Extending GPIO with an Arduino connected to the PINE64 using I2C

Although the PINE64 provides quite a decent number of GPIO pins, there are several reasons that you may want to have access to more pins. For example, the Arduino can provide an extra number of native PWM pins, or you may want to implement low-level control of a robot using the Arduino, with high-level operations being handled by the PINE. This post will cover how this can be achieved with the PINE64 and an Arduino Mega. We’ll also create a sketch for the Mega which for handling I2C communication. In the next post, we will write some C and C# code which will show how to send and receive data between the PINE and the Mega. Note that this can be done with any single board computer that supports I2C including any of the Raspberry Pis, the Beagleboard and others.

PINE64 connected to Arduino Mega over I2C

I2C stands for Inter-Integrated Circuit and it is a serial computer bus that enables communication between multiple devices that support the protocol. Every board that supports I2C will have 2 pins called SDA (serial data line) and SCL (serial clock line).

Pins 3 and 5 of the Pi 2 pinout on the PINE64 are the SDA and SCL pins respectively. On the Mega, they are pins 20 and 21. Connect the SDA and SCL pins from the PINE64 to the SDA and SCL pins respectively on the Arduino. I have also connected the 5V from the PINE to the Mega in order for the Mega to be powered by the PINE. If you decide to take this approach, one of the ground pins also has to be connected between both boards.

A closer look at the I2C connection between the PINE64 and the Arduino Mega

Before connecting the Mega, we’ll need to create and upload a sketch that will assign an I2C address which will be used to access the device. The sketch will make use of the Wire library which will be used for I2C communication. We will be making use of byte arrays to send and receive data over the I2C bus. You can come up with a fancy protocol for this, but I came up with the following simple rules.

Maximum length of 16 bytes.
First byte will always be the length (inclusive of the first byte) of the data sent or received.
Second byte is the command. We’ll support 3 simple commands, digital write (0x01 or 1), digital read (0x02 or 2) and analog write (0x03 or 3).
Third byte is the pin number.
For digital write only, fourth byte be a value of either 1 (for high) or 0 (for low).
For analog write only, the next four bytes after the third byte will store an integer value between 0 and 255 inclusive.

With that out of the way, let’s take a look at the sketch. First things first, define our constants and variables.

#include <Wire.h>

// I2C slave address
#define I2C_ADDRESS 0x08

// Custom I2C data commands
#define CMD_DIGITAL_WRITE 0x01
#define CMD_DIGITAL_READ 0x02
#define CMD_ANALOG_WRITE 0x03

// Pin states
#define IO_PIN_STATE_INPUT 0x01
#define IO_PIN_STATE_OUTPUT 0x02

// Buffer for reading and writing data from/to I2C
unsigned char bytes[16];
unsigned char sendBytes[5];

int analogValue = 0;
int lastReadCommand = 0;
int lastReadPin = 0;
int thisPin = 0;
int ioPinStates[53]; // pseudo hashmap
const int ioPinCount = 40;
const int ioPins[] = {
  2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32,
  33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52
};

#include <Wire.h>

// I2C slave address

#define I2C_ADDRESS 0x08

// Custom I2C data commands

#define CMD_DIGITAL_WRITE 0x01

#define CMD_DIGITAL_READ 0x02

#define CMD_ANALOG_WRITE 0x03

// Pin states

#define IO_PIN_STATE_INPUT 0x01

#define IO_PIN_STATE_OUTPUT 0x02

// Buffer for reading and writing data from/to I2C

unsigned char bytes[16];

unsigned char sendBytes[5];

int analogValue = 0;

int lastReadCommand = 0;

int lastReadPin = 0;

int thisPin = 0;

int ioPinStates[53]; // pseudo hashmap

const int ioPinCount = 40;

const int ioPins[] = {

2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32,

33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52

};

The code is straightforward. We define 0x08 as the I2C address that we want the Mega to use. We also define our commands, pin states (for digital read / write), buffers for storing data to be sent and received and other variables that will be used. The ioPins array is a list of all the pins available on the Mega. This will need to be changed to match the board that the sketch will be uploaded to. The ioPinStates is a pseudo hashmap which will map the pin number (used as the array index) to one of the defined pin states (IO_PIN_STATE_INPUT or IO_PIN_STATE_OUTPUT). We’re keeping track of the pin states so that we can activate the pins on demand, instead of activating them all at once in the setup() function.

