interrupt driven data reception

isr_rx.c

// the RX ring buffer
static volatile t_buffer g_rx_buff;

ISR(USART_RX_vect, ISR_BLOCK) {

	// no frame error
	// UCSR0A must be read before UDR0 !!!
	if (bit_is_clear(UCSR0A, FE0)) {
		/// must read the data in order to clear the interrupt flag
		volatile unsigned char data = UDR0;

		/// calculate the next available ring buffer data bucket index
		volatile unsigned char next =
		   	((g_rx_buff.head + 1) % SERIAL_RX_RING_SIZE);

		/// do not overflow the buffer
		if (next != g_rx_buff.tail) {
			g_rx_buff.ring[g_rx_buff.head] = data;
			g_rx_buff.head = next;			
		}
	}
	else {
		/// must read the data in order to clear the interrupt flag
		volatile unsigned char data __attribute__((unused)) = UDR0;
	}
}

unsigned char serial_getc(unsigned char *a_data) {
	
	if (g_rx_buff.head == g_rx_buff.tail)
		return 0;

	*a_data = g_rx_buff.ring[g_rx_buff.tail];
	g_rx_buff.tail = (g_rx_buff.tail + 1) % SERIAL_RX_RING_SIZE;
	return 1;
}