Controlling an Arduino connected to the PINE64 over I2C with C and C#

In a previous post, we connected an Arduino Mega to the PINE64 and wrote a sketch for the Mega for data communication. The data to be sent and received will follow the simple rules which were listed in the post. Next, we’re going to write a shared library with some C code to interface with I2C natively, and then a C# class which will call the shared library and bring everything together. Just a quick recap of the data rules:

Maximum length: 16 bytes.
First byte: Number of bytes sent/received.
Second byte: Control command. CMD_DIGITAL_WRITE (0x01 or 1), CMD_DIGITAL_READ (0x02 or 2) and CMD_ANALOG_WRITE (0x03 or 3).
Third byte: Pin number.
Fourth byte: Value of either 1 (for high) or 0 (for low) for CMD_DIGITAL_WRITE, or
Fourth to seventh bytes: Integer value for CMD_ANALOG_WRITE.

Linux has native I2C support which lets us communicate directly with any device connected to the bus. Since native communication is possible, we create a shared library with methods which can be invoked from our C# code. Since we can open the connection to a device on the I2C bus as a file descriptor, we will be making use of fcntl.h for the descriptor. Let’s take a look at the C library.

#include <fcntl.h>
#include <unistd.h>
#include <linux/i2c-dev.h>

#define MAX_BYTES 16

/**
 * Open an I2C connection using the specified file descriptor.
 */
int nanite_i2c_open(const char *filename, int slave_address)
{
    int fd;

    if ((fd = open(filename, O_RDWR)) < 0)
    {
        return -1;
    }

    if (ioctl(fd, I2C_SLAVE, slave_address) < 0)
    {
        return -1;
    }

    return fd;
}

#include <fcntl.h>

#include <unistd.h>

#include <linux/i2c-dev.h>

#define MAX_BYTES 16

/**

* Open an I2C connection using the specified file descriptor.

int nanite_i2c_open(const char *filename, int slave_address)

{

int fd;

if ((fd = open(filename, O_RDWR)) < 0)

{

return -1;

}

if (ioctl(fd, I2C_SLAVE, slave_address) < 0)

{

return -1;

}

return fd;

}

The first thing we do is include required headers and define a constant specifying the maximum number of bytes, which is 16. Then we create the nanite_i2c_open function try to open the specified device file (usually /dev/i2c-1). Next we call ioctl so that we will be able to control the device using a reference to the file descriptor, and the specified slave address. The file descriptor is returned for reference in other function calls. If any of the steps in the function fails, -1 is returned to indicate an error condition. Other functions in the library will handle the failure the same way.

The nanite_i2c_close function is fairly simple. It takes the file descriptor as an argument and checks if it is open in order to close it.

/**
 * Close the I2C connection using the specified file descriptor.
 */
int nanite_i2c_close(int fd)
{
    if (fd > 0)
    {
        close(fd);
    }

    return 0;
}

/**

* Close the I2C connection using the specified file descriptor.

int nanite_i2c_close(int fd)

{

if (fd > 0)

{

close(fd);

}

return 0;

}

Next is the nanite_i2c_send function which takes the file descriptor and the data to be sent as arguments. The bytes are written to the open file descriptor, and the number of bytes written is verified. If it does not match the length defined in the first byte, -1 is returned to indicate an error condition.

/**
 * Send up to the maximum number of bytes over I2C using the specified file descriptor.
 */
int nanite_i2c_send(int fd, const char bytes[MAX_BYTES])
{
    if (fd <= 0 || !bytes)
    {
        return -1;
    }

    int num_bytes;
    num_bytes = write(fd, bytes, bytes[0]);
    if (num_bytes != bytes[0])
    {
        return -1;
    }

    return num_bytes;
}

/**

* Send up to the maximum number of bytes over I2C using the specified file descriptor.

int nanite_i2c_send(int fd, const char bytes[MAX_BYTES])

{

if (fd <= 0 || !bytes)

{

return -1;

}

int num_bytes;

num_bytes = write(fd, bytes, bytes[0]);

if (num_bytes != bytes[0])

{

return -1;

}

return num_bytes;

}

The nanite_i2c_read function will be used to read data over I2C. The first byte is retrieved in order to determine how many more bytes to read. Then we validate the number of bytes that were received with the expected length. 0 is returned if the operation was successful.

