void __irq_usart3(void) {
   digitalWrite(33,HIGH);
   delay(20);
   digitalWrite(33,LOW);
}

void setup()
{

  pinMode(33,OUTPUT);
  
       USART3->regs->BRR = 0x120; // Baud Rate Register
    USART3->regs->CR1 = USART_CR1_TE | USART_CR1_RE | USART_CR1_RXNEIE;
    USART3->regs->CR1 |= USART_CR1_UE;
     USART3->regs->CR2 = 0b0010000000100000; // CR 2
     USART3->regs->CR3 = 0x0000; // CR 3
      
        nvic_irq_enable(NVIC_USART3);
        rcc_clk_enable(RCC_USART3);


  
  
}


void loop()
{
  digitalWrite(33,HIGH);
  delay(200);
  digitalWrite(33,LOW);
  delay(1000);
  
}