ESP32 (35) – BLE, scan response

In the previous posts I explained how to receive and send advertising packets based on the Bluetooth LE standard.

The payload (that is the amount of “useful” data) of those packets is at most 31 bytes. It isn’t much: if – for example – you want to include the device name, little place remains for other data.

The BLE standard allows peripherals to send additional data using the scan request – scan response process.

When a device receives an advertising packet, it can contact the transmitter by sending a scan request packet to request further information. When receiving a scan request package, the peripheral can respond with a scan response packet:

scan-response-001

Advertising and scan request packets have the same format; it’s therefore possible to transfer, using scan response, additional 31 bytes of data.

esp32

The esp framework offers two modes for configuring the content of a scan response packet: using the esp_ble_adv_data_t struct or creating a byte array (raw mode). These modes are similar to the ones used to configure advertising packets you learned in previous articles (struct and raw mode).

In the first case, you have to declare a second struct, in addition to the one related to the advertising packet, to define the content of the scan response packet:

static uint8_t manufacturer_data[6] = {0xE5,0x02,0x01,0x01,0x01,0x01};
static esp_ble_adv_data_t scan_rsp_data = {
  .set_scan_rsp = true,
  .manufacturer_len = 6,
  .p_manufacturer_data = manufacturer_data,
};

Very important is set to true the set_scan_rsp parameter. It’s indeed this parameter what tells the driver that this struct is related to the scan response packet.

You can then pass the new struct to the driver, with the same function used previously:

esp_ble_gap_config_adv_data(&scan_rsp_data);

The driver will call the callback function twice: one to indicate the successful configuration of the advertising packet and one for the configuration of the scan response one. The two events are different:

case ESP_GAP_BLE_ADV_DATA_SET_COMPLETE_EVT:
  [...]
case ESP_GAP_BLE_SCAN_RSP_DATA_SET_COMPLETE_EVT:
  [...]

You have to wait until both the events have triggered before starting the advertising process. In my example program (you can download the source code from my Github repository) I use two boolean variables:

bool adv_data_set = false;
bool scan_rsp_data_set = false;
[...]
case ESP_GAP_BLE_ADV_DATA_SET_COMPLETE_EVT:
  adv_data_set = true;
  if(scan_rsp_data_set) esp_ble_gap_start_advertising(&ble_adv_params); break;
 
case ESP_GAP_BLE_SCAN_RSP_DATA_SET_COMPLETE_EVT:
  scan_rsp_data_set = true;
  if(adv_data_set) esp_ble_gap_start_advertising(&ble_adv_params); break;

If you want to use the raw mode instead, you have to declare a byte array and fill it with the content of the payload of the packet. Then you can use a specific function of the framework to pass the array to the driver:

static uint8_t scan_rsp_raw_data[8] = {0x07,0xFF,0xE5,0x02,0x01,0x01,0x01,0x01};
[...]
esp_ble_gap_config_scan_rsp_data_raw(scan_rsp_raw_data, 8);

did you notice that the content of the scan response packet is the same in the two examples?

The driver will confirm the configuration of the packet with a dedicated event. Also in this case you have to wait for the end of both configurations (advertising and scan response):

case ESP_GAP_BLE_SCAN_RSP_DATA_RAW_SET_COMPLETE_EVT:
  scan_rsp_data_set = true;
  if(adv_data_set) esp_ble_gap_start_advertising(&ble_adv_params); break;
You can also mix the two modes in your program. For example you can configure the advertising packet using the struct and configure the scan response one using the raw mode.

Now with the nRF Connect app you can verify that your scan response packet is correctly received by your smartphone:

scan-response-002

In the following video I explain how I built the payload of the packet and how the program works:

ESP32 (34) – BLE, raw advertising

In the previous post, you learned how to send BLE advertising packets with the esp32 chip.

To define the content of the packet, you used a struct, of the esp_ble_adv_data_t type:

raw-adv-001

The struct’s definition is included in the esp_gap_ble_api.h file:

raw-adv-002

Although there are many fields available, sometimes it is necessary to be able to define the content of the advertising packet arbitrarily. For this reason, the esp-idf framework provides a raw mode.

