PacketSerial

PacketSerial is an small, efficient, library that allows Arduinos to send and receive serial data packets that include bytes with any value (0-255). A packet is simply an array of bytes.

"Why do I need this?" you may ask. The truth is that you may not need it if you are converting your values to ASCII strings and separating them with a known character (like a new line \n) before sending them. This is what happens if you call Serial.print() or Serial.println(). For instance, if you just want to send a byte with the value of 255 and follow it with a new line character (i.e. Serial.println(255)) the Arduino automatically converts the number to the equivalent printable ASCII characters, sending 4 bytes total. As a result the receiver won't just receive a byte for the number and a byte for the new line character. Instead it will receive a stream of 4 bytes:

50 // ASCII 2
53 // ASCII 5
53 // ASCII 5
10 // Serial.println() appends a new line \n character.

The receiver must then collect the 3 ASCII characters (2, 5, 5), combine them and convert them back into a single byte with a value of 255. This process can get complicated when the user wants to send large quantities of structured data between the Arduino and a receiver.

One way to send a packet of data without this library is to send each byte separated by a comma or space and terminate the sequence with a new line character. Thus, to send the value 255 and the value 10, one might call:

Serial.print(255);
Serial.print(',');
Serial.print(10);
Serial.print('\n');

The receiver will actually see a stream of 8 bytes:

50 // ASCII 2
53 // ASCII 5
53 // ASCII 5
44 // ASCII ,
49 // ASCII 1
48 // ASCII 0
10 // ASCII \n

In this case, the receiver must then collect the ASCII characters, combine them, skip the delimiter (the comma in this case) and then process the packet when a new line is encountered. While effective, this method doesn't scale well. Bytes with values larger than 9 require are encoded as 2 bytes and bytes with values larger than 99 are encoded as 3 bytes. If the user would like to send the number 4,294,967,295 (the maximum value of a 4 byte unsigned long), it would be encoded as 10 bytes. This means that there is an overhead of 6 extra bytes to transmit a 4 byte unsigned long.

An alternative to ASCII encoding is to write the bytes directly to using the Serial.write() methods. These methods do not convert the byte values to ASCII. So if the user wants to send a single byte with the value of 255 and follow it with a new line character (i.e. Serial.write(255); Serial.write('\n');), the receiver will see a stream of 2 bytes:

255 // The value transmitted.
10  // The new line character (\n).

This is much more compact but can create problems when the user wants to send a packet of data. If the user wants to send a packet consisting of two values such as 255 and 10, we run into problems if we also use the new line ('\n' ASCII 10) character as a packet boundary. This essentially means that the receiver will incorrectly think that a new packet is beginning when it receives the value of 10. Thus, to use this more compact form of sending bytes while reserving one value for a packet boundary marker. Several unambiguous packet boundary marking encodings exist, but one with a small predictable overhead is called Consistent Overhead Byte Stuffing. For a raw packet of length SIZE, the maximum encoded buffer size will only be SIZE + SIZE / 254 + 1. This is significantly less than ASCII encoding and the encoding / decoding algorithm is simple and fast. In its default mode, the COBS encoding process simply removes all zeros from the packet, allowing the sender and receiver to use the value of zero as a packet boundary marker.

PacketSerial

The PacketSerial class wraps the standard Arduino Serial class to automatically encode and decode byte packets automatically. Thus users can still call methods on the Serial object (e.g. Serial.write(), the built in serialEvent() callback etc), but it is not recommended. Users are advised to let PacketSerial manage all Serial communication via the packet handler callback for incoming packets and the send(const uint8_t* buffer, size_t size) method for outgoing packets.

Setup

For Arduino boards with more than one serial port, PacketSerial the desired serial port can be specified with the begin method, i.e.

    void begin(unsigned long baud, size_t port = 0)

Where:

Arduino Serial Port	PacketSerial Port Number
Serial	0
Serial1	1
Serial2	2
Serial3	3

To receive decoded packets automatically, the user should register a packet callback. The packet callback should be placed in your main Arduino Sketch and should have a method that looks like this signatur that looks like:

void onPacket(const uint8_t* buffer, size_t size)
{
    /// Process your incoming packet here.
}

Your callback can have any name and should be registered in the setup() method like this:

void setPacketHandler(PacketHandlerFunction onPacketFunction)

Main Loop

In order to processing incoming serial packets, the user must call the update() method at the end of the loop() method.

void loop()
{
    // Your program here. 
    
    serial.update();
}

Sending Packets

To send packets call the send() method. The send method will take a packet (an array of bytes, encode it, transmit the array and transmit the packet boundary marker (0). To send the values 255 and 10, one might do the following:

// Make an array.
uint8_t myPacket[] { 255, 10 };

serial.send(myPacket, 2);

Example

In this "reverse echo" example, we listen for incoming packets. When a new packet arrives in the onPacket method, we reverse the contents and send it back to the sender.

#include <PacketSerial.h>


// The PacketSerial object.
// It cleverly wraps one of the Serial objects.
// While it is still possible to use the Serial object
// directly, it is recommended that the user let the 
// PacketSerial object manage all serial communication.
// Thus the user should not call Serial.write(), etc.
// Additionally the user should not use the serialEvent()
// callbacks.
PacketSerial serial;


void setup()
{
  // We must specify a packet handler method so that
  serial.setPacketHandler(&onPacket);
  serial.begin(115200);
}


void loop()
{
  // Do other things here.
  
  // The update() method attempts to read in
  // any incoming serial data and emits packets via
  // the user's onPacket(const uint8_t* buffer, size_t size) 
  // method registered with the setPacketHandler() method.  
  //
  // The update() method should be called at the end of the loop().
  serial.update(); 
}

// This is our packet callback.
// The buffer is delivered already decoded.
void onPacket(const uint8_t* buffer, size_t size)
{
  // Make a temporary buffer.
  uint8_t tmp[size]; 
  
  // Copy the packet into our temporary buffer.
  memcpy(tmp, buffer, size); 
  
  // Reverse our  buffer.
  reverse(tmp, size);
  
  // Send the reversed buffer back.
  // The send() method will encode the buffer
  // as a packet, set packet markers, etc.
  serial.send(tmp, size);
}

/// \brief A simple array reversal method.
void reverse(uint8_t* buffer, size_t size)
{
  uint8_t tmp;
  
  for (int i=0; i < size / 2; i++)
  {
    tmp = buffer[i];
    buffer[i] = buffer[size-i-1];
    buffer[size-i-1] = tmp;
  }
}

Compatible libraries

• openFrameworks
  • https://github.com/bakercp/ofxSerial
  • See the ofx::IO::PacketSerial object which is directly compatible with this library.