void setup() {
  // put your setup code here, to run once:
  Wire.begin(I2C_ADDRESS);
  Serial.begin(9600);

  Wire.onReceive(onDataReceived);
  Wire.onRequest(onDataRequested);
}

void loop() {

}

bool isPinValid(int pin) {
  for (int i = 0; i < ioPinCount; i++) {
    if (ioPins[i] == pin) {
      return true;
    }
  }

  return false;
}

void setup() {

// put your setup code here, to run once:

Wire.begin(I2C_ADDRESS);

Serial.begin(9600);

Wire.onReceive(onDataReceived);

Wire.onRequest(onDataRequested);

}

void loop() {

}

bool isPinValid(int pin) {

for (int i = 0; i < ioPinCount; i++) {

if (ioPins[i] == pin) {

return true;

}

return false;

}

The setup function simply initialises the Wire library using the specified I2C address, and enables Serial output which will be used to output debug messages. Wire.onReceive registers the onDataReceived function which will be called when data is sent from the PINE64, while Wire.onRequest registers the onDataRequested function which will be called when the PINE64 requests data from the Mega. The isPinValid function is a helper method which checks if the pin specified as the parameter is valid for the board. It checks the pin against the ioPins array that we defined earlier.

Next is the onDataReceived function which handles most of the work. It accepts an argument which represents the number of bytes that were received.

void onDataReceived(int byteCount) {
  int i = 0;
  int idx = 0;

  while (Wire.available()) {
    bytes[idx] = Wire.read();
    
    if ((idx + 1) == bytes[0]) { // Length received
      int len = (int) bytes[0];
      if (len < 3) {
        Serial.print("Invalid data received.");
        return;
      }

void onDataReceived(int byteCount) {

int i = 0;

int idx = 0;

while (Wire.available()) {

bytes[idx] = Wire.read();

if ((idx + 1) == bytes[0]) { // Length received

int len = (int) bytes[0];

if (len < 3) {

Serial.print("Invalid data received.");

return;

}

The while loop checks if there is data available from the Wire library. If there is, the absolute minimum number of bytes received that can be considered valid based on the rules we defined earlier is 3 (length, command, pin). If the number of bytes received is less than 3, then the function ends at that point and output is written to the Serial console. The next step is to use a switch statement to check and handle the command that was received. The second byte (index 1) contains this data.

      switch (bytes[1]) {
        case CMD_DIGITAL_WRITE:
          if (len < 4) {
            Serial.println("Incomplete digital write data.");
            return;
          }
        
          thisPin = (int) bytes[2];
          if (!isPinValid(thisPin)) {
            Serial.println("Invalid pin specified for digital write.");
            return;
          }
      
          // do digital write
          if (ioPinStates[thisPin] != IO_PIN_STATE_OUTPUT) {
            pinMode(thisPin, OUTPUT);
            ioPinStates[thisPin] = IO_PIN_STATE_OUTPUT;
          }
          
          digitalWrite(thisPin, bytes[3]);
      
          break;

switch (bytes[1]) {

case CMD_DIGITAL_WRITE:

if (len < 4) {

Serial.println("Incomplete digital write data.");

return;

}

thisPin = (int) bytes[2];

if (!isPinValid(thisPin)) {

Serial.println("Invalid pin specified for digital write.");

return;

}

// do digital write

if (ioPinStates[thisPin] != IO_PIN_STATE_OUTPUT) {

pinMode(thisPin, OUTPUT);

ioPinStates[thisPin] = IO_PIN_STATE_OUTPUT;

}

digitalWrite(thisPin, bytes[3]);

break;

For digital write, the minimum number of bytes to be considered valid is 4 (length, command, pin, value). The function is terminated if we received less than 4 bytes for the command. The isPinValid is called to check if the pin received is valid, and if it isn’t, the function ends at that point. Next thing to be done is to check if the pin has been activated. We make use of the ioPinStates array to do this making use of the pin number as the index. If the pin has not yet been activated (IO_PIN_STATE_OUTPUT), then we activate the pin using pinMode. Once this check is complete, we can call digitalWrite using the pin and the value specified.