Instead of defining a struct, you create a byte array and fill it with the entire contents of the packet’s payload:

static uint8_t adv_raw_data[10] = 
  {0x09,0x09,0x4c,0x75,0x6b,0x45,0x53,0x50,0x33,0x32};

then you can use the esp_ble_gap_config_scan_rsp_data_raw() function to pass the array to the driver. You have to specify both the array and its size as parameters:

esp_ble_gap_config_scan_rsp_data_raw(scan_rsp_raw_data, 8);

When using this new function, it also changes the event that the driver passes to your callback function when the configuration is complete. The new event is ESP_GAP_BLE_ADV_DATA_RAW_SET_COMPLETE_EVT. As in the previous example, when this event is triggered you can start the advertising process:

case ESP_GAP_BLE_ADV_DATA_RAW_SET_COMPLETE_EVT: 
  esp_ble_gap_start_advertising(&ble_adv_params);
  break;

Raw data

For the advertising process to work, the data contained in the array must correspond to a valid payload.

In the blog post about the iBeacons, I’ve already shown you its structure. Let’s briefly review it:

ibeacon-002

The payload contains one or more AD (advertising data) structures. Each structure is made by 3 fields:

  • an initial byte that represents the length (in bytes) of the structure, excluding itself
  • a byte that represents the type of the data contained in the structure
  • a variable number of bytes which are the actual data

The codes that can be used to define the type of data can be found in the Bluetooth specifications. Depending on the type of data, it is then necessary to apply a particular format to the data that follows. The necessary information is found in the Core Specification Supplement document (available on the Bluetooth.com website).

Let’s see a simple example: the ADType 0x09 represents the complete local name, which is the name of the device. This name must be specified in AD data with simply a sequence of the ASCII codes that correspond to the different letters.

You can use a website to do the conversion:

raw-adv-003

The payload to transmit this name is therefore:

adv_raw_data[7] = {0x06,0x09,0x4d,0x79,0x42,0x4c,0x45};

The first byte has value 0x06 that is the sum of the name length (5 bytes) and 1 byte for the data type (0x09).

Demo

In the following video you can see how I use the raw advertising feature to simulate the advertising packet of my iBeacon and therefore I’m able to activate the relay as in the previous example.

The source code of the program is available in my Github repository.

Why electrolytic capacitors are actually kinda shitty

thumb332x187-1

Afroman writes, “Electrolytic capacitors are common, but knowledge of their limitations is uncommon. A demonstration is shown highlighting the difference in performance between electrolytic and ceramic capacitors in power supplies. Other topics discussed in the video: Electrolytic capacitor construction, ceramic capacitors, ESR, ESL, impedance curves, why “0.1uF”, and more.”

Via Afrotechmods.

Check out the video after the break.

Alexa (Echo) with ESP32 and ESP8266 – Voice controlled relay

p-esp8266-esp32-alexa-circuit-600

Rui Santos writes, “In this project, you’re going to learn how to control the ESP8266 or the ESP32 with voice commands using Alexa (Amazon Echo Dot). As an example, we’ll control two 12V lamps connected to a relay module. We’ll also add two 433 MHz RF wall panel switches to physically control the lamps.”

More info at randomnerdtutorials.com.

Check out the video after the break.

ESP32 (33) – BLE, advertising

In the previous posts you learned how to use the esp32 chip to receive and parse the advertising packets transmitted by BLE peripherals. As a practical example, I developed a program to detect the presence of a particular iBeacon and activate an output accordingly.

In today’s tutorial, you’ll learn how to transmit advertising packets instead.

Advertising process

You’ve already discovered that the Bluetooth driver included in the esp-idf stack is executed in a dedicated thread. Whenever the driver needs to send a notification to your program, it calls a callback function indicating which event has triggered.

The advertising process is very simple:

  • the program configures the data to be transmitted with esp_ble_gap_config_adv_data()
  • the driver reports that it has finished the configuration with the event ESP_GAP_BLE_ADV_DATA_SET_COMPLETE_EVT
  • the program can now start the advertising process with esp_ble_gap_start_advertising()
  • the driver reports that the process has started with the event ESP_GAP_BLE_ADV_START_COMPLETE_EVT

esp32-adv-001

Advertising DATA

It’s possible to tell the driver which data to include in the advertising packet with the following command:

esp_err_t esp_ble_gap_config_adv_data(esp_ble_adv_data_t *adv_data);

The command accepts as parameter a pointer to an esp_ble_adv_data_t struct:

esp32-adv-002

The meaning of the different fields is explained in the Supplement to the Bluetooth Core Specification document.

