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Mraa Usage

MRAA Introduction

Eclipse Mraa (Libmraa) is a C/C++ library with Java, Python, and JavaScript bindings for connecting I/O pins and buses on a variety of IoT and edge platforms, with a well-structured API and port names/numbers that match the boards on which they are located.

Install MRAA​

sudo apt-get update -y
sudo apt-get install libmraa2 libmraa-dev libmraa-java python3-mraa mraa-tools -y

MRAA Command Line Tools​

GPIO​

  • mraa-gpio list: List all available pins
  • mraa-gpio get pin: Get Pin Status
  • mraa-gpio set pin level: Set Pin Status
  • mraa-gpio version: Get MRAA Version

I2C​

  • mraa-i2c list: List available I2C buses
  • mraa-i2c version: Get mraa version and board name
  • mraa-i2c detect bus: List detected devices on specified bus
  • mraa-i2c get bus device reg: Get value from specified device register
  • mraa-i2c set bus device reg value: Set specified device register to value

UART​

  • mraa-uart list: Lists UARTs on the system
  • mraa-uart dev dev_num baud customized_baud send str: send str to port dev_num
  • mraa-uart dev dev_num baud customized_baud recv 1000: receive 1000 bytes from port dev_num

Sample Code​

GPIO​

blink.c

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "mraa/gpio.h"

/* gpio declaration */
#define GPIO_PIN 3

volatile sig_atomic_t flag = 1;

void sig_handler(int signum) {
    if(signum == SIGINT) {
        fprintf(stdout, "Exiting...\n");
        flag = 0;
    }
}

int main(void) {
    mraa_result_t status = MRAA_SUCCESS;
    mraa_gpio_context gpio;

    signal(SIGINT, sig_handler);

    mraa_init();

    /* initialize GPIO */
    gpio = mraa_gpio_init(GPIO_PIN);
    if(gpio == NULL) {
        fprintf(stderr, "Failed to initialize GPIO %d\n", GPIO_PIN);
        mraa_deinit();
        return EXIT_FAILURE;
    }

    /* set output */
    status = mraa_gpio_dir(gpio, MRAA_GPIO_OUT);
    if(status != MRAA_SUCCESS) {
        goto err_exit;
    }

    while(flag) {
        status = mraa_gpio_write(gpio, 1);
        if(status != MRAA_SUCCESS) {
            goto err_exit;
        }
        sleep(1);

        status = mraa_gpio_write(gpio, 0);
        if(status != MRAA_SUCCESS) {
            goto err_exit;
        }

        sleep(1);
    }

    /* release gpio's */
    status = mraa_gpio_close(gpio);
    if(status != MRAA_SUCCESS) {
        goto err_exit;
    }

    mraa_deinit();

    return EXIT_SUCCESS;

err_exit:
    mraa_result_print(status);

    mraa_deinit();

    return EXIT_FAILURE;
}

Usage:

  • Connect the led signal pin to PIN3, VCC to the board VCC, GND to the board GND

  • Test

gcc blink.c -lmraa -o blink && sudo . /blink

If the test is successful, the led will have a blinking effect.

I2C​

led_i2c_blink.c

#include <math.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "mraa/i2c.h"

#define I2C_BUS 2
#define LED_ADDRESS 0x20

volatile sig_atomic_t flag = 1;

void sig_handler(int signum) {
	if (signum == SIGINT) {
		fprintf(stdout, "Exiting...\n");
		flag = 0;
	}
}

int main() {
	mraa_result_t status = MRAA_SUCCESS;
	mraa_i2c_context i2c;

	signal(SIGINT, sig_handler);
	mraa_init();

	i2c = mraa_i2c_init(I2C_BUS);
	if(i2c == NULL) {
		fprintf(stderr, "Failed to initialize I2C\n");
		mraa_deinit();
		return EXIT_FAILURE;
	}

	/* set slave address */
	status = mraa_i2c_address(i2c, LED_ADDRESS);
	if(status != MRAA_SUCCESS) {
		return -1;
	}

	while(1) {
		/**
		 * Write a single word to an i2c context
		 *
		 * @param dev The i2c context
		 * @param data The word to write
		 * @param command The register
		 * @return Result of operation
		 */
		mraa_i2c_write_byte_data(i2c, 0x23, 0x08);
		sleep(1);
		mraa_i2c_write_byte_data(i2c, 0x21, 0x01);
		sleep(1);
		mraa_i2c_write_byte_data(i2c, 0x33, 0x12);
		sleep(1);
		mraa_i2c_write_byte_data(i2c, 0x1f, 0x42);
		sleep(1);
	}

	return 0;
}

Usage:

  • Turn on Overlay of I2C2 and reboot.

