Added CRC8 table-less (#32)

PJON.cpp

@@ -168,6 +168,18 @@ boolean PJON::can_start() {
}


/* Compute CRC8 with a table-less implementation: */

uint8_t PJON::compute_crc_8(char input_byte, uint8_t crc) {
  for (uint8_t i = 8; i; i--, input_byte >>= 1) {
    uint8_t result = (crc ^ input_byte) & 0x01;
    crc >>= 1;
    if(result) crc ^= 0x8C;
  }
  return crc;
};


/* Every byte is prepended with 2 synchronization padding bits. The first
   is a longer than standard logic 1 followed by a standard logic 0.
   __________ ___________________________
@@ -227,13 +239,13 @@ uint16_t PJON::send_string(uint8_t id, char *string, uint8_t length) {
  pinModeFast(_input_pin, OUTPUT);

  this->send_byte(id);
  CRC ^= id;
  CRC = this->compute_crc_8(id, CRC);
  this->send_byte(length + 3);
  CRC ^= length + 3;
  CRC = this->compute_crc_8(length + 3, CRC);

  for(uint8_t i = 0; i < length; i++) {
    this->send_byte(string[i]);
    CRC ^= string[i];
    CRC = this->compute_crc_8(string[i], CRC);
  }

  this->send_byte(CRC);
@@ -445,7 +457,7 @@ uint16_t PJON::receive() {
        package_length = data[i];
      else return FAIL;

    CRC ^= data[i];
    CRC = this->compute_crc_8(data[i], CRC);
  }

  pinModeFast(_input_pin, OUTPUT);

PJON.h

@@ -109,6 +109,9 @@ limitations under the License. */

      uint8_t device_id();
      void acquire_id();
      uint8_t device_id();

      uint8_t compute_crc_8(char input_byte, uint8_t crc);

      void set_id(uint8_t id);
      void set_negative_acknowledge(boolean state);

README.md

@@ -41,7 +41,7 @@ void loop() {
- Single wire (plus common ground) physical layer with up to 50 meters range.
- Device id implementation to enable univocal communication up to 254 devices.  
- Optional auto-addressing with id collision avoidance.
- Lightweight 1 byte XOR based error detection.
- CRC8 table-less cyclic redundancy check implementation.
- Acknowledgement of correct packet sending.
- Collision avoidance to enable multi-master capability.
- Broadcast functionality to contact all connected devices.