interrupt driven transmission

isr_tx.c

ISR(USART_UDRE_vect, ISR_BLOCK) {
	
	// proceed if there still is data to be send
	if (g_tx_buff.head != g_tx_buff.tail) {
		UDR0 = g_tx_buff.ring[g_tx_buff.tail];
		g_tx_buff.tail = (g_tx_buff.tail + 1) % SERIAL_TX_RING_SIZE;
	}
	else {
		// mask the interrupt everything has been send
		UCSR0B &= ~_BV(UDRIE0);
	}
}


unsigned char serial_sendc(unsigned char a_data) {

	uint8_t n = 0x00;
	uint8_t next =
		((g_tx_buff.head + 1) % SERIAL_TX_RING_SIZE);

	/// do not overflow the buffer
	if (next != g_tx_buff.tail) {
		g_tx_buff.ring[g_tx_buff.head] = a_data;
		g_tx_buff.head = next;
		n = 1;

		// enable data register empty interrupt
		UCSR0B |= _BV(UDRIE0);
	}

	return n;
}