この記事はプロトアウトスタジオのアドベントカレンダー2020 の6日目の記事です！

やったこと

M5Stackのビーコン化に挑戦してみました。

もう少し説明しますと、
M5Stack Gray のジャイロセンサーの値を別の M5Stack Gray へBLE通信で送ってみました。
なお、Twitterで「BLT通信」とはつぶやいてるのにはご容赦ください。おそらくBLEとLTEが混ざったようです。サンドイッチでもありませんw

なんとかM5StackどおしのBLT通信できたっぽい。　#protoout pic.twitter.com/wqukEAOW6O
— たつや@草刈りゲームのクラファン実施中（12/21まで） (@tatsuya1970) November 3, 2020

ついでに、電波強度（RSSI、TxPower）も取得して、距離を測ろうと試みしたが、以下のツイートのように正確な距離を測定することはできませんでした。
私のやり方がおかしいのか、周りの電波に影響されているのか分かりませんが、現状の私のスキルでは、ビーコンは正確な距離ではなく、アバウトに近いづいてきたかどうか判定するケースに使おうと思います。

とはいえ、せっかくここまでしましたので、備忘録をかねて、Qiitaに書きます。

なお、Twitterで今度は「LTE通信」とつぶやいてますね。通信のことをよく分かってないのがモロバレです。

M5stackをビーコン化するのに挑戦中。ジャイロセンサーのX軸の値をLTE送信。受信側でRSSIとTXPowerを取得して距離を求めるも、数値がバラバラ。うーむ。今日はこれまで。#protoout pic.twitter.com/tmw6ppG0lA
— たつや@草刈りゲームのクラファン実施中（12/21まで） (@tatsuya1970) December 6, 2020

発信(ビーコン）側

ビーコンについてのコードは、このサイトのコードをほぼそのまま使用しました。
M5StackとM5StickC間のarduino BLE通信

コード

BLEServer.ino


// <M5Stack.h>をincludeする前に、IMUモジュールを#defineしておく
#define M5STACK_MPU6886 
#include <Arduino.h>
#include <M5Stack.h>
#include <WiFi.h>
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

uint16_t Dvalue = 0;

BLEServer* pServer = NULL;
BLECharacteristic* pCharacteristic = NULL;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint32_t value = 0;

#define SERVICE_UUID        "7780" //適当なID(上のような長いUUIDだとうまく認識されなかったので適当に）
#define CHARACTERISTIC_UUID "7781" //適当なID

float gyroX　; //M5StackのジャイロセンサーX軸の値

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

void setup()
{
  M5.begin();
  M5.Power.begin();
  M5.IMU.Init();
  WiFi.mode(WIFI_OFF); 
  Serial.print("");
  // Create the BLE Device
  BLEDevice::init("M5STACK1G"); //適当な名前

  // Create the BLE Server
  pServer = BLEDevice::createServer();
  pServer->setCallbacks(new MyServerCallbacks());

  // Create the BLE Service
  BLEService *pService = pServer->createService(SERVICE_UUID);

  // Create a BLE Characteristic
  pCharacteristic = pService->createCharacteristic(
                      CHARACTERISTIC_UUID,
                      BLECharacteristic::PROPERTY_READ   |
                      BLECharacteristic::PROPERTY_WRITE  |
                      BLECharacteristic::PROPERTY_NOTIFY |
                      BLECharacteristic::PROPERTY_INDICATE
                    );

  // https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.descriptor.gatt.client_characteristic_configuration.xml
  // Create a BLE Descriptor
  pCharacteristic->addDescriptor(new BLE2902());

  // Start the service
  pService->start();

  // Start advertising
  BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
  pAdvertising->addServiceUUID(SERVICE_UUID);
  pAdvertising->setScanResponse(false);
  pAdvertising->setMinPreferred(0x0);  // set value to 0x00 to not advertise this parameter
  BLEDevice::startAdvertising();