/**
 * Read the byte data from I2C.
 */
int nanite_i2c_read(int fd, char bytes_read[MAX_BYTES])
{
    if (fd <= 0)
    {
        return -1;
    }

    char fbyte[1];
    // Read the first byte to get the total number of bytes to read
    if (read(fd, fbyte, 1) != 1)
    {
        return -1;
    }

    if (read(fd, bytes_read, fbyte[0]) != fbyte[0])
    {
        return -1;
    }

    return 0;
}

/**

* Read the byte data from I2C.

int nanite_i2c_read(int fd, char bytes_read[MAX_BYTES])

{

if (fd <= 0)

{

return -1;

}

char fbyte[1];

// Read the first byte to get the total number of bytes to read

if (read(fd, fbyte, 1) != 1)

{

return -1;

}

if (read(fd, bytes_read, fbyte[0]) != fbyte[0])

{

return -1;

}

return 0;

}

The final function in the library is nanite_i2c_max_bytes() which returns MAX_BYTES. This gives us a complete library that we can use for I2C data communication. You can create the shared library using gcc -shared -o libnanitei2c.so -fPIC main.c. The full code listing for the library is available at https://gitlab.com/akinwale/nanitei2c/blob/master/main.c.

Using DllImport, we can call functions from the shared library in the C# code which we’re going to look at next. We create our class, define the supported commands as an enumeration and specify the private members. We also define a couple of constructors with the default constructor using the default I2C device file which is /dev/i2c-1 on the PINE.

using System;
using System.Runtime.InteropServices;
using Nanite.Exceptions;

namespace Nanite.IO
{
    /// <summary>
    /// 
    /// </summary>
    public class I2CExtendedIO : IDisposable
    {
        public enum Commands
        {
            DigitalWrite = 1,
            DigitalRead = 2,
            AnalogWrite = 3
        }

        private const string DefaultI2CFilename = "/dev/i2c-1";

        private Object lockObject = new Object();

        private int fileDescriptor;

        private string i2cFilename;

        public I2CExtendedIO(string i2cFilename)
        {
            this.i2cFilename = i2cFilename;
        }

        public I2CExtendedIO()
            : this(DefaultI2CFilename)
        {

        }

using System;

using System.Runtime.InteropServices;

using Nanite.Exceptions;

namespace Nanite.IO

{

/// <summary>

///

/// </summary>

public class I2CExtendedIO : IDisposable

{

public enum Commands

{

DigitalWrite = 1,

DigitalRead = 2,

AnalogWrite = 3

}

private const string DefaultI2CFilename = "/dev/i2c-1";

private Object lockObject = new Object();

private int fileDescriptor;

private string i2cFilename;

public I2CExtendedIO(string i2cFilename)

{

this.i2cFilename = i2cFilename;

}

public I2CExtendedIO()

: this(DefaultI2CFilename)

{

}

The DllImport calls are defined within the class and are used to map the functions from the library to functions that we can call in the I2CExtendedIO class. For instance, to call the nanite_i2c_send in the class, we’ll make use of I2CSendBytes.

        [DllImport("libnanitei2c.so", EntryPoint="nanite_i2c_open")]
        internal static extern int OpenI2C(string filename, int slaveAddress);

        [DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_close")]
        internal static extern int CloseI2C(int fileDescriptor);

        [DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_send")]
        internal static extern int I2CSendBytes(int fileDescriptor, byte[] bytes);

        [DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_read")]
        internal static extern int I2CReadBytes(int fileDescriptor, byte[] bytesRead);

        [DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_max_bytes")]
        internal static extern int MaxByteCount();

[DllImport("libnanitei2c.so", EntryPoint="nanite_i2c_open")]

internal static extern int OpenI2C(string filename, int slaveAddress);

[DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_close")]

internal static extern int CloseI2C(int fileDescriptor);

[DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_send")]

internal static extern int I2CSendBytes(int fileDescriptor, byte[] bytes);

[DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_read")]

internal static extern int I2CReadBytes(int fileDescriptor, byte[] bytesRead);