        case CMD_DIGITAL_READ:
          if (len < 3) {
            Serial.println("Incomplete digital read data.");
            return;
          }
      
          thisPin = (int) bytes[2];
          if (!isPinValid(thisPin)) {
            Serial.println("Invalid pin specified for digital read.");
            return;
          }
      
          if (ioPinStates[thisPin] != IO_PIN_STATE_OUTPUT) {
            pinMode(thisPin, OUTPUT);
            ioPinStates[thisPin] = IO_PIN_STATE_OUTPUT;
          }
      
          lastReadCommand = CMD_DIGITAL_READ;
          lastReadPin = thisPin;
      
          break;

case CMD_DIGITAL_READ:

if (len < 3) {

Serial.println("Incomplete digital read data.");

return;

}

thisPin = (int) bytes[2];

if (!isPinValid(thisPin)) {

Serial.println("Invalid pin specified for digital read.");

return;

}

if (ioPinStates[thisPin] != IO_PIN_STATE_OUTPUT) {

pinMode(thisPin, OUTPUT);

ioPinStates[thisPin] = IO_PIN_STATE_OUTPUT;

}

lastReadCommand = CMD_DIGITAL_READ;

lastReadPin = thisPin;

break;

Digital read also follows the same set of steps as digital write (validate date length, validate pin, check pin state) but we will call the actual digitalRead function in onDataRequested. What we do here is store the command and the pin in variables (lastReadCommand and lastReadPin respectively) which we can then make use of in onDataRequested.

        case CMD_ANALOG_WRITE:
          if (len < 7) {
            Serial.println("Incomplete analog write data.");
            return;
          }
      
          thisPin = (int) bytes[2];
      
          if (!isPinValid(thisPin)) {
            Serial.println("Invalid pin specified for analog write.");
            return;
          }
      
          analogValue = 0;
          analogValue = (uint32_t) bytes[3] << 24;
          analogValue |= (uint32_t) bytes[4] << 16;
          analogValue |= (uint32_t) bytes[5] << 8;
          analogValue |= (uint32_t) bytes[6];
        
          if (analogValue < 0 || analogValue > 255) {
            Serial.println("Invalid value specified for analog write.");
            return;
          }
      
          analogWrite(thisPin, analogValue);
          
          break;

case CMD_ANALOG_WRITE:

if (len < 7) {

Serial.println("Incomplete analog write data.");

return;

}

thisPin = (int) bytes[2];

if (!isPinValid(thisPin)) {

Serial.println("Invalid pin specified for analog write.");

return;

}

analogValue = 0;

analogValue = (uint32_t) bytes[3] << 24;

analogValue |= (uint32_t) bytes[4] << 16;

analogValue |= (uint32_t) bytes[5] << 8;

analogValue |= (uint32_t) bytes[6];

if (analogValue < 0 || analogValue > 255) {

Serial.println("Invalid value specified for analog write.");

return;

}

analogWrite(thisPin, analogValue);

break;

Similar to digital write, analog write follows a couple of steps (validate data length and validate pin). We don’t need to check or set the pin state before calling the analogWrite function. We check that the value is between 0 and 255 inclusive before calling analogWrite with the pin and the value as the arguments.

        default:
          Serial.println("Unrecognised command.");
          break;
      }

      idx = 0;
    }
    
    idx++;
  }
}

default:

Serial.println("Unrecognised command.");

break;

}

idx = 0;

}

idx++;

}

If the data sent did not match any of the defined commands, the code falls back to the default statement which outputs Unrecognised command. to the serial command, and then the onDataReceived function will be called again when new data is received.

Finally, we have the onDataRequested function which makes use of lastReadCommand and lastReadPin. The function is straightforward, as it uses the Wire library to send data back to the PINE following our simple rules.

void onDataRequested() {
  switch (lastReadCommand)
  {
    case CMD_DIGITAL_READ:
        // Send the value over I2C
        sendBytes[0] = 3; // length
        sendBytes[1] = lastReadPin; // the pin that was read
        sendBytes[2] = digitalRead(lastReadPin); // the pin value (0/1)
        Wire.write(sendBytes, sendBytes[0]);
        break;
  }
}

void onDataRequested() {

switch (lastReadCommand)

{

case CMD_DIGITAL_READ:

// Send the value over I2C

sendBytes[0] = 3; // length

sendBytes[1] = lastReadPin; // the pin that was read

sendBytes[2] = digitalRead(lastReadPin); // the pin value (0/1)

Wire.write(sendBytes, sendBytes[0]);

break;

}

And that’s it! Compile the sketch using the Arduino IDE and then upload it to your board. Connect your Arduino to the PINE after the sketch is successfully uploaded, and boot up the PINE. You can obtain the full code listing for the sketch at https://gitlab.com/akinwale/nanitei2c/blob/master/nanitei2c_mega.ino.

