The Arduino IDE supports all of these boards, but BLE is supported by two different libraries. The Arduino Nano 33 BLE Sense and Seeed Xiao BLE nRF52840 use the ArduinoBLE library with the low level functions handled by the Mbed OS. To implement iBeacon there is an additional small BeaconNano library used. The RAK4631 uses the Adafruit Bluefruit52lib library with FreeRTOS. The iBeacon is implemented within the Bluefruit library. Note: in the case of the RAK4631, the BLE library is automatically installed as part of the board library installation.

nRF52 TX power

Supported tx_power values depending on mcu:

nRF52832: -40dBm, -20dBm, -16dBm, -12dBm, -8dBm, -4dBm, 0dBm, +3dBm and +4dBm.
nRF52840: -40dBm, -20dBm, -16dBm, -12dBm, -8dBm, -4dBm, 0dBm, +2dBm, +3dBm, +4dBm, +5dBm, +6dBm, +7dBm and +8dBm.

Default is 0 dBm.

I had problems setting the TX power with the two boards using the Mbed OS. The TX power is set within the Mbed Cordio BLE stack. I found that the boards do have individual mbed_config.h files in their variants directory that include #defines for various Cordio parameters including TX power. Unfortunately, changing that parameter definition (MBED_CONF_CORDIO_PREFERRED_TX_POWER) did not produce a change in measured TX power.

I've included the Cordio section of the mbed_config.h file below.

#define #define MBED_CONF_CORDIO_LL_DEFAULT_EXTENDED_ADV #define MBED_CONF_CORDIO_LL_EXTENDED_ADVERTISING_SIZE #define MBED_CONF_CORDIO_LL_MAX_ACL_SIZE #define MBED_CONF_CORDIO_LL_MAX_ADVERTISING_REPORTS #define MBED_CONF_CORDIO_LL_MAX_ADVERTISING_SETS #define MBED_CONF_CORDIO_LL_MAX_SCAN_REQUEST_EVENTS #define MBED_CONF_CORDIO_LL_NRF52840_CRYPTOCELL3 #define MBED_CONF_CORDIO_LL_PHY_2M_SUPPORT #define MBED_CONF_CORDIO_LL_PHY_CODED_SUPPORT #define MBED_CONF_CORDIO_LL_RX_BUFFERS #define MBED_CONF_CORDIO_LL_TX_BUFFERS #define MBED_CONF_CORDIO_MAX_PREPARED_WRITES #define MBED_CONF_CORDIO_NORDIC_LL_HCI_DRIVER_BUFFER_SIZE #define MBED_CONF_CORDIO_NORDIC_LL_WSF_POOL_BUFFER_SIZE #define MBED_CONF_CORDIO_PREFERRED_TX_POWER #define MBED_CONF_CORDIO_ROUTE_UNHANDLED_COMMAND #define MBED_CONF_CORDIO_RX_ACL_BUFFER_SIZE #define MBED_CONF_CORDIO_TRACE_HCI_PACKETS #define MBED_CONF_CORDIO_TRACE_PAL_ECHOES

MBED_CONF_CORDIO_DESIRED_ATT_MTU 23 // set by library:cordio ERTISING_FRAGMENTATION_SIZE 64 // set by library:cordio-ll 512 // set by library:cordio-ll[MCU_NRF52840] 256 // set by library:cordio-ll 4 // set by library:cordio-ll 3 // set by library:cordio-ll 4 // set by library:cordio-ll 10_ACCELERATION 1 // set by library:cordio-ll-nrf52840 1 // set by library:cordio-ll 1 // set by library:cordio-ll[MCU_NRF52840] 4 // set by library:cordio-ll 4 // set by library:cordio-ll 4 // set by library:cordio 16056 // set by library:cordio-nordic-ll[MCU_NRF52840] 4900 // set by library:cordio-nordic-ll 8 // set by library:cordio _COMPLETE_EVENTS 1 // set by library:cordio 70 // set by library:cordio 0 // set by library:cordio 0 // set by library:cordio

The Mbed library is a pre-compiled module that is linked during sketch compilation. I've tried to find the correct method to call the Cordio function setTxPower() from my sketch code, but haven't succeeded yet. For now, I'll run some close proximity tests with the default 0 dBm default power.

