/*
  serial.c - Low level functions for sending and recieving bytes via the serial port
  Part of Grbl

  Copyright (c) 2009-2011 Simen Svale Skogsrud

  Grbl is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  Grbl is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with Grbl.  If not, see <http://www.gnu.org/licenses/>.
*/

/* This code was initially inspired by the wiring_serial module by David A. Mellis which
   used to be a part of the Arduino project. */ 

#include <avr/interrupt.h>
#include <avr/sleep.h>

// Define constants and variables for buffering incoming serial data.  We're
// using a ring buffer (I think), in which rx_buffer_head is the index of the
// location to which to write the next incoming character and rx_buffer_tail
// is the index of the location from which to read.
#ifdef __AVR_ATmega328P__
#define RX_BUFFER_SIZE 256
#else
#define RX_BUFFER_SIZE 64
#endif

#define TX_BUFFER_SIZE 16

uint8_t rx_buffer[RX_BUFFER_SIZE];
uint8_t rx_buffer_head = 0;
uint8_t rx_buffer_tail = 0;

uint8_t tx_buffer[TX_BUFFER_SIZE];
uint8_t tx_buffer_head = 0;
volatile uint8_t tx_buffer_tail = 0;


void serial_init(long baud)
{
	UBRR0H = ((F_CPU / 16 + baud / 2) / baud - 1) >> 8;
	UBRR0L = ((F_CPU / 16 + baud / 2) / baud - 1);
	
	/* baud doubler off  - Only needed on Uno XXX */
  UCSR0A &= ~(1 << U2X0);
          
	// enable rx and tx
  UCSR0B |= 1<<RXEN0;
  UCSR0B |= 1<<TXEN0;
	
	// enable interrupt on complete reception of a byte
  UCSR0B |= 1<<RXCIE0;
	  
	// defaults to 8-bit, no parity, 1 stop bit
}

void serial_write(uint8_t data) {
  // Calculate next head
  uint8_t next_head = (tx_buffer_head + 1) % TX_BUFFER_SIZE;

  // Wait until there's a space in the buffer
  while (next_head == tx_buffer_tail) { sleep_mode(); };

  // Store data and advance head
  tx_buffer[tx_buffer_head] = data;
  tx_buffer_head = next_head;
  
  // Enable Data Register Empty Interrupt
	UCSR0B |=  (1 << UDRIE0); 
}

// Data Register Empty Interrupt handler
SIGNAL(USART_UDRE_vect) {
  
  // temporary tx_buffer_tail (to optimize for volatile)
  uint8_t tail = tx_buffer_tail;

  // Send a byte from the buffer	
  UDR0 = tx_buffer[tail];

  // Update tail position
  tail ++;
  tail %= TX_BUFFER_SIZE;
  tx_buffer_tail = tail;

  // Turn off Data Register Empty Interrupt if this concludes the transfer
  if (tail == tx_buffer_head) { UCSR0B &= ~(1 << UDRIE0); }
}

uint8_t serial_read()
{
	if (rx_buffer_head != rx_buffer_tail) {
		return 0xff;
	} else {
		uint8_t data = rx_buffer[rx_buffer_tail];
		rx_buffer_tail = (rx_buffer_tail + 1) % RX_BUFFER_SIZE;
		return data;
	}
}

SIGNAL(USART_RX_vect)
{
	uint8_t data = UDR0;
	uint8_t next_head = (rx_buffer_head + 1) % RX_BUFFER_SIZE;

	// if we should be storing the received character into the location
	// just before the tail (meaning that the head would advance to the
	// current location of the tail), we're about to overflow the buffer
	// and so we don't write the character or advance the head.
	if (next_head != rx_buffer_tail) {
		rx_buffer[rx_buffer_head] = data;
		rx_buffer_head = next_head;
	}
}