[DllImport("libnanitei2c.so", EntryPoint = "nanite_i2c_max_bytes")]

internal static extern int MaxByteCount();

Here, we’ve defined our class dispose function which closes the file descriptor if it is open. We also have a bunch of simple helper functions which will be called within the class.

        private void ValidateDigitalReadData(int pin, byte[] bytesReceived)
        {
            if (bytesReceived[0] < 3)
            {
                throw new ExtendedIOException("Invalid response data received over the I2C connection.");
            }

            if (bytesReceived[1] != pin)
            {
                throw new ExtendedIOException(string.Format("Pin mismatch for the DigitalRead command. Expected {0} but got {1}.", pin, bytesReceived[1]));
            }

            if (!IsGPIOValueValid(bytesReceived[2]))
            {
                throw new ExtendedIOException("The returned pin value is not valid.");
            }
        }


        private static bool IsGPIOValueValid(int value)
        {
            return ( (value == (int) GPIO.Value.Low) || (value == (int) GPIO.Value.High) );
        }

        private void CheckFileDescriptor()
        {
            if (fileDescriptor <= 0)
            {
                throw new ExtendedIOException("I2C connection is not open.");
            }
        }

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        ~I2CExtendedIO()
        {
            // Finalizer calls Dispose(false)
            Dispose(false);
        }

        protected virtual void Dispose(bool disposing)
        {
            // Close the file descriptor if it's open
            if (fileDescriptor > 0)
            {
                CloseI2C(fileDescriptor);
                fileDescriptor = -1;
            }
        }

private void ValidateDigitalReadData(int pin, byte[] bytesReceived)

{

if (bytesReceived[0] < 3)

{

throw new ExtendedIOException("Invalid response data received over the I2C connection.");

}

if (bytesReceived[1] != pin)

{

throw new ExtendedIOException(string.Format("Pin mismatch for the DigitalRead command. Expected {0} but got {1}.", pin, bytesReceived[1]));

}

if (!IsGPIOValueValid(bytesReceived[2]))

{

throw new ExtendedIOException("The returned pin value is not valid.");

}

private static bool IsGPIOValueValid(int value)

{

return ( (value == (int) GPIO.Value.Low) || (value == (int) GPIO.Value.High) );

}

private void CheckFileDescriptor()

{

if (fileDescriptor <= 0)

{

throw new ExtendedIOException("I2C connection is not open.");

}

public void Dispose()

{

Dispose(true);

GC.SuppressFinalize(this);

}

~I2CExtendedIO()

{

// Finalizer calls Dispose(false)

Dispose(false);

}

protected virtual void Dispose(bool disposing)

{

// Close the file descriptor if it's open

if (fileDescriptor > 0)

{

CloseI2C(fileDescriptor);

fileDescriptor = -1;

}

The Open function is simple as it delegates to the OpenI2C function with the specified device filename and the slave address, and assigns the returned file descriptor to a private member.

        public void Open(int slaveAddress)
        {
            fileDescriptor = OpenI2C(i2cFilename, slaveAddress);
            if (fileDescriptor <= 0)
            {
                throw new ExtendedIOException(string.Format("Unable to open the I2C connection to slave address: {0}", slaveAddress));
            }
        }

public void Open(int slaveAddress)

{

fileDescriptor = OpenI2C(i2cFilename, slaveAddress);

if (fileDescriptor <= 0)

{

throw new ExtendedIOException(string.Format("Unable to open the I2C connection to slave address: {0}", slaveAddress));

}

With DigitalWrite, we build the data to be sent based on the specified rules. Then we delegate to I2CSendBytes using the specified arguments, which calls the corresponding library function.

        public void DigitalWrite(int pin, GPIO.Value value)
        {
            CheckFileDescriptor();

            // Build the command to send
            byte[] bytes = new byte[4];
            bytes[0] = (byte) bytes.Length;
            bytes[1] = (byte) Commands.DigitalWrite;
            bytes[2] = (byte) pin;
            bytes[3] = (byte) value;

            int numBytes = I2CSendBytes(fileDescriptor, bytes);
            if (numBytes != bytes.Length)
            {
                throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));
            }
        }

public void DigitalWrite(int pin, GPIO.Value value)

