calculator.c

#include <avr/io.h> 
 
void USART_Init( unsigned int ubrr){
	/* Set baud rate */
	UBRR1H = (unsigned char)(ubrr>>8);
	UBRR1L = (unsigned char)ubrr;
	/* Enable receiver and transmitter */
	UCSR1B = (1<<RXEN1)|(1<<TXEN1);
	/* Set frame format: 8data, 2stop bit */
	UCSR1C = (1<<USBS1)|(1<<UCSZ11)|(1<<UCSZ10);
} // USART_Init

void USART_Transmit( unsigned char data )
{
	/* Wait for empty transmit buffer */
	while ( !( UCSR1A & (1<<UDRE1)) )
	;
	/* Put data into buffer, sends the data */
	UDR1 = data;
}

void USART_TransmitStr ( char *data )
{
	for (; *data; data++) {
		USART_Transmit(*data);
	}
}

unsigned char USART_Receive( void )
{
	/* Wait for data to be received */
	while ( !(UCSR1A & (1<<RXC1)) )
	;
	/* Get and return received data from buffer */
	return UDR1;
}

int main() {
	USART_Init(51);
	
	uint32_t a = 0, b = 0, result = 0;
	unsigned char operator = 0;
	
	while (1) {
		unsigned char ch = USART_Receive();
		
		switch (ch) {
			case 8:
				// Backspace.
				if (!operator) {
					a /= 10;
				} else {
					b /= 10;
				}
				break;
				
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9':
				if (!operator) {
					a = a * 10 + (ch - '0');
				} else{
					b = b * 10 + (ch - '0');
				}
				break;
				
			case '+':
			case '-':
			case '*':
			case '/':
			case '%':
				operator = ch;
				break;
				
			case '\r':
			case '=':
				result = 0;
				switch (operator) {
					case '+':
						result = a + b;
						break;
					case '-':
						result = a - b;
						break;
					case '*':
						result = a * b;
						break;
					case '/':
						result = a / b;
						break;
					case '%':
						result = a % b;
						break;
				}
				
				if (ch == '\r') {
					USART_Transmit('=');
				} else {
					USART_Transmit(ch);
				}
				
				char buffer[20];
				sprintf(buffer, "%d\r\n", result);
				USART_TransmitStr(buffer);
				
				operator = 0;
				a = b = 0;
				continue;
				
			default:
				continue;
		}
		
		USART_Transmit(ch);
	}
}