Allows you to specify which channels are used to advertise.

[raulcigil]

In some use cases getting rssi signal from one channel is more stable (less variance) than rssi from the three advertising channels.

This change allows you to specify which channels are used to advertise.

[me-no-dev]

thanks :)

[selman-nus]

an example code would be very nice to understand @raulcigil

[raulcigil]

Good idea.
Using BleServer.ino from examples:

BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
  pAdvertising->addServiceUUID(SERVICE_UUID);
  pAdvertising->setScanResponse(true);
  pAdvertising->setMinPreferred(0x06);  // functions that help with iPhone connections issue
  pAdvertising->setMaxPreferred(0x12);

  //Setting the advertising channel
  pAdvertising->setAdvertisementChannelMap(ADV_CHNL_37);
  
  BLEDevice::startAdvertising();

You can set the channel with this enumeration:

https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/bluetooth/esp_gap_ble.html#_CPPv421esp_ble_adv_channel_t