Install i2c-tools using sudo apt-get install i2c-tools. By default, only root can use the I2C commands, but you can add the user account with useradd -G i2c ubuntu (replace ubuntu with the username that you want to use to access I2C). Reboot the PINE and then scan the I2C bus with the command, i2cdetect -y 1. You should get output should be similar to the following:

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- 08 -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

0 1 2 3 4 5 6 7 8 9 a b c d e f

00: -- -- -- -- -- 08 -- -- -- -- -- -- --

10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

70: -- -- -- -- -- -- -- --

Based on this output, we can see that the Mega was recognised over the I2C bus with the configured address in our sketch (0x08 or 8). With this, we have access to the extra pins which we will be able to control directly from the PINE. That’s pretty neat. In the next post, we will write the C and C# code for the PINE for handling I2C communication.

Control GPIO pins on the PINE64 with C#

Similar to the Raspberry Pi, GPIO pins on the PINE64 can be controlled through sysfs. You can refer to my previous post which goes into the concept in detail, and the C# code remains the same for the PINE64. However, with the longsleep Ubuntu image, root access is required to control the GPIO pins. You will need to grant the necessary permissions in order to be able to control GPIO pins as a normal user.

Granting user permissions
We’ll assume that we want to be able to control the pins as the default ubuntu user. Follow these steps to grant the necessary permissions.

Create a user group called gpio.
groupadd gpio
Add the ubuntu user to the gpio group.
useradd -G gpio ubuntu

Add a udev rule to run chown on the sysfs files. The chown command will set group ownership to the gpio group. Adding the udev rule will run the chown command automatically whenever you export a pin. Create a file called 99-com.rules in /etc/dev/rules.d and paste the following contents.

SUBSYSTEM=="gpio*", PROGRAM="/bin/sh -c 'chown -R root:gpio /sys/class/gpio && chmod -R 770 /sys/class/gpio; chown -R root:gpio /sys/devices/soc.0/1c20800.pinctrl/gpio && chmod -R 770 /sys/devices/soc.0/1c20800.pinctrl/gpio; chown -R root:gpio /sys/devices/soc.0/1f02c00.pinctrl/gpio && chmod -R 770 /sys/devices/soc.0/1f02c00.pinctrl/gpio'"

SUBSYSTEM=="gpio*", PROGRAM="/bin/sh -c 'chown -R root:gpio /sys/class/gpio && chmod -R 770 /sys/class/gpio; chown -R root:gpio /sys/devices/soc.0/1c20800.pinctrl/gpio && chmod -R 770 /sys/devices/soc.0/1c20800.pinctrl/gpio; chown -R root:gpio /sys/devices/soc.0/1f02c00.pinctrl/gpio && chmod -R 770 /sys/devices/soc.0/1f02c00.pinctrl/gpio'"

Physical pin to GPIO number mapping
I took some time to test the physical pins on the PINE in order to determine the sysfs gpio numbers and came up with this table. As an example, physical pin 22 on the Pi 2 pinout corresponds to /sys/class/gpio/gpio79.

Pin #	GPIO #
Pi 2 pinout
3	227
5	226
8	32
10	33
11	71
12	72
13	233
15	76
16	77
18	78
19	64
21	65
22	79
23	66
24	67
26	231
27	361
28	360
29	229
31	230
32	68
33	69
35	73
36	70
37	80
38	74
40	75

Euler pinout
7	363
10	232
11	35
12	36
13	37
15	38
16	39
18	100
19	98
21	99
22	101
23	97
24	96
26	102
27	34
28	103
29	40
30	41

Exp pinout
2	359
7	40
8	41

How to control GPIO pins on the Raspberry Pi 3 using C#

GPIO pins on the Raspberry Pi can be controlled using the sysfs interface, which is a virtual filesystem that the Linux kernel provides. In this guide, we will write a basic C# class to control available pins on the Pi through sysfs.

