XIAO nRF52840/CircuitPython implement BLE UART Service, linked with XIAO ESP32C3/Arduino BLE UART Client.

 

The BLE GATT Nordic UART Service (NUS) is a custom service that receives and writes data and serves as a bridge to the UART interface.
ref: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/2.3.0/nrf/libraries/bluetooth_services/services/nus.html

In this exercise, XIAO nRF52840 Sense running CircuitPython, implement BLE UART Service.

cpynRF_ble.py

"""
Circuitpython BLE exercise
Run on XIAO nRF52840 Sense/CircuitPython 8.1.0-beta.1

Provide an echo service over BLE UART.
Act as BLE UART server,
receive from BLE UART and echo back

ref:
https://learn.adafruit.com/circuitpython-ble-libraries-on-any-computer/ble-uart-example
"""
import os, sys
import time
import board, busio

from adafruit_ble import BLERadio
from adafruit_ble.advertising.standard import ProvideServicesAdvertisement
from adafruit_ble.services.nordic import UARTService

#------------------------
print("========================================")
print(sys.implementation[0], os.uname()[3],
      "\nrun on", os.uname()[4])
print("========================================")

ble = BLERadio()
print(ble.name)

uart = UARTService()
advertisement = ProvideServicesAdvertisement(uart)

while True:
    ble.start_advertising(advertisement)
    print("Waiting to connect")
    while not ble.connected:
        pass
    print("Connected")
    while ble.connected:
        line = uart.readline()
        if line:
            print(type(line), line)
            stringout = str(line)
            uart.write(stringout.encode("utf-8"))

In the BLE client side, basically copy from my ole exercise BLE UART Communication between XIAO ESP32C3 (client/central) and nRF52840 (server/peripheral), in Arduino Framework.

XESP32C3_BLE_client.ino

/*
 * https://coxxect.blogspot.com/2022/10/ble-uart-communication-between-xiao.html
 *
 * Modify Examples from XIAO_ESP32C3
 *        > ESP32 BLE Arduino 
 *        > BLE_client
 * Run on Xiao ESP32C3.
 * With UUID of Nordic UART Service (NUS), to work with 
 * XnRF52_bleuart_OLED_Terminal run on Xiao nRF52840 Sensse.
 * Read from Serial and send to BLE UART server/peripheral.
 */

#include "BLEDevice.h"
//#include "BLEScan.h"

// UUID of Nordic UART Service (NUS)
#define SERVICE_UUID           "6E400001-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"

// The remote service we wish to connect to.
static BLEUUID serviceUUID(SERVICE_UUID);
// The characteristic of the remote service we are interested in.
static BLEUUID    charUUID_RX(CHARACTERISTIC_UUID_RX);

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

static void notifyCallback(
  BLERemoteCharacteristic* pBLERemoteCharacteristic,
  uint8_t* pData,
  size_t length,
  bool isNotify) {
    Serial.print("Notify callback for characteristic ");
    Serial.print(pBLERemoteCharacteristic->getUUID().toString().c_str());
    Serial.print(" of data length ");
    Serial.println(length);
    Serial.print("data: ");
    Serial.println((char*)pData);
}

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");
    pClient->setMTU(517); //set client to request maximum MTU from server (default is 23 otherwise)
  
    // 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_RX = pRemoteService->getCharacteristic(charUUID_RX);
    if (pRemoteCharacteristic_RX == nullptr) {
      Serial.print("Failed to find our characteristic UUID: ");
      Serial.println(charUUID_RX.toString().c_str());
      pClient->disconnect();
      return false;
    }
    Serial.println(" - Found our characteristic");

    // Read the value of the characteristic.
    if(pRemoteCharacteristic_RX->canRead()) {
      std::string value = pRemoteCharacteristic_RX->readValue();
      Serial.print("The characteristic value was: ");
      Serial.println(value.c_str());
    }

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

    connected = true;
    return 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());

    // 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() {
  Serial.begin(115200);
  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.
  BLEScan* pBLEScan = BLEDevice::getScan();
  pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks());
  pBLEScan->setInterval(1349);
  pBLEScan->setWindow(449);
  pBLEScan->setActiveScan(true);
  
  //pBLEScan->start(5, false);
  pBLEScan->start(0, false);

} // End of setup.


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

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

    if (Serial.available() > 0) {
      int incomingByte = Serial.read();
      Serial.println(incomingByte, DEC);
    if (connected) {
      pRemoteCharacteristic_RX->writeValue(incomingByte);  
    }
  }
  }else if(doScan){
    BLEDevice::getScan()->start(0);  // this is just example to start scan after disconnect, most likely there is better way to do it in arduino
  }
  
  delay(1000); // Delay a second between loops.
} // End of loop