c / f = l
300 / 433 = 0.692

c / l = f
300 / 0.6928 = 433.02

300/433/4 = 0.173

  Atmega328p
  Ports register reference:
  Port D
  1   2   3   4   5   6   7   8
  --  --  --  --  --  --  --  --
  D7  D6  D5  D4  D3  D2  D1  D0

  Port B
  1   2   3   4   5   6   7   8
  --- --- --- --- --- --- --- ---
          D13 D12 D11 D10 D9  D8

  Port C
  1   2   3   4   5   6   7   8
  --  --  --  --  --  --  --  --
          A5  A4  A3  A2  A1  A0

// setup()
  // Set D8+D9+D10+D11+D13 as OUTPUTs
  DDRB = 0b00101111 ;
  // Set all bits in register LOW
  PORTB = 0b00000000 ;

// ...

// Functions that belong elsewhere
void bridgeOFF( void ) {
  // Set all output pins LOW
  PORTB = 0b00000000 ;
  // Motor Spin-down grace period
  delay( 300 ) ;
}

void bridgeForward( void ) {
  bridgeOFF() ;
  // Forward, D8+D9+D13 HIGH, D13 is the onboard LED.
  PORTB = 0b00100011 ;
}

void bridgeReverse( void ) {
  bridgeOFF() ;
  // Reverse, D10+D10 HIGH, D13 is set LOW here.
  PORTB = 0b00001100 ;
}

// Handheld unit?
#define IS_GARAGE_OPENER 	0
// Garage base station?
#define IS_GARAGE_STATION	1

/*
Constants from Arduino_CC1101/ELECHOUSE_CC1101.h:
Name   	Pin  Comment
SCK_PIN	13   MUST LEVELSHIFT FROM Arduino 5V to CC1101 SPI 3V3!
MISO_PIN   12
MOSI_PIN   11   MUST LEVELSHIFT FROM Arduino 5V to CC1101 SPI 3V3!
SS_PIN 	10   CSN/SS; MUST LEVELSHIFT FROM Arduino 5V to CC1101 SPI 3V3!
GDO0   	2
GDO2   	9    
*/

// Handheld unit?
#define IS_GARAGE_OPENER 	0
// Garage base station
#define IS_GARAGE_STATION	1
#if IS_GARAGE_OPENER
// Interrupt/sleep related libraries, not used.
//#include <avr/wdt.h>       	 
//#include <avr/interrupt.h> 	 
//#include <avr/sleep.h>     	 
//#include <avr/power.h>     	 
//const byte wakePin = 3 ; // INT1
#elif IS_GARAGE_STATION
// Motor open/close drive times usually differ,
// time both separately with a stopwatch app.
#define  motor_open_drive_time 100 // ms, change this
#define  motor_close_drive_time 100 // ms, change this
#endif
#include 
const byte buttonPin = 6 ;
bool buttonState = LOW ;        	 
bool previousButtonState = LOW;   
unsigned long previousDebounceTime = 0 ;  
//unsigned long debounceDelay = 50;    
unsigned long debounceDelay = 100 ;    
const int magic_token_len = 6 ;
// PING LIKE A SUBMARINE!
byte magic_token[ magic_token_len ] = { "PING!" } ;
const int input_buffer_len = 6 ;
const byte ledPin = 4 ;
bool is_garage_open = false ;
// 0: No Serial.printing, except in serialEvent()
// 1: Lots of useful information, such as received data, events etc.
const bool debug = 1 ;

void setup( void ) {
  Serial.begin( 115200 ) ;
  pinMode( ledPin, OUTPUT ) ;
  ELECHOUSE_cc1101.Init( F_433 ); // Frequency: 433MHz
  //ELECHOUSE_cc1101.Init( F_868 ) ; // Frequency: 868MHZ
  //ELECHOUSE_cc1101.Init( F_915 ) ; // Frequency: 915MHz
  if ( IS_GARAGE_STATION )
	ELECHOUSE_cc1101.SetReceive() ; // Do listen
  else if ( IS_GARAGE_OPENER ) {
      // 10K pull-down from D6->GND, then D6->SW1->5V
	pinMode( buttonPin, INPUT ) ; 
  }
}

void loop( void ) {
  // Begin Garage Opener
  if ( IS_GARAGE_OPENER ) {
	int read = digitalRead( buttonPin ) ;
	if( read != previousButtonState ) {
  	// Reset the debouncing timer
  	previousDebounceTime = millis() ;
	}
	if ( (millis() - previousDebounceTime ) > debounceDelay ) {
  	if ( read != buttonState ) {
    buttonState = read ;
    // Still HIGH?
    if (buttonState == HIGH) {
      digitalWrite( ledPin, HIGH ) ;

      Serial.println( "[!] Sending magic_token three times ..." ) ;
      // Now send magic_token with moderate confidence
      for ( int it = 0 ; it < 3 ; it++ ) { ELECHOUSE_cc1101.SendData( magic_token, magic_token_len ) ; } delay( 50 ) ; digitalWrite( ledPin, LOW ) ; } } } previousButtonState = read ; } // End Garage Opener // Begin Garage Station else if ( IS_GARAGE_STATION ) { byte input_buffer[ input_buffer_len ] = { 0 } ; bool token_matched = false ; if ( ELECHOUSE_cc1101.CheckReceiveFlag() ) { int len = 0 ; len = ELECHOUSE_cc1101.ReceiveData( input_buffer ) ; if ( debug ) { Serial.print( F( "[!] RX data => " ) ) ;
    Serial.println( (const char *)input_buffer ) ;
    Serial.print( F( "[!] RX data length => " ) ) ;
    Serial.println( len, DEC ) ;
  	}
  	delay( 100 ) ;
  	// Are magic_token and input_buffer identical?
  	if( strncmp( (const char *)magic_token, (const char*)input_buffer, 2 ) == 0 ) {
    token_matched = true ;
    // If open, close
    // If closed, open
    // set is_garage_open accordingly
  	}
  	else token_matched = false ;
  	// End token_match
  	// Begin open/close actions
  	if ( token_matched ) {
    if ( is_garage_open ) {
      if( debug ) Serial.println( F( "[!] Garage door CLOSE action!" ) ) ;
      // Drive closed ...
      is_garage_open = false ;
      digitalWrite( ledPin, LOW ) ;
      delay( motor_close_drive_time ) ;
    }
    else if ( ! is_garage_open ) {
      if ( debug ) Serial.println( F( "[!] Garage door OPEN action!" ) ) ;
      // Drive open ...
      is_garage_open = true ;
      digitalWrite( ledPin, HIGH ) ;
      delay( motor_open_drive_time ) ;
    }
  	}
  	// End open/close actions
	}
	ELECHOUSE_cc1101.SetReceive() ; // Do continue listening
  }
}

// Called if Arduino receives data over serial link
void serialEvent( void ) {
  printFreeRAM() ;
  while ( Serial.available() > 0 )
	Serial.read() ;
}

// Print free SRAM in bytes
void printFreeRAM( void ) {
  extern int __heap_start, *__brkval ;
  int v ;
  v = (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval ) ;
  Serial.print( F( "[!] Free RAM => " ) ) ;
  Serial.println( v, DEC ) ;
}

#define  motor_open_drive_time 5000 // ms
#define  motor_close_drive_time 5000 // ms