Understanding the sysfs interface
sysfs provides access to the GPIO pins at the path /sys/class/gpio. You can cd into this path and ls to list files in the directory. There are two special files here which are export and unexport. You write to the export file to activate a particular pin, while writing to unexport deactivates the pin. The following example activates GPIO pin 18.

echo 18 > /sys/class/gpio/export

1	echo 18 > /sys/class/gpio/export

You can verify that the pin is activated by listing the files in the /sys/class/gpio directory. You should see a gpio18 folder in the directory listing. After the pin has been activated, you should specify whether the pin should be an input or output pin before you can read or write values. You do this for input like so:

echo in > /sys/class/gpio/gpio18/direction

1	echo in > /sys/class/gpio/gpio18/direction

Or for output:

echo out > /sys/class/gpio/gpio18/direction

1	echo out > /sys/class/gpio/gpio18/direction

If the pin is specified as an output pin, you can write a value of either 0 (low) or 1 (high) for the pin. If a LED is connected to the pin for this example, a value of 0 will turn the LED off, while a value of 1 will turn the LED on. To specify the pin value, you can do this:

echo 1 > /sys/class/gpio/gpio18/value
echo 0 > /sys/class/gpio/gpio18/value

1 2	echo 1 > /sys/class/gpio/gpio18/value echo 0 > /sys/class/gpio/gpio18/value

Once you are done with the pin, you can deactivate it using:

echo 18 > /sys/class/gpio/unexport

1	echo 18 > /sys/class/gpio/unexport

Writing the C# class
Now that we have an idea of how sysfs works, we can create a class to implement the necessary steps. The sysfs approach basically requires writing values to the file, so we can use simple file I/O operations to achieve the desired result. The full listing for the GPIO class can be found at https://gitlab.com/akinwale/NaniteIo/blob/master/Nanite/IO/GPIO.cs.

The first thing we’ll do is add the using statements for the namespaces. System.IO is required for FileStream, StreamReader and StreamWriter which are used for file I/O. System.Threading is required for the Thread class, while Nanite.Exceptions contains the custom exceptions defined for our project. We’ll also define enumerations for the GPIO direction and value, and a few constants for strings like the GPIO path and other special files. The class will be defined as static, because we do not need to create an instance of the class.

using System.IO;
using System.Threading;
using Nanite.Exceptions;

namespace Nanite.IO
{
    public static class GPIO
    {
        public enum Direction
        {
            Input = 0,
            Output = 1
        };

        public enum Value
        {
            Low = 0,
            High = 1
        };

        private const string GPIOPath = "/sys/class/gpio";

        private const string GPIODirection = "direction";

        private const string GPIOExport = "export";

        private const string GPIOUnexport = "unexport";

        private const string GPIOValue = "value";

using System.IO;

using System.Threading;

using Nanite.Exceptions;

namespace Nanite.IO

{

public static class GPIO

{

public enum Direction

{

Input = 0,

Output = 1

};

public enum Value

{

Low = 0,

High = 1

};

private const string GPIOPath = "/sys/class/gpio";

private const string GPIODirection = "direction";

private const string GPIOExport = "export";

private const string GPIOUnexport = "unexport";

private const string GPIOValue = "value";

Pretty straightforward so far. The first method we’re going to define is the PinMode method, which will take the pin number and direction as parameters. This method will activate the pin and then set the direction to either in or out depending on the specified parameter value.

public static void PinMode(int pin, Direction direction)
{
    ClosePin(pin);

    string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));
    if (!Directory.Exists(pinPath))
    {
        try
        {
            using (StreamWriter writer = new StreamWriter(new FileStream(
                Path.Combine(GPIOPath, GPIOExport), FileMode.Open, FileAccess.Write, FileShare.ReadWrite)))
            {
                writer.Write(pin);
            }
        }
        catch (IOException ex)
        {
            throw new GPIOException("Unable to export the pin.", ex);
        }
    }

    do
    {
        // Wait until the pin has been initialised properly before setting the direction
        Thread.Sleep(500);
    } while (!Directory.Exists(pinPath));

    try
    {
        using (StreamWriter writer = new StreamWriter(new FileStream(
            Path.Combine(pinPath, GPIODirection), FileMode.Truncate, FileAccess.Write, FileShare.ReadWrite)))
        {
            writer.Write(direction == Direction.Input ? "in" : "out");
        }
    }
    catch (IOException ex)
    {
        throw new GPIOException("Unable to set the pin direction.", ex);
    }
}

public static void PinMode(int pin, Direction direction)