}

void loop()
{
  M5.update();
  uint8_t buf[2]; //要素数２

  M5.IMU.getGyroData(&gyroX); //ジャイロデータ取得

    M5.Lcd.clear(BLACK);
    M5.Lcd.setCursor(10, 0);
    M5.Lcd.setTextSize(5);
    M5.Lcd.setTextColor(YELLOW);
    M5.Lcd.println("GyroX:");
    M5.Lcd.setTextColor(RED);
    M5.Lcd.println(String(gyroX));
    Serial.println(gyroX);
    delay(300);
    Dvalue=gyroX; 

    if (deviceConnected) {
        memset(buf,0,sizeof buf);  
        buf[0]=(uint8_t)(Dvalue & 0xff);
        buf[1]=(uint8_t)((Dvalue >>8) & 0xff);
        // 数値は0～500少しまであるので、2バイト使います

        pCharacteristic->setValue(buf,sizeof buf);
        pCharacteristic->notify();
        value++;
        delay(3); // bluetooth stack will go into congestion, if too many packets are sent, in 6 hours test i was able to go as low as 3ms
    }
    // disconnecting
    if (!deviceConnected && oldDeviceConnected) {
        delay(500); // give the bluetooth stack the chance to get things ready
        pServer->startAdvertising(); // restart advertising
        oldDeviceConnected = deviceConnected;
    }
    // connecting
    if (deviceConnected && !oldDeviceConnected) {
        // do stuff here on connecting
        oldDeviceConnected = deviceConnected;
    }
}

受信側

ビーコンについてのコードは、発信側と同じく、こちらのサイトのコードをほぼそのまま使用しました。
・M5StackとM5StickC間のarduino BLE通信

RSSI値とTXPower値は、BLEAdvertisedDevice の getRSSI() 、getTXPower（）で呼び出しで取得できます。以下のサイトを参考にしました。
・M5StackでBLEタグのRSSIを取得する
・M5StickC非公式日本語リファレンス

ビーコンの距離についてはこちらのサイトを参考にしました
・RSSI と TxPower からビーコンとの距離および近接度（Proximity）を推定する

コード

BLECLient.ino

#include <Arduino.h>
#include <M5Stack.h>
#include "BLEDevice.h"

// The remote service we wish to connect to.
static BLEUUID serviceUUID("7780");
// The characteristic of the remote service we are interested in.
static BLEUUID    charUUID("7781");

static boolean doConnect = false;
static boolean connected = false;
static boolean doScan = false;
static BLERemoteCharacteristic* pRemoteCharacteristic;
static BLEAdvertisedDevice* myDevice;

uint16_t val; 

static void notifyCallback(
  BLERemoteCharacteristic* pBLERemoteCharacteristic,  uint8_t* pData,  size_t length,  bool isNotify) {
    val=0;
    val=(uint16_t)(pData[1]<<8 | pData[0]); //ペリフェラルから受信した値（2バイト）を数値に変換
    Serial.println(val);

}

class MyClientCallback : public BLEClientCallbacks {
  void onConnect(BLEClient* pclient) {
  }

  void onDisconnect(BLEClient* pclient) {
    connected = false;
    Serial.println("onDisconnect");
  }
};

bool connectToServer() {
    Serial.print("Forming a connection to ");
    Serial.println(myDevice->getAddress().toString().c_str());

    BLEClient*  pClient  = BLEDevice::createClient();
    Serial.println(" - Created client");

    pClient->setClientCallbacks(new MyClientCallback());

    // Connect to the remove BLE Server.
    pClient->connect(myDevice);  // if you pass BLEAdvertisedDevice instead of address, it will be recognized type of peer device address (public or private)
    Serial.println(" - Connected to server");

    // Obtain a reference to the service we are after in the remote BLE server.
    BLERemoteService* pRemoteService = pClient->getService(serviceUUID);
    if (pRemoteService == nullptr) {
      Serial.print("Failed to find our service UUID: ");
      Serial.println(serviceUUID.toString().c_str());
      pClient->disconnect();
      return false;
    }
    Serial.println(" - Found our service");

    // Obtain a reference to the characteristic in the service of the remote BLE server.
    pRemoteCharacteristic = pRemoteService->getCharacteristic(charUUID);
    if (pRemoteCharacteristic == nullptr) {
      Serial.print("Failed to find our characteristic UUID: ");
      Serial.println(charUUID.toString().c_str());
      pClient->disconnect();
      return false;
    }
    Serial.println(" - Found our characteristic");

    // Read the value of the characteristic.
    if(pRemoteCharacteristic->canRead()) {
      //サンプルだと下の行を実行するタイミングでリブートかかるのでコメント化。
      //notifyCallbackのほうでCharacteristicから取得するように変更
      //std::string value = pRemoteCharacteristic->readValue();
      //Serial.print("The characteristic value was: ");
      //Serial.println(value.c_str());
    }

    if(pRemoteCharacteristic->canNotify())
      pRemoteCharacteristic->registerForNotify(notifyCallback);

    connected = true;