BeaconNano code (same for both Nano 33 and Xiao boards)

arduino_nano_ble33_ibeacon.ino

// GGBeaconNano - Version: Latest 

#include <ArduinoBLE.h>
#include <BeaconNano.h>

void setup() { 
  BeaconNano bn;
  bn.setUuid("c336aa38054bb0483b0ae75027061982");
  bn.setMajor(1);
  bn.setMinor(1);
  bn.setTx(197);  // TX power (0xC5 = 197, 2's complement = 256-197 = -59 dBm) is the strength of the signal measured at 1 meter from the device
  bn.startBeacon();


}

void loop(){
  
}

Arduino Nano 33 BLE Sense

Seeed Xiao BLE

The results are similar as might be expected, the Nano has a PCB antenna and the Xiao has a chip antenna.

RAK4531

I had more success being able to modify the TX power on the RAK4631. The Bluefruit library has a setTxPower() function and that works properly with the sketch code.

I took a few data points between 0 and 8 dBm to check it out. At 0 dBm the external antenna shows 7 to 9 dBm improvement over the chip/PCB antennas.

RAK4631_ble_beacon.ino

/*********************************************************************
 This is an example for our nRF52 based Bluefruit LE modules

 Pick one up today in the adafruit shop!

 Adafruit invests time and resources providing this open source code,
 please support Adafruit and open-source hardware by purchasing
 products from Adafruit!

 MIT license, check LICENSE for more information
 All text above, and the splash screen below must be included in
 any redistribution
*********************************************************************/
#include <bluefruit.h>

// Beacon uses the Manufacturer Specific Data field in the advertising
// packet, which means you must provide a valid Manufacturer ID. Update
// the field below to an appropriate value. For a list of valid IDs see:
// https://www.bluetooth.com/specifications/assigned-numbers/company-identifiers
// 0x004C is Apple
// 0x0822 is Adafruit
// 0x0059 is Nordic
#define MANUFACTURER_ID   0x0059

// "nRF Connect" app can be used to detect beacon
uint8_t beaconUuid[16] =
{
  0x01, 0x12, 0x23, 0x34, 0x45, 0x56, 0x67, 0x78,
  0x89, 0x9a, 0xab, 0xbc, 0xcd, 0xde, 0xef, 0xf0
};

// A valid Beacon packet consists of the following information:
// UUID, Major, Minor, RSSI @ 1M
BLEBeacon beacon(beaconUuid, 0x0102, 0x0304, -54);

void setup() 
{
  Serial.begin(115200);

  // Uncomment to blocking wait for Serial connection
  // while ( !Serial ) delay(10);

  Serial.println("Bluefruit52 Beacon Example");
  Serial.println("--------------------------\n");

  Bluefruit.begin();

  // off Blue LED for lowest power consumption
  Bluefruit.autoConnLed(false);
  Bluefruit.setTxPower(4);    // Check bluefruit.h for supported values

  // Manufacturer ID is required for Manufacturer Specific Data
  beacon.setManufacturer(MANUFACTURER_ID);

  // Setup the advertising packet
  startAdv();

  Serial.println("Broadcasting beacon, open your beacon app to test");

  // Suspend Loop() to save power, since we didn't have any code there
  suspendLoop();
}

void startAdv(void)
{  
  // Advertising packet
  // Set the beacon payload using the BLEBeacon class populated
  // earlier in this example
  Bluefruit.Advertising.setBeacon(beacon);

  // Secondary Scan Response packet (optional)
  // Since there is no room for 'Name' in Advertising packet
  Bluefruit.ScanResponse.addName();
  
  /* Start Advertising
   * - Enable auto advertising if disconnected
   * - Timeout for fast mode is 30 seconds
   * - Start(timeout) with timeout = 0 will advertise forever (until connected)
   * 
   * Apple Beacon specs
   * - Type: Non connectable, undirected
   * - Fixed interval: 100 ms -> fast = slow = 100 ms
   */
  //Bluefruit.Advertising.setType(BLE_GAP_ADV_TYPE_ADV_NONCONN_IND);
  Bluefruit.Advertising.restartOnDisconnect(true);
  Bluefruit.Advertising.setInterval(160, 160);    // in unit of 0.625 ms
  Bluefruit.Advertising.setFastTimeout(30);      // number of seconds in fast mode
  Bluefruit.Advertising.start(0);                // 0 = Don't stop advertising after n seconds  
}

void loop() 
{
  // loop is already suspended, CPU will not run loop() at all
}

The Eye Diagrams at 0 and 8 dBm TX power

0 dBm