Import grbl v1.1h
diff --git a/grbl/serial.c b/grbl/serial.c
new file mode 100644
index 0000000..cf5f35e
--- /dev/null
+++ b/grbl/serial.c
@@ -0,0 +1,204 @@
+/*
+  serial.c - Low level functions for sending and recieving bytes via the serial port
+  Part of Grbl
+
+  Copyright (c) 2011-2016 Sungeun K. Jeon for Gnea Research LLC
+  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/>.
+*/
+
+#include "grbl.h"
+
+#define RX_RING_BUFFER (RX_BUFFER_SIZE+1)
+#define TX_RING_BUFFER (TX_BUFFER_SIZE+1)
+
+uint8_t serial_rx_buffer[RX_RING_BUFFER];
+uint8_t serial_rx_buffer_head = 0;
+volatile uint8_t serial_rx_buffer_tail = 0;
+
+uint8_t serial_tx_buffer[TX_RING_BUFFER];
+uint8_t serial_tx_buffer_head = 0;
+volatile uint8_t serial_tx_buffer_tail = 0;
+
+
+// Returns the number of bytes available in the RX serial buffer.
+uint8_t serial_get_rx_buffer_available()
+{
+  uint8_t rtail = serial_rx_buffer_tail; // Copy to limit multiple calls to volatile
+  if (serial_rx_buffer_head >= rtail) { return(RX_BUFFER_SIZE - (serial_rx_buffer_head-rtail)); }
+  return((rtail-serial_rx_buffer_head-1));
+}
+
+
+// Returns the number of bytes used in the RX serial buffer.
+// NOTE: Deprecated. Not used unless classic status reports are enabled in config.h.
+uint8_t serial_get_rx_buffer_count()
+{
+  uint8_t rtail = serial_rx_buffer_tail; // Copy to limit multiple calls to volatile
+  if (serial_rx_buffer_head >= rtail) { return(serial_rx_buffer_head-rtail); }
+  return (RX_BUFFER_SIZE - (rtail-serial_rx_buffer_head));
+}
+
+
+// Returns the number of bytes used in the TX serial buffer.
+// NOTE: Not used except for debugging and ensuring no TX bottlenecks.
+uint8_t serial_get_tx_buffer_count()
+{
+  uint8_t ttail = serial_tx_buffer_tail; // Copy to limit multiple calls to volatile
+  if (serial_tx_buffer_head >= ttail) { return(serial_tx_buffer_head-ttail); }
+  return (TX_RING_BUFFER - (ttail-serial_tx_buffer_head));
+}
+
+
+void serial_init()
+{
+  // Set baud rate
+  #if BAUD_RATE < 57600
+    uint16_t UBRR0_value = ((F_CPU / (8L * BAUD_RATE)) - 1)/2 ;
+    UCSR0A &= ~(1 << U2X0); // baud doubler off  - Only needed on Uno XXX
+  #else
+    uint16_t UBRR0_value = ((F_CPU / (4L * BAUD_RATE)) - 1)/2;
+    UCSR0A |= (1 << U2X0);  // baud doubler on for high baud rates, i.e. 115200
+  #endif
+  UBRR0H = UBRR0_value >> 8;
+  UBRR0L = UBRR0_value;
+
+  // enable rx, tx, and interrupt on complete reception of a byte
+  UCSR0B |= (1<<RXEN0 | 1<<TXEN0 | 1<<RXCIE0);
+
+  // defaults to 8-bit, no parity, 1 stop bit
+}
+
+
+// Writes one byte to the TX serial buffer. Called by main program.
+void serial_write(uint8_t data) {
+  // Calculate next head
+  uint8_t next_head = serial_tx_buffer_head + 1;
+  if (next_head == TX_RING_BUFFER) { next_head = 0; }
+
+  // Wait until there is space in the buffer
+  while (next_head == serial_tx_buffer_tail) {
+    // TODO: Restructure st_prep_buffer() calls to be executed here during a long print.
+    if (sys_rt_exec_state & EXEC_RESET) { return; } // Only check for abort to avoid an endless loop.
+  }
+
+  // Store data and advance head
+  serial_tx_buffer[serial_tx_buffer_head] = data;
+  serial_tx_buffer_head = next_head;
+
+  // Enable Data Register Empty Interrupt to make sure tx-streaming is running
+  UCSR0B |=  (1 << UDRIE0);
+}
+
+
+// Data Register Empty Interrupt handler
+ISR(SERIAL_UDRE)
+{
+  uint8_t tail = serial_tx_buffer_tail; // Temporary serial_tx_buffer_tail (to optimize for volatile)
+
+  // Send a byte from the buffer
+  UDR0 = serial_tx_buffer[tail];
+
+  // Update tail position
+  tail++;
+  if (tail == TX_RING_BUFFER) { tail = 0; }
+
+  serial_tx_buffer_tail = tail;
+
+  // Turn off Data Register Empty Interrupt to stop tx-streaming if this concludes the transfer
+  if (tail == serial_tx_buffer_head) { UCSR0B &= ~(1 << UDRIE0); }
+}
+
+
+// Fetches the first byte in the serial read buffer. Called by main program.
