Introduction

Welcome to Part - 3 of Our path to libmraa with 96Boards series. This blog series is intended to provide a roadmap of our work towards standardizing libmraa library for 96Boards. Before going into the current part, I’d like to give a quick recap of the previous parts.

1. Part 1 - Getting started with libmraa for 96Boards - This is the introductory blog focussed on providing a roadmap to libmraa from 96Boards perspective. Also, this blog provided library breakdown, how to get started instructions, introduction to libupm and much more.
2. Part 2 - Our Path to libmraa with 96Boards - Here we discussed briefly about the organization of libupm on top of libmraa. Towards the end, libupm library breakdown was also provided along with an example to interface Accelerometer and LCD with Dragonboard.

GPIO input pull-up/down support

GPIO (General Purpose Input Output) is one of the most common subsystem present in all Single Board Computers. It provides the IO access to control LED’s, switches etc… A single GPIO line can be configured as Input or Output depending upon the configuration (It might not be the case sometimes).

|     Output    |     Input     |
| ------------- | ------------- |
|     High      |     Pull Up   |
|     Low       |     Pull Down |
---------------------------------

You might be wondering about the need of Pull-up/down modes in input, since we are only sampling the pin. But there are times when you would want the input pin to be in a stable state (High/Low) in order to detect an event.

Following code snippet is one of the use case:

While (gpio->HIGH); // Wait here until GPIO becomes low
/* Do something */

If the pin is in floating state (neither high nor low), it will be difficult to provide waiting condition for detecting an event. For aiding this purpose, there are two methods employed.

1. Pull-up/down in Software
2. External Pull-up/down circuitry

First method requires no external circuit to be present but the second one requires some crafty circuitry work to be present. This might increase the BoM cost unnecessarily.

As we saw in Part 1, libmraa still uses the sysfs interface for accessing GPIO’s and our support for adding chardev interface is in progress. Sysfs interface supports the GPIO input to be configured as pull-up/down mode using the active_low node present under /sys/class/gpio/gpioN/.

But there was no support present in libmraa for GPIO pull-up/down mode [Thanks to Sahaj Sarup for pointing this]. Then we decided to add this support to libmraa. So, I created an initial Pull Request consists of support for pull-up/down mode along with C++ binding.

Pull Request: (https://github.com/intel-iot-devkit/mraa/pull/768)

After few days of the submission, it was accepted into libmraa by one of its maintainers.

Pull request breakdown

API for setting pull-up/down mode is mraa_gpio_input_mode. You need to specify the input mode using two enum types:

C:

1. MRAA_GPIO_ACTIVE_HIGH
2. MRAA_GPIO_ACTIVE_LOW

C++:

1. MODE_IN_ACTIVE_HIGH
2. MODE_IN_ACTIVE_LOW

Core support has been added to gpio source in src/gpio/gpio.c and C++ binding in api/mraa/gpio.hpp.

Example Usage:

The following code snippet could be used to leverage the pull-up/down support in libmraa.

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

#include "mraa.h"

#define PIN 23

int
main(int argc, char** argv)
{
    mraa_result_t res = MRAA_SUCCESS;
    int iopin = PIN;

    mraa_init();
    mraa_gpio_context gpio;
    gpio = mraa_gpio_init(iopin);
    if (gpio == NULL) {
        fprintf(stderr, "Are you sure that pin%d you requested is valid on your platform?",    iopin);
        exit(1);
    }
    printf("Initialised pin%d\n", iopin);

    // set direction to IN
    res = mraa_gpio_dir(gpio, MRAA_GPIO_IN);
    if (res != MRAA_SUCCESS) {
        mraa_result_print(res);
        exit(1);
    }

    //Set pull up mode
    res = mraa_gpio_input_mode(gpio, MRAA_GPIO_ACTIVE_LOW);
    if (res != MRAA_SUCCESS) {
        mraa_result_print(res);
        exit(1);
    }

    printf("Pin%d set to pull up mode\n", iopin);

    return res;
}

There are further plans to add more binding support to this API and to read the state of pull-up/down mode.

Conclusion

That’s it for the Part 3 of Our Path to libmraa with 96Boards blog series. I hope this blog series will help libmraa to emerge as a de-facto standard for our 96Boards. See you with something more in next part :-) As we always say, please provide your valuable feedback/queries in comments.

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