Rx    --  Tx
Tx    --  Rx
XCK  --  XCK

/* Initialize UART */
void USART0_Init( unsigned int baudrate )
{
        /* Set the baud rate */
        UBRR0H = (unsigned char) (baudrate>>8);
        UBRR0L = (unsigned char) baudrate;

        /* Enable UART receiver and transmitter */
        UCSR0B = ( ( 1 << RXEN0 ) | ( 1 << TXEN0 ) );

        /* Set frame format: 8 data 2stop */
        UCSR0C = (1<<USBS0)|(1<<UCSZ01)|(1<<UCSZ00);              //For devices with Extended IO
        //UCSR0C = (1<<URSEL)|(1<<USBS0)|(1<<UCSZ01)|(1<<UCSZ00);   //For devices without Extended IO
}

/* Read and write functions */
unsigned char USART0_Receive( void )
{
        /* Wait for incomming data */
        while ( !(UCSR0A & (1<<RXC0)) )
              ;
        /* Return the data */
        return UDR0;
}

void USART0_Transmit( unsigned char data )
{
        /* Wait for empty transmit buffer */
        while ( !(UCSR0A & (1<<UDRE0)) )
              ;
        /* Start transmittion */
        UDR0 = data;
}

В main:

USART0_Init( 500 );

передаю число так :

USART0_Transmit(0xAA);

принимаю так:

char data=USART0_Receive();

#define F_CPU 12500000UL  // 12.5 MHz

/* Initialize UART */
void USART0_Init( unsigned int baudrate )
{
        /* Set the baud rate */
        UBRR0H = (unsigned char) (baudrate>>8);
        UBRR0L = (unsigned char) baudrate;

        /* Enable UART receiver and transmitter */
        UCSR0B = ( ( 1 << RXEN0 ) | ( 1 << TXEN0 ) );

        /* Set frame format: 8 data 2stop */
        UCSR0C = (1<<USBS0)|(1<<UCSZ01)|(1<<UCSZ00);              //For devices with Extended IO
}


/* Read and write functions */
unsigned char USART0_Receive( void )
{
        /* Wait for incomming data */
        while ( !(UCSR0A & (1<<RXC0)) );
        /* Return the data */
        return UDR0;
}

void USART0_Transmit( unsigned char data )
{
        /* Wait for empty transmit buffer */
        while ( !(UCSR0A & (1<<UDRE0)) );
        /* Start transmittion */
        UDR0 = data;
}


void init_all(void){

        /***initialize clock***/  // Set the clock speed to 8MHz
        // set the clock prescaler. First write CLKPCE to enable setting of clock the next four instructions.
        CLKPR=(1<<CLKPCE);
        CLKPR=0;                                // 8 MHz
        delay_ms(20);

       // change clkout from 6.25MHz to 12.5MHz. Тактуется все от 25MHz. Получается 25/2=12.5Mhz
        delay_ms(10);
        USART0_Init(162);  /* 12500000/(16*(162+1.0))/4800*100=99.853 Set the baudrate to 4800 using a 12,5MHz int osc */

        /***enable interrupt***/
        sei();
}

int main(void){
        init_all();

        USART0_Transmit(3); // передать 2 МК число 3
        uint8_t XXX[3];
        XXX[0]= USART0_Receive;

        //записать XXX[0]  в память

        return (0);
}                               //end main

#define F_CPU 12500000UL  // 12.5 MHz

/* Prototypes */
void USART0_Init( unsigned int baudrate );
unsigned char USART0_Receive( void );
void USART0_Transmit( unsigned char data );

/* Initialize UART */
void USART0_Init( unsigned int baudrate )
{
        /* Set the baud rate */
        UBRR0H = (unsigned char) (baudrate>>8);
        UBRR0L = (unsigned char) baudrate;

        /* Enable UART receiver and transmitter */
        UCSR0B = ( ( 1 << RXEN0 ) | ( 1 << TXEN0 ) );

        /* Set frame format: 8 data 2stop */
        UCSR0C = (1<<USBS0)|(1<<UCSZ01)|(1<<UCSZ00);              //For devices with Extended IO
}

void USART0_Transmit( unsigned char data )
{

        /* Wait for empty transmit buffer */
        while ( !(UCSR0A & (1<<UDRE0)) );

        /* Start transmittion */
        UDR0 = data;
}

/* Read and write functions */
unsigned char USART0_Receive( void )
{

        /* Wait for incomming data */
        while ( !(UCSR0A & (1<<RXC0)) );

        /* Return the data */
        return UDR0;
}

void init_all(void){

        /***initialize clock***/                //Set the clock speed to 8MHz
        // set the clock prescaler. First write CLKPCE to enable setting of clock the next four instructions.
        CLKPR=(1<<CLKPCE);
        CLKPR=0;                                // 8 MHz
        delay_ms(3000);                         //change clkout from 6.25MHz to 12.5MHz
        USART0_Init(162);                       /* 12500000/(16*(162+1.0))/4800*100=99.853 error Set the baudrate to 4800 using a 12,5MH*/

        /***enable interrupt***/
        sei();
}


void led_on( unsigned int cikle){

        uint8_t i=0;
        while(i<cikle){
                /* led on, pin=0 */
                PORTC &= ~(1<<PC5);
                delay_ms(500);
                /* set output to 5V, LED off */
                PORTC|= (1<<PC5);
                delay_ms(500);
                i++;
        }
}

/* Main - a simple test program*/
void main( void )
{
        init_all();

        /* INITIALIZE */
        /* enable PC5 as output */
        DDRC|= (1<<DDC5);

        led_on(3);                                   //Тут все верно при старте мигает 3 раза
        delay_ms(1000);                        

        for(;;){                                        /* Forever */

                unsigned charinput=USART0_Receive();
                led_on(charinput);
        }
}