+uint8_t serial_read()
+{
+  uint8_t tail = serial_rx_buffer_tail; // Temporary serial_rx_buffer_tail (to optimize for volatile)
+  if (serial_rx_buffer_head == tail) {
+    return SERIAL_NO_DATA;
+  } else {
+    uint8_t data = serial_rx_buffer[tail];
+
+    tail++;
+    if (tail == RX_RING_BUFFER) { tail = 0; }
+    serial_rx_buffer_tail = tail;
+
+    return data;
+  }
+}
+
+
+ISR(SERIAL_RX)
+{
+  uint8_t data = UDR0;
+  uint8_t next_head;
+
+  // Pick off realtime command characters directly from the serial stream. These characters are
+  // not passed into the main buffer, but these set system state flag bits for realtime execution.
+  switch (data) {
+    case CMD_RESET:         mc_reset(); break; // Call motion control reset routine.
+    case CMD_STATUS_REPORT: system_set_exec_state_flag(EXEC_STATUS_REPORT); break; // Set as true
+    case CMD_CYCLE_START:   system_set_exec_state_flag(EXEC_CYCLE_START); break; // Set as true
+    case CMD_FEED_HOLD:     system_set_exec_state_flag(EXEC_FEED_HOLD); break; // Set as true
+    default :
+      if (data > 0x7F) { // Real-time control characters are extended ACSII only.
+        switch(data) {
+          case CMD_SAFETY_DOOR:   system_set_exec_state_flag(EXEC_SAFETY_DOOR); break; // Set as true
+          case CMD_JOG_CANCEL:   
+            if (sys.state & STATE_JOG) { // Block all other states from invoking motion cancel.
+              system_set_exec_state_flag(EXEC_MOTION_CANCEL); 
+            }
+            break; 
+          #ifdef DEBUG
+            case CMD_DEBUG_REPORT: {uint8_t sreg = SREG; cli(); bit_true(sys_rt_exec_debug,EXEC_DEBUG_REPORT); SREG = sreg;} break;
+          #endif
+          case CMD_FEED_OVR_RESET: system_set_exec_motion_override_flag(EXEC_FEED_OVR_RESET); break;
+          case CMD_FEED_OVR_COARSE_PLUS: system_set_exec_motion_override_flag(EXEC_FEED_OVR_COARSE_PLUS); break;
+          case CMD_FEED_OVR_COARSE_MINUS: system_set_exec_motion_override_flag(EXEC_FEED_OVR_COARSE_MINUS); break;
+          case CMD_FEED_OVR_FINE_PLUS: system_set_exec_motion_override_flag(EXEC_FEED_OVR_FINE_PLUS); break;
+          case CMD_FEED_OVR_FINE_MINUS: system_set_exec_motion_override_flag(EXEC_FEED_OVR_FINE_MINUS); break;
+          case CMD_RAPID_OVR_RESET: system_set_exec_motion_override_flag(EXEC_RAPID_OVR_RESET); break;
+          case CMD_RAPID_OVR_MEDIUM: system_set_exec_motion_override_flag(EXEC_RAPID_OVR_MEDIUM); break;
+          case CMD_RAPID_OVR_LOW: system_set_exec_motion_override_flag(EXEC_RAPID_OVR_LOW); break;
+          case CMD_SPINDLE_OVR_RESET: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_RESET); break;
+          case CMD_SPINDLE_OVR_COARSE_PLUS: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_COARSE_PLUS); break;
+          case CMD_SPINDLE_OVR_COARSE_MINUS: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_COARSE_MINUS); break;
+          case CMD_SPINDLE_OVR_FINE_PLUS: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_FINE_PLUS); break;
+          case CMD_SPINDLE_OVR_FINE_MINUS: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_FINE_MINUS); break;
+          case CMD_SPINDLE_OVR_STOP: system_set_exec_accessory_override_flag(EXEC_SPINDLE_OVR_STOP); break;
+          case CMD_COOLANT_FLOOD_OVR_TOGGLE: system_set_exec_accessory_override_flag(EXEC_COOLANT_FLOOD_OVR_TOGGLE); break;
+          #ifdef ENABLE_M7
+            case CMD_COOLANT_MIST_OVR_TOGGLE: system_set_exec_accessory_override_flag(EXEC_COOLANT_MIST_OVR_TOGGLE); break;
+          #endif
+        }
+        // Throw away any unfound extended-ASCII character by not passing it to the serial buffer.
+      } else { // Write character to buffer
+        next_head = serial_rx_buffer_head + 1;
+        if (next_head == RX_RING_BUFFER) { next_head = 0; }
+
+        // Write data to buffer unless it is full.
+        if (next_head != serial_rx_buffer_tail) {
+          serial_rx_buffer[serial_rx_buffer_head] = data;
+          serial_rx_buffer_head = next_head;
+        }
+      }
+  }
+}
+
+
+void serial_reset_read_buffer()
+{
+  serial_rx_buffer_tail = serial_rx_buffer_head;
+}