{

ClosePin(pin);

string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));

if (!Directory.Exists(pinPath))

{

try

{

using (StreamWriter writer = new StreamWriter(new FileStream(

Path.Combine(GPIOPath, GPIOExport), FileMode.Open, FileAccess.Write, FileShare.ReadWrite)))

{

writer.Write(pin);

}

catch (IOException ex)

{

throw new GPIOException("Unable to export the pin.", ex);

}

{

// Wait until the pin has been initialised properly before setting the direction

Thread.Sleep(500);

} while (!Directory.Exists(pinPath));

try

{

using (StreamWriter writer = new StreamWriter(new FileStream(

Path.Combine(pinPath, GPIODirection), FileMode.Truncate, FileAccess.Write, FileShare.ReadWrite)))

{

writer.Write(direction == Direction.Input ? "in" : "out");

}

catch (IOException ex)

{

throw new GPIOException("Unable to set the pin direction.", ex);

}

We build the pinPath string making use of Path.Combine(GPIOPath, string.Format("gpio{0}", pin));. If the value specified for the pin parameter is 18, pinPath will contain the string, "/sys/class/gpio/gpio18". The ClosePin method call is optional, but the idea behind this is that the pin should be deactivated first before activating. We also check if the gpio pin directory exists using if (!Directory.Exists(pinPath)) before activating to make sure we are not activating a pin that has already been activated.

After the request for pin activation, there may be a small delay which is why we have a while loop which waits until the corresponding gpio pin directory has been created before we set the pin direction. Thread.Sleep(500) makes the program wait 500 milliseconds before proceeding to the next statement. Note that this while loop is completely optional, but it acts as a safeguard against setting the pin direction before the gpio pin directory has been created by the system. One thing to take note of is if the gpio pin directory never gets created (for instance, if the pin is invalid), the loop may end up running forever. To fix this, we can set a maximum number of times the loop should run before ending the loop.

int loopCount = 1;
do
{
    if (loopCount > 5)
    {
        // Exit the loop after it has been run 5 times (waited for 2.5s)
        break;
    }

    // Wait until the pin has been initialised properly before setting the direction
    Thread.Sleep(500);
    loopCount++;
} while (!Directory.Exists(pinPath));

int loopCount = 1;

{

if (loopCount > 5)

{

// Exit the loop after it has been run 5 times (waited for 2.5s)

break;

}

// Wait until the pin has been initialised properly before setting the direction

Thread.Sleep(500);

loopCount++;

} while (!Directory.Exists(pinPath));

The next method is the ClosePin method which takes the pin number as a parameter. This method checks if the pin directory exists before it writes the pin number to the /sys/class/gpio/unexport file.

public static void ClosePin(int pin)
{
    string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));
    if (Directory.Exists(pinPath))
    {
        try
        {
            using (StreamWriter writer = new StreamWriter(new FileStream(
                Path.Combine(GPIOPath, GPIOUnexport), FileMode.Open, FileAccess.Write, FileShare.ReadWrite)))
            {
                writer.Write(pin);
            }
        }
        catch (IOException ex)
        {
            throw new GPIOException("Unable to close the pin.", ex);
        }
    }
}

public static void ClosePin(int pin)

{

string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));

if (Directory.Exists(pinPath))

{

try

{

using (StreamWriter writer = new StreamWriter(new FileStream(

Path.Combine(GPIOPath, GPIOUnexport), FileMode.Open, FileAccess.Write, FileShare.ReadWrite)))

{

writer.Write(pin);

}

catch (IOException ex)

{

throw new GPIOException("Unable to close the pin.", ex);

}

We create the Write method to write a value to a pin. It takes two parameters, the pin number and the value which is of the Value enumerator type with possible values Value.Low or Value.High. In this method, we make use Path.Combine to create the full path to the value file in the gpio pin directory. For pin 18, this will be "/sys/class/gpio/gpio18/value". If value for the value parameter is Value.Low, we write 0 to the file, otherwise if it’s Value.High, we write 1 to the file.

