text: #ifndef _SERIAL_H_
  #define _SERIAL_H_
  
  #include <stdbool.h>
  
  #include "avr_mcu.h"
  
  void serial_init();
  bool serial_has_next_byte();
  unsigned char serial_read_next_byte();
  void serial_write(const unsigned char* data, short len);
  void serial_write_newline();
  void serial_tx_byte(unsigned char data);
  
  #ifdef SERIAL_EXTRA_SUPPORT
  void serial_extra_start();
  void serial_extra_stop();
  bool serial_extra_has_next_byte();
  unsigned char serial_extra_read_next_byte();
  void serial_extra_tx_byte(unsigned char data);
  #endif // SERIAL_EXTRA_SUPPORT
  
  #endif // _SERIAL_H_
  