static unsigned char d2led[]={0x3F,0x6,0x5b,0x4F,0x66,0x6D,0x7D,0x7,0x7f,0x6F};

int main(void)
{
   //Проверяю терминал, всё нормально показывает
   USART_putstring("POEHALI! ");
   _delay_ms(100);

   //Для проверки дисплея показываю числа, всё отображается нормально.
   int i;
   for (i=0; i<10; i++)
   {
      upd7segment(i);
      _delay_ms(500);
   }
   while(1)
   {
   }
}

ISR(USART_RXC_vect)
{
   char ReceivedByte;
   ReceivedByte = UDR;
   uint8_t i = 0;
   i = (i<<8) + ReceivedByte;
   // надо показать полученное число (0...9) на дисплее. Ничего не показывает.
   upd7segment(i);
   // хотя назад в ком уходит правильное
   UDR = i;
}

inline void upd7segment(int16_t number)
{
   //if((number<0)||(number>9)) number=0;
   PORTB=d2led[number];      
}

ISR(USART_RXC_vect)
{
   char ReceivedByte;
   ReceivedByte = UDR; // Fetch the received byte value into the variable "ByteReceived"
   uint8_t i = 0;
   //i = (i<<8) + ReceivedByte;
   i=atoi(ReceivedByte);
   newMode = i;
   upd7segment(i);
   UDR = ReceivedByte;
}

inline void upd7segment(unsigned char number)
{
   //if((number<0)||(number>9)) number=0;
   PORTB=d2led[number];     
}

ISR(USART_RXC_vect)
{
   char ReceivedByte;
   ReceivedByte = UDR;
   PORTB=d2led[ReceivedByte - 32];
   UDR = ReceivedByte;
}

ISR(USART_RXC_vect)
{
   char ReceivedByte;
   ReceivedByte = UDR; // Fetch the received byte value into the variable "ByteReceived"
   uint8_t i = 0;
   i=ReceivedByte-32;
   UDR = ReceivedByte;
   PORTB=i;
}

   i=ReceivedByte-32;
   i &= ~(0x10);
   newMode = i;
   upd7segment(i);

ISR(USART_RXC_vect)
{
   unsigned char ReceivedByte;
   ReceivedByte = UDR;
   PORTB=d2led[ReceivedByte - 32];
   UDR = ReceivedByte;
}