}
/**
 * Scan for BLE servers and find the first one that advertises the service we are looking for.
 */
class MyAdvertisedDeviceCallbacks: public BLEAdvertisedDeviceCallbacks {
 /**
   * Called for each advertising BLE server.
   */
  void onResult(BLEAdvertisedDevice advertisedDevice) {
    Serial.print("BLE Advertised Device found: ");
    Serial.println(advertisedDevice.toString().c_str());
    Serial.println(advertisedDevice.getRSSI());
    Serial.println(advertisedDevice.getTXPower());

    M5.Lcd.setTextSize(2);
    M5.Lcd.setTextColor(WHITE);

    //M5.Lcd.print(advertisedDevice.getName().c_str());
    M5.Lcd.print("RSSI:");
    M5.Lcd.print(advertisedDevice.getRSSI());

    M5.Lcd.print(" / TxPower:");
    M5.Lcd.println(advertisedDevice.getTXPower());

    float RSSI = advertisedDevice.getRSSI();
    float tx = advertisedDevice.getTXPower();
    //距離を求めてるつもりだけど自信なし
    float distance = pow(10.0, (tx - RSSI) / 20.0); 

    M5.Lcd.setTextColor(YELLOW);
    M5.Lcd.print("Distance:");
    M5.Lcd.println(distance);

    // We have found a device, let us now see if it contains the service we are looking for.

    if (advertisedDevice.haveServiceUUID() && advertisedDevice.isAdvertisingService(serviceUUID)) {

      BLEDevice::getScan()->stop();
      myDevice = new BLEAdvertisedDevice(advertisedDevice);
      doConnect = true;
      doScan = true;

    } // Found our server
  } // onResult
}; // MyAdvertisedDeviceCallbacks


void setup() {
  M5.begin();
  Serial.begin(1500000);
  Serial.println("Starting Arduino BLE Client application...");
  BLEDevice::init("");

  // Retrieve a Scanner and set the callback we want to use to be informed when we
  // have detected a new device.  Specify that we want active scanning and start the
  // scan to run for 5 seconds.
  BLEScan* pBLEScan = BLEDevice::getScan();
  pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks());

  pBLEScan->setInterval(1349);
  pBLEScan->setWindow(449);
  pBLEScan->setActiveScan(true);
  pBLEScan->start(5, false);
} // End of setup.


// This is the Arduino main loop function.
void loop() {

  M5.Lcd.fillScreen(BLACK); //この辺の表示は接続確認後、にしたほうがいいかも
  M5.Lcd.setCursor(10, 0);
  M5.Lcd.setTextSize(3);
  M5.Lcd.setTextColor(YELLOW);
  M5.Lcd.print("GyroX:");
  M5.Lcd.setTextColor(RED);   
  M5.Lcd.println(val);


  // If the flag "doConnect" is true then we have scanned for and found the desired
  // BLE Server with which we wish to connect.  Now we connect to it.  Once we are 
  // connected we set the connected flag to be true.
  if (doConnect == true) {
    if (connectToServer()) {
      Serial.println("We are now connected to the BLE Server.");
    } else {
      Serial.println("We have failed to connect to the server; there is nothin more we will do.");
    }
    doConnect = false;
  }

  // If we are connected to a peer BLE Server, update the characteristic each time we are reached
  // with the current time since boot.
  if (connected) {
    String newValue = "Time since boot: " + String(millis()/1000);
    Serial.println("Setting new characteristic value to \"" + newValue + "\"");

    M5.Lcd.setTextSize(1);
    M5.Lcd.setTextColor(BLUE);
    M5.Lcd.print("newValue:");
    M5.Lcd.println(newValue);

    // Set the characteristic's value to be the array of bytes that is actually a string.
    pRemoteCharacteristic->writeValue(newValue.c_str(), newValue.length());


  }else if(doScan){
    BLEDevice::getScan()->start(0);  // this is just eample to start scan after disconnect, most likely there is better way to do it in arduino
  }

  BLEScan* pBLEScan = BLEDevice::getScan();
  pBLEScan->start(1);

  delay(1000); // Delay a second between loops.
}