GRBL分析：serial文件

宏定義：

RX_BUFFER_SIZE   128

  #ifdef USE_LINE_NUMBERS
    #define TX_BUFFER_SIZE    112
  #else
    #define TX_BUFFER_SIZE    104

SERIAL_NO_DATA     0xff

#define RX_RING_BUFFER (RX_BUFFER_SIZE+1)
#define TX_RING_BUFFER (TX_BUFFER_SIZE+1)

使用的變量：

uint8_t serial_rx_buffer[RX_RING_BUFFER]; //讀取buf
uint8_t serial_rx_buffer_head = 0;        //讀取buf的頭
volatile uint8_t serial_rx_buffer_tail = 0; //讀取buf的尾   使用volatile關鍵字讓編譯器將該變量放到cpu的cache裏面方便快速讀取
uint8_t serial_tx_buffer[TX_RING_BUFFER]; //發送buf
uint8_t serial_tx_buffer_head = 0;        //發送buf的頭
volatile uint8_t serial_tx_buffer_tail = 0; //發送buf的尾

實際上是兩個環形數組，一個爲發送的緩衝區、另一個爲接受緩衝區

函數說明：

void serial_init();  //主要設置串口的波特率、停止位、校驗位、流控制

// 獲取串口接收緩衝區中可用的字節數
uint8_t serial_get_rx_buffer_available();

// // 獲取讀取緩衝區中已使用的字節數
// NOTE: Deprecated. Not used unless classic status reports are enabled in config.h.
uint8_t serial_get_rx_buffer_count();

// 獲取發送緩衝區中已使用的字節數
// NOTE: Not used except for debugging and ensuring no TX bottlenecks.
uint8_t serial_get_tx_buffer_count();

// 寫入一個字節到發送緩衝區中
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
  // 在打印較長的信息時由於緩衝區的大小不夠因此需要等待緩衝區有空閒再繼續
  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;  //發送數據寫入到發送緩衝區中
  serial_tx_buffer_head = next_head;
  // Enable Data Register Empty Interrupt to make sure tx-streaming is running
  UCSR0B |=  (1 << UDRIE0);
}

uint8_t serial_read()；   /讀取接受緩衝區中的接受數據部分的第一個字節
void serial_reset_read_buffer()； //重置接收緩衝區，讀寫指針相等即可


//讀取中斷----支持實時指令並在幾毫秒內響應是通過接收數據中斷來實現的，由於實時指令均是單個ASCII碼，因此在中斷中接收到之後無需存儲即可立刻分析，並立即執行大大的節約了時間
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               //這部分判斷出不是實時指令，就存儲到接收緩衝區中，等待後面的處理
        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;
        }
      }
  }
}