begin method added - (#56) (#43)

PJON.cpp

@@ -36,7 +36,7 @@ limitations under the License. */
PJON::PJON(uint8_t input_pin) {
  _input_pin = input_pin;
  _device_id = NOT_ASSIGNED;
  this->initialize();
  this->set_default();
}


@@ -45,16 +45,13 @@ PJON::PJON(uint8_t input_pin) {
PJON::PJON(uint8_t input_pin, uint8_t device_id) {
  _input_pin = input_pin;
  _device_id = device_id;
  this->initialize();
  this->set_default();
}


/* Initialization tasks: */
/* Set bus state default configuration: */

void PJON::initialize() {
  /* Initial random delay to avoid startup collision */
  randomSeed(analogRead(A0));
  delayMicroseconds(random(0, INITIAL_MAX_DELAY));
void PJON::set_default() {
  _negative_acknowledge = true;

  this->set_error(dummy_error_handler);
@@ -68,6 +65,15 @@ void PJON::initialize() {
}


/* Initialization tasks: */

void PJON::begin() {
  /* Initial random delay to avoid startup collision */
  randomSeed(analogRead(A0));
  delay(random(0, INITIAL_MAX_DELAY));
}


/* Set the device id, passing a single byte (watch out to id collision) */

void PJON::set_id(uint8_t id) {

PJON.h

@@ -105,17 +105,18 @@ limitations under the License. */
    public:
      PJON(uint8_t input_pin);
      PJON(uint8_t input_pin, uint8_t id);
      void initialize();
      void begin();

      void acquire_id();
      uint8_t device_id();

      uint8_t compute_crc_8(char input_byte, uint8_t crc);

      void set_default();
      void set_error(error e);
      void set_id(uint8_t id);
      void set_negative_acknowledge(boolean state);
      void set_receiver(receiver r);
      void set_error(error e);

      uint8_t  read_byte();
      uint16_t receive_byte();

README.md

@@ -2,25 +2,26 @@
PJON (Padded Jittering Operative Network) is an Arduino compatible single wire, multi-master communications bus system implemented in  270 lines of C++ and Wiring (not considering comments). It is designed as an alternative to i2c, 1-Wire, Serial and other Arduino compatible protocols. If you are interested to know more about the PJON standard, visit the [Wiki](https://github.com/gioblu/PJON/wiki). If you need a wireless multimaster implementation check [PJON_ASK](https://github.com/gioblu/PJON_ASK). If you need help or something is not working visit the [Troubleshooting Wiki page](https://github.com/gioblu/PJON/wiki/Troubleshooting). If you own a Saleae Logic Analyzer you can scope communication with [saleae-pjon-protocol-analyzer](https://github.com/aperepel/saleae-pjon-protocol-analyzer) crafted by Andrew Grande.

```cpp  
#include <PJON.h>     // Transmitter board code
PJON network(12, 45); // Bus connection to pin 12, device id 45
#include <PJON.h> // Transmitter board code
PJON bus(12, 45); // Bus connection to pin 12, device id 45

void setup() {
  network.send(44, "B", 1, 1000000);
  // Send to device 44, "B" content of 1 byte length every 1000000 microseconds (1 second)
  bus.begin(); // Initialize PJON bus
  bus.send(44, "B", 1, 1000000); // Send to 44 "B" of length 1 every second
}

void loop() {
  network.update();
  bus.update();
}

/* ---------------------------------------------------------------------------- */

#include <PJON.h>     // Receiver board code
PJON network(12, 44); // Bus connection to pin 12, device id 44
#include <PJON.h> // Receiver board code
PJON bus(12, 44); // Bus connection to pin 12, device id 44

void setup() {
  network.set_receiver(receiver_function); // Set the function used to receive messages
  bus.begin(); // Initialize PJON bus
  bus.set_receiver(receiver_function); // Set the function used to receive messages
};

void receiver_function(uint8_t length, uint8_t *payload) {
@@ -32,7 +33,7 @@ void receiver_function(uint8_t length, uint8_t *payload) {
}

void loop() {
  network.receive(1000);
  bus.receive(1000);
}
```

@@ -84,23 +85,25 @@ PJON application example made by the user [Michael Teeuw](http://michaelteeuw.nl
#### How to start
The first step is the physical layer: connect with a wire two boards using a digital pin on both boards. After this you should have both arduino boards connected by the wire on the same pin. The selected pins are the same only for simplicity and to avoid mistakes, PJON works fine on every Arduino digital or analog (used as digital) I/O pin.

Lets start coding, instantiate the `PJON` object that in the example is called network. To initialize a network based on PJON you need only to define the communication pin (any free digital pin on your board) and a unique ID (0 - 255):
Lets start coding, instantiate the `PJON` object that in the example is called bus. To initialize a bus based on PJON you need only to define the communication pin (any free digital pin on your board) and a unique ID (0 - 255):

```cpp  
  PJON network(12);
  network.set_id(123); // Set id later
  PJON bus(12);
  bus.begin();     // initialize PJON bus
  bus.set_id(123); // Set id later

  // or

  PJON network(12, 123);
  PJON bus(12, 123);
  bus.begin();     // initialize PJON bus
```

If you are interested auto-addressing is really easy to use:
```cpp  
  PJON network(12);
  network.acquire_id();

  Serial.println(network.device_id()); // Device id found with scan
  PJON bus(12);
  bus.begin();     // initialize PJON bus
  bus.acquire_id();
  Serial.println(bus.device_id()); // Device id found with scan
```
All ids are scanned sending a packet containing the `ACQUIRE_ID` constant. If no answer is received from an id, it is considered free.
If auto-addressing approach is your choice, you should never have a blind timeframe longer than 1.5 seconds (i.e. `delay(2000)`) between every `receive` function call. This constrain is imposed by the necessity of having the device able to receive incoming packets (as `ACQUIRE_ID` used to determine if a device id is free or not). If a device is executing something for too much time while not reading the bus for incoming packets, its device id could be stolen by another device. There is still no device id collision detection / correction, but respecting the described rules, collision should not happen.