UCSRB= (1 << TXEN )|(1 << RXEN )|(1 << TXIE )|(1 << RXIE );

/*********************************************
* Chip type           : ATmega16
* Clock frequency     : 16,000000 MHz
*********************************************/
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/signal.h>
#include <inttypes.h>
#include <avr/iom16.h>

#define F_OSC 16000000                   /* oscillator-frequency in Hz */
#define UART_BAUD_RATE 9600
#define UART_BAUD_CALC(UART_BAUD_RATE,F_OSC) ((F_OSC)/((UART_BAUD_RATE)*16l)-1)

void delay_ms(unsigned short ms) {
    unsigned short outer1, outer2;
    outer1 = 200;
    while (outer1) {
        outer2 = 1000;
        while (outer2) {
            while ( ms ) ms--;
            outer2--;
        }
        outer1--;
    }
}

void usart_putc(unsigned char c) {
   // wait until UDR ready
    while(!(UCSRA & (1 << UDRE)));
    UDR = c;    // send character
}

void uart_puts (char *s) {
    //  loop until *s != NULL
    while (*s) {
        usart_putc(*s);
        s++;
    }
}

void init(void) {
    // set baud rate
    UBRRH = (uint8_t)(UART_BAUD_CALC(UART_BAUD_RATE,F_OSC)>>8);
    UBRRL = (uint8_t)UART_BAUD_CALC(UART_BAUD_RATE,F_OSC);

    // Enable receiver and transmitter; enable RX interrupt
    UCSRB = (1 << RXEN) | (1 << TXEN) | (1 << RXCIE);

    //asynchronous 8N1
    UCSRC = (1 << URSEL) | (3 << UCSZ0);
}

// INTERRUPT can be interrupted
// SIGNAL can't be interrupted
SIGNAL (SIG_UART_RECV) { // USART RX interrupt
    unsigned char c;
    c = UDR;
    usart_putc(c);
}

int main(void) {
    init(); // init USART

    // send initial character
    while(!(UCSRA & (1 << UDRE)));
    UDR = 0x43; // "C"
    while(!(UCSRA & (1 << UDRE)));
    UDR = 0x0d;

    uart_puts("Test\n\r");

    sei();  // enable interrupts

    // enable  PD5 as output
    DDRD |= (1<<PD5);
    while (1) {
        // PIN5 PORTD clear -> LED off
        PORTD &= ~(1 << PD5);
        delay_ms(500);
        // PIN5 PORTD set -> LED on
        PORTD |= (1 << PD5);
        delay_ms(500);    
    }
    return 0;
}