gpio.c

gpio.c
§

Jonas Altrock ew20b126@technikum-wien.at

To overview

main.c

This is basically the same as in assigment 2 and 3, but includes code to read the value of the first analog input AIN0. Also I got a BeagleBone Black which has different wiring than the PocketBeagle, so I added the possibility to override which GPIOs to use for the LEDs.
```
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
```
§

gpio.h
```
#include "gpio.h"
```

If this should run on a different board, we can override the GPIO pins connected to the LEDs with compiler arguments lik -DLED1_GPIO=53.

To stringify the preprocessor constants, we have to do a little dance with two macro expansions. And because the chosen GPIOs could be on another gpiochip, we have to do a little arithmetic as well.

#ifndef LED1_GPIO
#define LED1_GPIO 87
#endif

#ifndef LED2_GPIO
#define LED2_GPIO 89
#endif

#ifndef POTI_AIN
#define POTI_AIN 0
#endif

#define _STR(x) #x
#define STR(x) _STR(x)

#if (LED1_GPIO / 32 == 0)
#define LED1_GPIO_CHIP 0
#elif (LED1_GPIO / 32 == 1)
#define LED1_GPIO_CHIP 1
#elif (LED1_GPIO / 32 == 2)
#define LED1_GPIO_CHIP 2
#elif (LED1_GPIO / 32 == 3)
#define LED1_GPIO_CHIP 3
#endif

#if (LED2_GPIO / 32 == 0)
#define LED2_GPIO_CHIP 0
#elif (LED2_GPIO / 32 == 1)
#define LED2_GPIO_CHIP 1
#elif (LED2_GPIO / 32 == 2)
#define LED2_GPIO_CHIP 2
#elif (LED2_GPIO / 32 == 3)
#define LED2_GPIO_CHIP 3
#endif

I changed the structure here to accomodate GPIO and AIN.

typedef struct gpio_file {
    char *path;
    FILE *file;
} gpio_file_t;

A GPIO pin is two files,

typedef struct gpio_pin {
    gpio_file_t direction;
    gpio_file_t value;
} gpio_t;

gpio_t gpios[2] = {
    {
        .direction.path = "/sys/bus/gpio/devices/gpiochip" STR(LED1_GPIO_CHIP) "/gpio/gpio" STR(LED1_GPIO) "/direction",
        .value.path = "/sys/bus/gpio/devices/gpiochip" STR(LED1_GPIO_CHIP) "/gpio/gpio" STR(LED1_GPIO) "/value",
    },
    {
        .direction.path = "/sys/bus/gpio/devices/gpiochip" STR(LED2_GPIO_CHIP) "/gpio/gpio" STR(LED2_GPIO) "/direction",
        .value.path = "/sys/bus/gpio/devices/gpiochip" STR(LED2_GPIO_CHIP) "/gpio/gpio" STR(LED2_GPIO) "/value",
    }
};

and an AIN input is only one file.

gpio_file_t AIN = {
    .path = "/sys/bus/iio/devices/iio:device0/in_voltage" STR(POTI_AIN) "_raw"
};

void _gpio_deinit()
{
    for (int i = 0; i < 2; i++) {
        fclose(gpios[i].direction.file);
        fclose(gpios[i].value.file);
    }

    fclose(AIN.file);
}

int gpio_init()
{
    for (int i = 0; i < 2; i++) {
        if (NULL == (gpios[i].direction.file = fopen(gpios[i].direction.path, "w+"))) {
            perror("Could not open GPIO direction file");
            return -1;
        }
        if (NULL == (gpios[i].value.file = fopen(gpios[i].value.path, "w+"))) {
            perror("Could not open GPIO value file");
            return -1;
        }
    }

    if (NULL == (AIN.file = fopen(AIN.path, "r"))) {
        perror("Could not open ADC AIN file");
        return -1;
    }

    atexit(_gpio_deinit);

    return 0;
}

void _gpio_write(FILE *file, const char *value)
{
    fprintf(file, "%s\n", value);
    fflush(file);
}

Reading a GPIO/AIN file must accommodate at most 4095 + \n + \0 bytes.

#define _GPIO_READ_VALUE_LEN 6

void _gpio_read(FILE *file, char *value)
{
    rewind(file);
    fscanf(file, "%5s", value);
    fflush(file);
}

enum Direction gpio_get_dir(enum GPIO gpio)
{
    char str[_GPIO_READ_VALUE_LEN];
    _gpio_read(gpios[gpio].direction.file, str);

    if (str[0] == 'o') {
        return OUT;
    }

    return IN;
}

void gpio_set_dir(enum GPIO gpio, enum Direction dir)
{
    _gpio_write(gpios[gpio].direction.file, dir == OUT ? "out" : "in");
}

enum Value gpio_get_value(enum GPIO gpio)
{
    char str[_GPIO_READ_VALUE_LEN] = "";
    _gpio_read(gpios[gpio].value.file, str);

    if (str[0] == '0') {
        return OFF;
    }

    return ON;
}

void gpio_set_value(enum GPIO gpio, enum Value value)
{
    _gpio_write(gpios[gpio].value.file, value == ON ? "1" : "0");
}

§

ain_get_value()

int ain_get_value()
{
    char value[_GPIO_READ_VALUE_LEN] = "";
    _gpio_read(AIN.file, value);

§

We know the value in the file will always be numeric, as it is continuously updated from the hardware, so we can use atoi and don’t need strtol with rest check.
```
    return atoi(value);
}
```