/*
 * Minimalistic TX-only compile-time configured asynchronous buffered
 * UART communication for Arduino Nano (Atmega 88/168/328).
 */
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/atomic.h>
#include <util/delay.h>

// user settable parameters start
#define UART_BAUD	115200
#define TX_BUFFER	(1 << 4)	// this has to be a power of two
// user settable parameters end

#define TX_BUFFER_MASK	(TX_BUFFER-1)

#if (TX_BUFFER & TX_BUFFER_MASK)
#error TX buffer size TX_BUFFER must be a power of two
#endif

#ifndef F_CPU
#error F_CPU not defined, set it in Makefile
#endif

#define UBRR_VAL        ((F_CPU + 8UL * UART_BAUD) / (16UL*UART_BAUD) - 1)

static volatile unsigned char buffer[TX_BUFFER];
static volatile unsigned char b_head, b_tail, b_overrun;
/*
 * b_head == b_tail: empty queue
 * b_head + 1 == b_tail: full queue
 * b_overrun is set but not currently handled in any way
 */

void uart_init()
{
	UBRR0H = (unsigned char)(UBRR_VAL >> 8);
	UBRR0L = (unsigned char)(UBRR_VAL & 0xFF);

	UCSR0C = _BV(USBS0) | _BV(UCSZ00) | _BV(UCSZ01);
	UCSR0B = _BV(TXEN0); // enable TX only

	b_head = 0;
	b_tail = 0;
	b_overrun = 0;
}

ISR(USART_UDRE_vect) // tx end irq
{
	// maybe handle overrun here?
	if (b_head == b_tail) {	// queue empty
		UCSR0B &= ~_BV(UDRIE0); // disable IRQ
	} else {
		UDR0 = buffer[b_tail];
		b_tail = (b_tail + 1) & TX_BUFFER_MASK;
	}
}

// low-level byte sending
unsigned char uart_tx_byte(unsigned char byte, unsigned char wait)
{
	ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
	retry:
		if (((b_head + 1) & TX_BUFFER_MASK) == b_tail) {
			if (wait) {
				NONATOMIC_BLOCK(NONATOMIC_FORCEOFF) {
					while (((b_head + 1) & TX_BUFFER_MASK)
						== b_tail)
						;
				}
				goto retry;
			}

			b_overrun = 1;
			return 0;
		}

		buffer[b_head] = byte;
		b_head = (b_head + 1) & TX_BUFFER_MASK;

		UCSR0B |= _BV(UDRIE0);
	}
	return 1;
}

// ASCII hex bytes, for debugging purposes

#define hex_nibble(x) ((x) < 10 ? '0' + (x) : 'A' + (x) - 10)

unsigned char uart_tx_hex(unsigned char byte, unsigned char wait)
{
	if (
		   uart_tx_byte(hex_nibble(byte >> 4), wait)
		&& uart_tx_byte(hex_nibble(byte & 0xF), wait)
	)
		return 1;
	else
		return 0;
}

unsigned char uart_tx_hex2(unsigned short word, unsigned char wait)
{
	if (
		   uart_tx_byte(hex_nibble((word >> 12) & 0xF), wait)
		&& uart_tx_byte(hex_nibble((word >>  8) & 0xF), wait)
		&& uart_tx_byte(hex_nibble((word >>  4) & 0xF), wait)
		&& uart_tx_byte(hex_nibble( word        & 0xF), wait)
	)
		return 1;
	else
		return 0;
}