{

CheckFileDescriptor();

// Build the command to send

byte[] bytes = new byte[4];

bytes[0] = (byte) bytes.Length;

bytes[1] = (byte) Commands.DigitalWrite;

bytes[2] = (byte) pin;

bytes[3] = (byte) value;

int numBytes = I2CSendBytes(fileDescriptor, bytes);

if (numBytes != bytes.Length)

{

throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));

}

AnalogWrite is similar to DigitalWrite, except that we convert the integer value to 4 bytes instead of the single byte for low (0) or high (1). Valid values are between 0 and 255 inclusive.

        public void AnalogWrite(int pin, int value)
        {
            CheckFileDescriptor();

            // Build the command to send
            byte[] bytes = new byte[7];
            bytes[0] = (byte) bytes.Length;
            bytes[1] = (byte) Commands.AnalogWrite;
            bytes[2] = (byte) pin;
            bytes[3] = (byte) (value >> 24);
            bytes[4] = (byte) (value >> 16);
            bytes[5] = (byte) (value >> 8);
            bytes[6] = (byte) value;

            int numBytes = I2CSendBytes(fileDescriptor, bytes);
            if (numBytes != bytes.Length)
            {
                throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));
            }
        }

public void AnalogWrite(int pin, int value)

{

CheckFileDescriptor();

// Build the command to send

byte[] bytes = new byte[7];

bytes[0] = (byte) bytes.Length;

bytes[1] = (byte) Commands.AnalogWrite;

bytes[2] = (byte) pin;

bytes[3] = (byte) (value >> 24);

bytes[4] = (byte) (value >> 16);

bytes[5] = (byte) (value >> 8);

bytes[6] = (byte) value;

int numBytes = I2CSendBytes(fileDescriptor, bytes);

if (numBytes != bytes.Length)

{

throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));

}

To wrap it all up, we have the DigitalRead function which is a little different because we have to send data and then immediately receive a response. We obtain a mutual-exclusion lock so that the send and receive process completes before any subsequent operations are run. Then we validate the received data and return the value for the pin that was read.

        /// <summary>
        /// Reads the pin value
        /// 
        /// Expected byte data always has a length of 3
        ///     bytesReceived[0] - the number of bytes
        ///     bytesReceived[1] - the pin number
        ///     bytesReceived[2] - the pin value
        /// </summary>
        /// <returns>GPIO.Value.High if the specified pin is HIGH, or GPIO.Value.Low if the specified pin is LOW.</returns>
        /// <param name="pin">Pin.</param>
        public GPIO.Value DigitalRead(int pin)
        {
            CheckFileDescriptor();

            // Build the command to send
            byte[] bytes = new byte[3];
            bytes[0] = (byte) bytes.Length;
            bytes[1] = (byte) Commands.DigitalRead;
            bytes[2] = (byte) pin;

            // The array to store received byte data
            byte[] bytesRecvd = new byte[MaxByteCount()];

            // Obtain lock for send/receive
            lock (lockObject)
            {
                // Send the DigitalRead command
                int numBytes = I2CSendBytes(fileDescriptor, bytes);
                if (numBytes != bytes.Length)
                {
                    throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));
                }

                // Read the response immediately after the command
                I2CReadBytes(fileDescriptor, bytesRecvd);
            }

            ValidateDigitalReadData(pin, bytesRecvd);

            return (GPIO.Value) bytesRecvd[2];
        }

/// <summary>

/// Reads the pin value

///

/// Expected byte data always has a length of 3

/// bytesReceived[0] - the number of bytes

/// bytesReceived[1] - the pin number

/// bytesReceived[2] - the pin value

/// </summary>

/// <returns>GPIO.Value.High if the specified pin is HIGH, or GPIO.Value.Low if the specified pin is LOW.</returns>

/// <param name="pin">Pin.</param>

public GPIO.Value DigitalRead(int pin)

{

CheckFileDescriptor();

// Build the command to send

byte[] bytes = new byte[3];

bytes[0] = (byte) bytes.Length;

bytes[1] = (byte) Commands.DigitalRead;

bytes[2] = (byte) pin;

