# 11. 蓝牙低功耗（BLE）串口透传

蓝牙低功耗（BLE， Bluetooth Low Energy）串口透传使用较为广泛。在苹果的iOS平台，经典蓝牙需要有MFi认证才可以和苹果的iOS设备进行连接。而蓝牙低功耗设备并没有这个限制。

蓝牙低功耗的协议栈和原理这里就不再赘述了，相关的文章和视频较多。简单的来说就是蓝牙服务以Profile的方式提供，每个服务的Profile下有N个具有独立ID（UUID）的character。每个character有不同的权限（read，write，notify，indicate）。用户定义character并且和权限组合后，就可以提供完整的服务了。

BLE透传的实际上是建立了一个BLE的Service，这个Profile下有2个character。

```
#define SERVICE_UUID           "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" // UART service UUID
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"
```

一个用于TX（发送数据），一个用于RX（接收数据）。为此他们有不同的权限。下面的代码是新建service和character的：

```
  // Create the BLE Service
  BLEService *pService = pServer->createService(SERVICE_UUID);
  
  // Create a BLE Characteristic
  pTxCharacteristic = pService->createCharacteristic(
										CHARACTERISTIC_UUID_TX,
										BLECharacteristic::PROPERTY_NOTIFY
									);
                      
  pTxCharacteristic->addDescriptor(new BLE2902());

  BLECharacteristic * pRxCharacteristic = pService->createCharacteristic(
											 CHARACTERISTIC_UUID_RX,
											BLECharacteristic::PROPERTY_WRITE
										);
```

接下来是2个回调函数，分别在有连接时，以及有write RX character时进行的操作：

```
class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;
    };

    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;
    }
};

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic *pCharacteristic) {
      std::string rxValue = pCharacteristic->getValue();

      if (rxValue.length() > 0) {
        Serial.println("*********");
        Serial.print("Received Value: ");
        for (int i = 0; i < rxValue.length(); i++)
          Serial.print(rxValue[i]);

        Serial.println();
        Serial.println("*********");
      }
    }
};
```

最后主循环是连接的控制，判断是否又连接，是否断开连接。

```
    if (deviceConnected) {
        pTxCharacteristic->setValue(&txValue, 1);
        pTxCharacteristic->notify();
        txValue++;
		    delay(10); // bluetooth stack will go into congestion, if too many packets are sent
	  }
    
    // disconnecting
    if (!deviceConnected && oldDeviceConnected) {
        delay(500); // give the bluetooth stack the chance to get things ready
        pServer->startAdvertising(); // restart advertising
        Serial.println("start advertising");
        oldDeviceConnected = deviceConnected;
    }
    // connecting
    if (deviceConnected && !oldDeviceConnected) {
		// do stuff here on connecting
        oldDeviceConnected = deviceConnected;
    }
```

完整的代码见官方库的示例：ble\_uart，调试工具可以使用LightBlue。


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