl/fsl,imx6q-pinctrl.txt in the Linux kernel source tree, the number to the right of the pin control spec can be used for additional attributes like pull-ups, pull-downs, keepers, drive strength, etc.
The value 0x80000000 is "don't know value please use the default". Otherwise, the value consists of a bitwise-OR combination of the following values.

CONFIG bits definition
 value
PAD_CTL_HYS (1 << 16)
PAD_CTL_PUS_100K_DOWN (0 << 14)
PAD_CTL_PUS_47K_UP (1 << 14)
PAD_CTL_PUS_100K_UP (2 << 14)
PAD_CTL_PUS_22K_UP (3 << 14)
PAD_CTL_PUE (1 << 13)
PAD_CTL_PKE (1 << 12)
PAD_CTL_ODE (1 << 11)
PAD_CTL_SPEED_LOW (1 << 6)
PAD_CTL_SPEED_MED (2 << 6)
PAD_CTL_SPEED_HIGH (3 << 6)
PAD_CTL_DSE_DISABLE (0 << 3)
PAD_CTL_DSE_240ohm (1 << 3)
PAD_CTL_DSE_120ohm (2 << 3)
PAD_CTL_DSE_80ohm (3 << 3)
PAD_CTL_DSE_60ohm (4 << 3)
PAD_CTL_DSE_48ohm (5 << 3)
PAD_CTL_DSE_40ohm (6 << 3)
PAD_CTL_DSE_34ohm (7 << 3)
PAD_CTL_SRE_FAST (1 << 0)
PAD_CTL_SRE_SLOW (0 << 0)

4.3 Device Tree GPIO default Value

The Bluetooth is a good example to see how to set the default value during boot. variscite-bluetooth
For example how to reset the Bluetooth: 

echo 178 >/sys/class/gpio/export
echo "out" > /sys/class/gpio/gpio178/direction
echo 0 > /sys/class/gpio/gpio178/value
sleep 1
echo 1 > /sys/class/gpio/gpio178/value
sleep 1

You can also add it to your default build file system: initscripts

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This page was last edited on 6 August 2021, at 16:56.

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