/*  S+4Bits comm        RX   BY M.Kikuchi
    Tiny13a   1.2MHz    2013/12/6
    ROM   190 Bytes     conf=1F 6A (BOD=0 Start=14+64 Int9.6M)
    V2 2014/11/6        11/7 Interrupt
*/

char     RXDAT,Bc;
sbit     RXPin at PINB4_bit;

void cm_rx()                // 420 ck+
{
      while(RXPin);         // 1 to 0 Stert bit
      Delay_us(40);
      while(~RXPin);        // 0 to 1
      Delay_us(75);
      Bc=4;              // bit set
      RXDAT=0;
      do{
         RXDAT <<=1;
         if(RXPin)    
            RXDAT++;
         Bc--;
         Delay_us(50);
      } while(Bc);
      PORTB=RXDAT;       //Display
}

void PIN_in() iv IVT_ADDR_PCINT0{
      PCIF_bit=0;
      cm_rx();
}

void main() 
{
      DDRB=0b00001111;       //1=output
      PORTB=0;
      PCMSK=0b00010000;      //PIN Change mask
      PCIE_bit=1;
      SREG_I_bit=1;          //Global interrupt
      while(1);              // endless
}
---------------------------------------------------
/*  S+4Bits comm        TX   BY M.Kikuchi
    Tiny13a   1.2MHz    2013/12/6      V2-2014/11/6
    ROM   196 Bytes     conf=1F 6A (BOD=0 Start=14+64 Int9.6M)
*/

char     Bc,LC;
sbit     TXPIN at PORTB4_bit;

void cm_tx(char TXDA)
{
    TXPIN=0;                 //Start bit
    Delay_us(50);
    TXPIN=1;
    Delay_us(50);
    Bc=4;
    do {
         if((TXDA & 8)==8)
             TXPIN=1;
         else
             TXPIN=0;
         TXDA <<=1;
         Bc--;
         Delay_us(50);
    }  while(Bc);
    TXPIN=1;
}

void main()
{
      DDRB  =0b00010000;     //1=output
      TXPIN=1;
      Delay_ms(10);          //Wait
      while(1){
         for (LC=0;LC<8;LC++){
            cm_tx(LC);
            Delay_ms(200);
         }
      }
}