  • Wiring: SDA <--> SDA, SCL <--> SCL, GDN <--> GND, VCC <--> VCC

  • Open the terminal and enter the following command to test

gcc led_i2c_blink.c -lmraa -o led_i2c_blink && sudo ./led_i2c_blink

If the test is successful, the I2C Led will have a running light effect.

PWM​

led_pwm_fade_out.c

#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include "mraa/pwm.h"

/* PWM period in us */
#define PWM_FREQ 1e3

char *usage =
	"Usage: %s pwm_pin\n"
	"Example: %s 3\n";

int main(int argc, char *argv[]) {
	char usagestr[130];
	char *str = NULL;
	int invalid = 0, pwm_num = 0;
	mraa_result_t status = MRAA_SUCCESS;
	mraa_pwm_context pwm;
	float output = 0.0;
	float duty_cycle = 0.0;
	float step = 0.05;

	memset(usagestr, '\0', 130);
	// expect only 1 argument => argc must be 2
	if(argc != 2) {
		snprintf(usagestr, 129, usage, argv[0], argv[0]);
		puts(usagestr);
		return -1;
	}
	// check for a valid, numeric argument
	str = argv[1];
	while(*str != '\0') {
		if(!isdigit(*str)) {
			invalid = 1;
		}
		str++;
	}
	if(invalid == 1) {
		printf("%s: Invalid GPIO %s\n", argv[0], argv[2]);
		return -1;
	}
	pwm_num = atoi(argv[1]);

	mraa_init();

	/* init pwm */
	if((pwm = mraa_pwm_init(pwm_num)) == NULL) {
		printf("Failed to initialize PWM\n");
		mraa_deinit();
		return -1;
	}
	/* set period */
	if((status = mraa_pwm_period_us(pwm, PWM_FREQ))) {
		printf("Failed to set pwm period\n");
		return -1;
	}
	/* enable pwm */
	if((status = mraa_pwm_enable(pwm, 1)) != MRAA_SUCCESS) {
		printf("Failed to Enable PWM\n");
		return -1;
	}

	while (1) {
		// Set pwm duty cycle
		mraa_pwm_write(pwm, duty_cycle);

		// sleep 100 ms
		usleep(100000);

		// change the duty cycle
		duty_cycle += step;
		if (duty_cycle > 1.0) {
			duty_cycle = 0.0;
		}

		printf("PWM value: %f\n", mraa_pwm_read(pwm));
	}

	mraa_pwm_close(pwm);

	return 0;
}

Usage:

  • Turn on the PWM Overlay of a certain PIN and restart, take the PWM of PIN28 as an example.

  • Connect the signal pin of the led to the PIN28 pin, VCC to the VCC of the board, and GDN to the GND of the board.

  • Test

gcc led_pwm_fade_out.c -lmraa -o led_pwm_fade_out && sudo ./led_pwm_fade_out 28

If the test is successful, the led will have a breathing light effect.

SPI​

spi_test.c

#include <signal.h>
#include <stdlib.h>
#include <unistd.h>

/* mraa header */
#include "mraa/spi.h"

/* SPI declaration */
#define SPI_BUS 0

/* SPI frequency in Hz */
#define SPI_FREQ 400000

int main(int argc, char** argv) {
	mraa_result_t status = MRAA_SUCCESS;
	mraa_spi_context spi;
	int i, j;

	/* initialize mraa for the platform (not needed most of the times) */
	mraa_init();

	//! [Interesting]
	/* initialize SPI bus */
	spi = mraa_spi_init(SPI_BUS);
	if (spi == NULL) {
		fprintf(stderr, "Failed to initialize SPI\n");
		mraa_deinit();
		return EXIT_FAILURE;
	}

	/* set SPI frequency */
	status = mraa_spi_frequency(spi, SPI_FREQ);
	if (status != MRAA_SUCCESS)
		goto err_exit;
	/* set big endian mode */
	status = mraa_spi_lsbmode(spi, 0);
	if (status != MRAA_SUCCESS) {
		goto err_exit;
	}

	while(1) {
		printf("0x%x\n",mraa_spi_write(spi, 0xaa));
		sleep(1);
	}
err_exit:
	mraa_result_print(status);

	/* stop spi */
	mraa_spi_stop(spi);

	/* deinitialize mraa for the platform (not needed most of the times) */
	 mraa_deinit();

	return EXIT_FAILURE;
}

Usage:

  • Turn on spi0 i.e. spidev0.0's Overlay and reboot!