// The array to store received byte data

byte[] bytesRecvd = new byte[MaxByteCount()];

// Obtain lock for send/receive

lock (lockObject)

{

// Send the DigitalRead command

int numBytes = I2CSendBytes(fileDescriptor, bytes);

if (numBytes != bytes.Length)

{

throw new ExtendedIOException(string.Format("Could not send {0} bytes over the I2C connection.", bytes.Length));

}

// Read the response immediately after the command

I2CReadBytes(fileDescriptor, bytesRecvd);

}

ValidateDigitalReadData(pin, bytesRecvd);

return (GPIO.Value) bytesRecvd[2];

}

You can obtain the full code listing for the C# class at https://gitlab.com/akinwale/NaniteIo/blob/master/Nanite/IO/I2CExtendedIO.cs.

We can put this all together and test with a simple interactive console application.

public static void Main (string[] args)
{
    // Extended IO via I2C
    using (I2CExtendedIO eio = new I2CExtendedIO())
    {
        try
        {
            eio.Open(0x08);

            string input;
            do
            {
                Console.Write("Enter PIN and value: ");
                input = Console.ReadLine();

                if (input == "-1")
                {
                    break;
                }

                string[] inputParts = input.Split(new char[] { ' ' });
                if (inputParts.Length != 2 && inputParts.Length != 3)
                {
                    Console.WriteLine("Invalid PIN and value. Try again.");
                    continue;
                }

                try
                {
                    int pin = int.Parse(inputParts[0].Trim());
                    string value = inputParts[1];
                    if (value != "off" && value != "on" && inputParts.Length == 2)
                    {
                        Console.WriteLine("Invalid PIN and value. Try again.");
                        continue;
                    }

                    if (inputParts.Length == 3)
                    {
                        int analogValue = int.Parse(inputParts[2].Trim());
                        if (value != "analog")
                        {
                            Console.WriteLine("Invalid PIN and value. Try again.");
                            continue;
                        }

                        eio.AnalogWrite(pin, analogValue);
                    }
                    else
                    {
                        eio.DigitalWrite(pin, "on" == value ? GPIO.Value.High : GPIO.Value.Low);
                    }
                }
                catch (FormatException)
                {
                    Console.WriteLine("Invalid PIN and value. Try again.");
                    continue;
                }
            } while (input != "-1");
        }
        catch (ExtendedIOException ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }
}

public static void Main (string[] args)

{

// Extended IO via I2C

using (I2CExtendedIO eio = new I2CExtendedIO())

{

try

{

eio.Open(0x08);

string input;

{

Console.Write("Enter PIN and value: ");

input = Console.ReadLine();

if (input == "-1")

{

break;

}

string[] inputParts = input.Split(new char[] { ' ' });

if (inputParts.Length != 2 && inputParts.Length != 3)

{

Console.WriteLine("Invalid PIN and value. Try again.");

continue;

}

try

{

int pin = int.Parse(inputParts[0].Trim());

string value = inputParts[1];

if (value != "off" && value != "on" && inputParts.Length == 2)

{

Console.WriteLine("Invalid PIN and value. Try again.");

continue;

}

if (inputParts.Length == 3)

{

int analogValue = int.Parse(inputParts[2].Trim());

if (value != "analog")

{

Console.WriteLine("Invalid PIN and value. Try again.");

continue;

}

eio.AnalogWrite(pin, analogValue);

}

else

{

eio.DigitalWrite(pin, "on" == value ? GPIO.Value.High : GPIO.Value.Low);

}

catch (FormatException)

{

Console.WriteLine("Invalid PIN and value. Try again.");

continue;

}

} while (input != "-1");

}

catch (ExtendedIOException ex)

{

Console.WriteLine(ex.ToString());

}

The console application accepts inputs like 13 on, 18 off or 9 analog 172 which makes it easy to test the Arduino pins. Although this is practically a complete solution for most requirements with respect to controlling an Arduino connected to the PINE (or Pi or any other SBC) over I2C using C#, you could choose to implement an additional command for analogRead. All you would have to do is follow the logic for digitalRead and add the necessary code to the sketch and the C# class.

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.