.equ    SYSCLK = 20000000;System Clock [Hz]
.equ    BaudSpeed0=1200    ;speed of UART  
.equ     bauddivider = (SYSCLK/(16*BaudSpeed0))-1

;************************************************************************
; USART0_Init
;
        lds    R16,PRR
        andi    R16,255-(1<<PRUSART0);USART0 wake up
        sts    PRR,R16

        ldi    R16,0b00000000    ;RXCn, TXCn, UDREn, FEn, DORn, UPEn, U2Xn, MPCMn
        sts    UCSR0A,R16

        ldi    R16,0b00011000    ;RXCIE,TXCIE,UDRIE,RXEN,TXEN,UCSZn2,RXB8,TXB8
        sts    UCSR0B,R16

        ldi    R16,0b00000110    ;-, UMSELn, UPMn1, UPMn0, USBSn, UCSZn1, UCSZn0, UCPOLn
        sts    UCSR0C,R16

        ldi    R16,High(bauddivider)
        sts    UBRR0H,R16
        ldi    R16,Low(bauddivider)
        sts    UBRR0L,R16    ;BaudRate Gen.

(AVR USART 0)Baud rate = 54212 b/s

#include <ioavr.h>

#define F_CPU 14745600
#define BAUD 1200


void USART_Init()
{
  UCSRB = (1<<RXCIE)|(1<<TXCIE)|(1<<RXEN)|(1<<TXEN); //enable interrupts during recieve/transmit, enable recieve, enable transmit
  UCSRC = (1<<UCSZ1)|(1<<UCSZ0); //asynchronous mode, 8 data, 1 stop bit

  UBRRH = ((F_CPU/(16*BAUD)-1)>>8);
  UBRRL = ((F_CPU/(16*BAUD)-1)&0xFF);
}


int main( void )
{
  USART_Init();
  asm("sei");
  
  for(;;)
  {
    while ( !( UCSRA & (1<<UDRE)) )
;
    UDR = 0xAA;
    for(long i = 0; i<7372;i++);
  }

}

  UBRRH = ((((F_CPU) + 8UL * (BAUD)) / (16UL * (BAUD)) -1UL) >> 8);
  UBRRL = ((((F_CPU) + 8UL * (BAUD)) / (16UL * (BAUD)) -1UL) & 0xff);

#define BYTE0(X)                    ((uint8_t)((X)>>0))
#define BYTE1(X)                    ((uint8_t)((X)>>8))
#define BYTE2(X)                    ((uint8_t)((X)>>16))
#define BYTE3(X)                    ((uint8_t)((X)>>24))

void uart_init_tx_8n1(void)
{
    static const uint16_t ubrr = (F_CPU + 8UL * BAUD) / (16UL * BAUD) - 1UL; // эта формула верна ???

    UCSRA = 0;
    UCSRB = 0;
    UCSRC = (1<<UCSZ0)|(1<<UCSZ1);

    UBRRH = BYTE1(ubrr);
    UBRRL = BYTE0(ubrr);


    UCSRB = (1<<TXEN);
}

void uart_tx_byte(uint8_t x)
{
    while (!( UCSRA & (1<<UDRE)))   {;}

    UDR = x;
}