  • Test

Open a terminal and enter the following commands to test

gcc spi_test.c -o spi_test && sudo ./spi_test

If the test is successful, the terminal will output "0xaa".

UART​

snd.c

#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include "mraa/uart.h"

#ifndef FALSE
#define FALSE 0
#define TRUE (!FALSE)
#endif

char *usage =
	"Usage: %s uart_path_id msg\n"
	"Example: %s 4 'this is test message'\n";

int main(int argc, char *argv[]) {
	char usagestr[130];
	char *str = NULL;
	int invalid = 0;
	mraa_uart_context snd_uart;
	mraa_result_t status = MRAA_SUCCESS;

	memset(usagestr, '\0', 130);
	if(argc != 3) {
		snprintf(usagestr, 129, usage, argv[0], argv[0]);
		puts(usagestr);
		return -1;
	}

	mraa_init();

    char uart_path[15];
    snprintf(uart_path, sizeof(uart_path), "/dev/ttyS%d", atoi(argv[1]));
	printf("uart_path: %s, msg: %s\n", uart_path, argv[2]);

	if((snd_uart = mraa_uart_init_raw(uart_path)) == NULL) {
		printf("Failed to initialize UART\n");
		mraa_deinit();
		return -1;
	}

    if ((status = mraa_uart_set_baudrate(snd_uart, 115200)) != MRAA_SUCCESS) {
        return -1;
    }

    if ((status = mraa_uart_set_mode(snd_uart, 8, MRAA_UART_PARITY_NONE, 1)) != MRAA_SUCCESS) {
        return -1;
    }

    if ((status = mraa_uart_set_flowcontrol(snd_uart, FALSE, FALSE)) != MRAA_SUCCESS) {
        return -1;
    }

	while(1) {
		mraa_uart_write(snd_uart, argv[2], strlen(argv[2]));
		sleep(1);
	}

	mraa_uart_stop(snd_uart);
	mraa_deinit();

	return 0;
}

recv.c

#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include "mraa/uart.h"

#ifndef FALSE
#define FALSE 0
#define TRUE (!FALSE)
#endif

char *usage =
	"Usage: %s uart_path_id\n"
	"Example: %s 4\n";


int main(int argc, char *argv[]) {
	char usagestr[130];
	char *str = NULL;
	int invalid = 0, uart_id = 0;
	mraa_uart_context recv_uart;
	mraa_result_t status = MRAA_SUCCESS;

	memset(usagestr, '\0', 130);
	if(argc != 2) {
		snprintf(usagestr, 129, usage, argv[0], argv[0]);
		puts(usagestr);
		return -1;
	}
	// check for a valid, numeric argument
	str = argv[1];
	while(*str != '\0') {
		if(!isdigit(*str)) {
			invalid = 1;
		}
		str++;
	}
	if(invalid == 1) {
		printf("%s: Invalid GPIO %s\n", argv[0], argv[1]);
		return -1;
	}

	mraa_init();

	char uart_path[15];
	snprintf(uart_path, sizeof(uart_path), "/dev/ttyS%d", atoi(argv[1]));
	printf("uart_path: %s\n", uart_path);

	if((recv_uart = mraa_uart_init_raw(uart_path)) == NULL) {
		printf("Failed to initialize UART\n");
		mraa_deinit();
		return -1;
	}

	if ((status = mraa_uart_set_baudrate(recv_uart, 115200)) != MRAA_SUCCESS) {
		return -1;
	}

	if ((status = mraa_uart_set_mode(recv_uart, 8, MRAA_UART_PARITY_NONE, 1)) != MRAA_SUCCESS) {
		return -1;
	}

	if ((status = mraa_uart_set_flowcontrol(recv_uart, FALSE, FALSE)) != MRAA_SUCCESS) {
		return -1;
	}

	char recv_buff[1024];
	while(1) {
		if (mraa_uart_data_available(recv_uart, 1) == TRUE) {
			mraa_uart_read(recv_uart, recv_buff, sizeof(recv_buff));
			printf("recv: %s\n", recv_buff);
			sleep(1);
		}
	}
}

Usage:

  • Open the overlay of two UARTs, take uart4 and uart6 as an example, i.e. /dev/ttyS4 and /dev/ttyS6.

  • Cross-wire the TX and RX pins of the two UARTs.

  • Test

Open two terminals and enter the following commands to test respectively.

gcc recv.c -lmraa -o recv && sudo ./recv 6 # /dev/ttyS6
gcc snd.c -lmraa -o  snd && sudo ./snd 4 "hello, this is test"  # /dev/ttyS4

If the test is successful, uart6 receives the cyclic message "hello, this is test" from uart4.

More​