public static void Write(int pin, Value value)
{
    try
    {
        string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));
        using (StreamWriter writer = new StreamWriter(new FileStream(
            Path.Combine(pinPath, GPIOValue), FileMode.Truncate, FileAccess.Write, FileShare.ReadWrite)))
        {
            writer.Write(value == Value.Low ? 0 : 1);
        }
    }
    catch (IOException ex)
    {
        throw new GPIOException("Unable to write the pin value.", ex);
    }
}

public static void Write(int pin, Value value)

{

try

{

string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));

using (StreamWriter writer = new StreamWriter(new FileStream(

Path.Combine(pinPath, GPIOValue), FileMode.Truncate, FileAccess.Write, FileShare.ReadWrite)))

{

writer.Write(value == Value.Low ? 0 : 1);

}

catch (IOException ex)

{

throw new GPIOException("Unable to write the pin value.", ex);

}

Finally, we have our Read method to read a value from a pin. It will return either Value.Low or Value.High depending on what the pin has been set to. The question mark at the end of the method return type indicates that we can return null for the method if the value retrieved is invalid.

public static Value? Read(int pin)
{
    int pinValue = -1;

    try
    {
        string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));
        using (StreamReader reader = new StreamReader(new FileStream(
            Path.Combine(pinPath, GPIOValue), FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
        {
            int.TryParse(reader.ReadToEnd(), out pinValue);
        }
    }
    catch (IOException ex)
    {
        throw new GPIOException("Unable to read the pin value.", ex);
    }

    if (pinValue != 0 && pinValue != 1)
    {
        return null;
    }

    return (Value) pinValue;
}

public static Value? Read(int pin)

{

int pinValue = -1;

try

{

string pinPath = Path.Combine(GPIOPath, string.Format("gpio{0}", pin));

using (StreamReader reader = new StreamReader(new FileStream(

Path.Combine(pinPath, GPIOValue), FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))

{

int.TryParse(reader.ReadToEnd(), out pinValue);

}

catch (IOException ex)

{

throw new GPIOException("Unable to read the pin value.", ex);

}

if (pinValue != 0 && pinValue != 1)

{

return null;

}

return (Value) pinValue;

}

To determine if the retrieved value is valid, we add a couple of checks in the method. The first is the int.TryParse method, which returns false if the retrieved value is not a valid integer. Then verify that the value is either 0 or 1 using if (pinValue != 0 && pinValue != 1). If it’s neither 0 nor 1, null is returned. Otherwise, the corresponding enumeration value is returned by casting the integer to GPIO.Value.

Finally, we can put this all together in a sample program. If a LED is connected to pin 18, the LED will light up when the value is set to High and turn off when the value is set to Low.

using System;
using System.Threading;
using Nanite.IO;
using Nanite.Exceptions;

namespace NaniteIoSample
{
    class Program
    {
        public static void Main(string[] args)
        {
            // Activate the pin and set the direction as Output
            GPIO.PinMode(18, GPIO.Direction.Output);

            // Blink a LED connected to pin 18 up to 50 times
            for (int i = 0; i < 50; i++)
            {
                GPIO.Write(18, GPIO.Value.High);
                Thread.Sleep(200);

                GPIO.Write(18, GPIO.Value.Low);
                Thread.Sleep(200);
            }

            // Close the pin after we're done
            GPIO.ClosePin(18);
        }
    }
}

using System;

using System.Threading;

using Nanite.IO;

using Nanite.Exceptions;

namespace NaniteIoSample

{

class Program

{

public static void Main(string[] args)

{

// Activate the pin and set the direction as Output

GPIO.PinMode(18, GPIO.Direction.Output);

// Blink a LED connected to pin 18 up to 50 times

for (int i = 0; i < 50; i++)

{

GPIO.Write(18, GPIO.Value.High);

Thread.Sleep(200);

GPIO.Write(18, GPIO.Value.Low);

Thread.Sleep(200);

}

// Close the pin after we're done

GPIO.ClosePin(18);

}

Source Code
The full code listing for the GPIO class can be obtained from https://gitlab.com/akinwale/NaniteIo/blob/master/Nanite/IO/GPIO.cs.