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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