#define RXD 4
#define TXD 3
#define CLK 2

#define WAIT 500

char no = 0;
uint8_t receive;

void setup()
{
  pinMode(TXD, OUTPUT);
  pinMode(RXD, INPUT);
  pinMode(CLK, OUTPUT); 
  digitalWrite(CLK, LOW);

  Serial.begin(19200);
  
  //ノイズ対策(?)のため7bit送る
  for(int i=0; i<7; i++)
  {
    digitalWrite(TXD, HIGH);
    delay(10);
    digitalWrite(CLK, HIGH);
    delay(10);
    digitalWrite(CLK, LOW);
  }
}

void loop()
{
  sendByte(no);
  Serial.print(receive, HEX);
  Serial.print(" ");
  if(++no == 255)
    no = 0;
}

void sendByte(byte b)
{
  uint8_t onebit;

  receive = 0;
  
  for(int i=0; i<8; i++)
  {
    onebit = (b >> (7 - i)) & 1;
    digitalWrite(TXD, onebit);
    delayMicroseconds(WAIT);
    digitalWrite(CLK, HIGH);
    delayMicroseconds(WAIT);
    receive += (digitalRead(RXD) << (7 - i));
    digitalWrite(CLK, LOW);
  }
}