multiple-bit-communication.cpp

const int buttonPin = 4;
const int ss = 10;
const int mosi = 11;
const int miso = 12;
const int sclk = 13;

int lastState = LOW;
int ledState = LOW;

void setup()
{ 
  Serial.begin(9600);
  
  pinMode(buttonPin, INPUT);
  pinMode(ss, OUTPUT);
  pinMode(mosi, OUTPUT);
  pinMode(miso, INPUT);
  pinMode(sclk, OUTPUT);
  digitalWrite(ss, HIGH);
}


void sendMessage(String message) {
  digitalWrite(ss, LOW); // SELECT
  delay(10);
  
  for (int i = 0; i < message.length(); i++) {
    digitalWrite(mosi, message.charAt(i) == '1' ? HIGH : LOW);
    delay(10);
    digitalWrite(sclk, HIGH); // CLOCK HIGH (slave reads)
    delay(10);
    digitalWrite(sclk, LOW); // CLOCK LOW
    delay(10);
  }

  digitalWrite(ss, HIGH); // STOPS LISTENING
}

void loop()
{
  int currentState = digitalRead(buttonPin);
  
  if(currentState != lastState && currentState == HIGH){
    if (ledState == LOW){
      Serial.print("Send 10 to turn on");
      sendMessage("1000");
    } else {
      Serial.print("Send 01 to turn on");
      sendMessage("0001");
    }
    
    ledState = !ledState;
  }
  
  lastState = currentState;
}

///

const int ss = 10;
const int mosi = 11;
const int miso = 12;
const int sclk = 13;
const int LEDPin = 7;

int LEDState = LOW;
int last = LOW;
int lastSS = HIGH;
String message = "";

void setup()
{
  Serial.begin(9600);
  pinMode(LEDPin, OUTPUT);
  pinMode(mosi, INPUT);
  pinMode(miso, OUTPUT);
  pinMode(sclk, INPUT);
  pinMode(ss, INPUT);
}

void loop()
{
  int clockState = digitalRead(sclk);
  int ssState = digitalRead(ss);

  if (ssState == HIGH) {
    if (ssState != lastSS) {
      if (message == "1000") {
	  	digitalWrite(LEDPin, HIGH); 
      } else if (message == "0001") {
      	digitalWrite(LEDPin, LOW); 
      }
      
	  message = "";
    }
  } else {
	if(clockState != last && clockState == HIGH){
      Serial.print(digitalRead(mosi));
      if(digitalRead(mosi) == HIGH) {
      	message += "1";
      } else {
      	message += "0";
      }

      digitalWrite(miso, HIGH);
    }

    last = clockState;
  }
  
  lastSS = ssState;
}