First let’s find out how to transmit the device name. You have to use the esp_ble_gap_set_device_name() function to pass the name to the driver and set the field include_name to true in the struct:

static esp_ble_adv_data_t adv_data = {
  .include_name = true,
};
[...]
ESP_ERROR_CHECK(esp_ble_gap_set_device_name("ESP32_BLE"));
ESP_ERROR_CHECK(esp_ble_gap_config_adv_data(&adv_data));

Using the flags, you can publish some features of your device. The available constants are:

esp32-adv-003

you can combine them with the OR operator. If, for example, you want to tell the world that your device is limited discoverable (i.e. it sends the advertising packets only for a limited time, usually 30 seconds) and that it doesn’t support classic Bluetooth (BR/EDR, Basic Rate/Enhanced Data Rate) you’ll write:

static esp_ble_adv_data_t adv_data = {
  .flag = ESP_BLE_ADV_FLAG_LIMIT_DISC | ESP_BLE_ADV_FLAG_BREDR_NOT_SPT,
};

Advertising PARAMETERS

After configuring the content of the advertising packet, you have also to tell the driver how to send the packet.

The command:

esp_err_t esp_ble_gap_start_advertising(esp_ble_adv_params_t *adv_params);

accepts as parameter an esp_ble_adv_params_t struct:

esp32-adv-004

You can configure the minimum and maximum transmission interval of the packet. The two parameters can assume a value from 0x20 to 0x4000. To calculate the interval in milliseconds, the value specified must be multiplied for 0.625. This means that the minimum value (0x20) corresponds to an interval of 12.5ms.

The esp_gap_ble_api.h file lists the constants that can be used for the other parameters (esp_ble_adv_type_t, esp_ble_addr_type_t …).

For example, let’s configure the advertising process as it follows:

  • minimum transmission interval: 0x20, maximum: 0x40
  • non connectable device (it doesn’t accept incoming connections and only sends data in broadcast)
  • public MAC address
  • transmission on all the 3 channels dedicated to advertising packets
  • no filter on devices who can perform a scan or connect
static esp_ble_adv_params_t ble_adv_params = {
  .adv_int_min = 0x20,
  .adv_int_max = 0x40,
  .adv_type = ADV_TYPE_NONCONN_IND,
  .own_addr_type  = BLE_ADDR_TYPE_PUBLIC,
  .channel_map = ADV_CHNL_ALL,
  .adv_filter_policy  = ADV_FILTER_ALLOW_SCAN_ANY_CON_ANY,
};
[...]
esp_ble_gap_start_advertising(&ble_adv_params);

Demo

I prepared a program that includes what explained above. The source code is available in my Github repository.

Here’s how it works:

Arduino bootloader and ISP

After having developed a sketch using the Arduino IDE, you can compile and load it on the Arduino board connected to your PC with just a click on the upload button:

isp-002

The program is stored in the flash memory of the microcontroller (on Arduino Uno boards this is the ATmega328p).

You can upload your program on the microcontroller without the need of a dedicated programmer because of the microcontroller itself runs a small program named bootloader.

The bootloader is indeed a program, already flashed in every microcontroller of the Arduino boards, that is executed everytime the microcontroller is reset. The first operation the bootloader does is to check if on the serial/USB connection there’s a request to flash a new program. If so, the bootloader talks with the Arduino IDE, receives the new program and stores it in the flash memory:

isp-001

The bootloader used as of now on the Arduino Uno boards is named optiboot.

If you buy an ATmega328p chip and you want to upload your sketches on it using the Arduino IDE you have first to flash the bootloader in the chip. You usually need a dedicated programmer to program the chip. ATmega microcontrollers support a programming method named In System Programming (ISP), designed to program the chips directly on the board where theyt are located.

This method requires to connect 6 pins of the chip to the programmer:

  • 5V and ground
  • reset
  • MISO, MOSI and SCK

For example every Arduino Uno has a dedicated connector which allows to re-program the ATmega328p chip without removing it from the board:

isp-003

On the Internet you can find several ATmega programmers (for example this by Adafruit); very interesting is the possibility to use an Arduino as a programmer, thanks to the work of Randall Bohn.

First you have to upload on your Arduino the ArduinoISP sketch which is included in the Examples shipped with the IDE:

isp-004

then you have to create – for example on a breadboard – a minimal circuit with the chip to be programmed, a 16MHz crystal and two 22pF capacitors:

isp-013

using some jumpers, connect your Arduino Uno to this circuit as it follows:

  • 5V -> pin 7 and 20
  • Ground -> pin 8, 22 and the two capacitors
  • pin 10 of Arduino or RESET of the ICSP connector -> pin 1
  • pin 11 of Arduino or MOSI of the ICSP connector -> pin 17
  • pin 12 of Arduino or MISO of the ICSP connector -> pin 18
  • pin 13 of Arduino or CLK of the ICSP connector -> pin 19

isp-014 isp-015

Configure the IDE to use Arduino as programmer:

isp-005

then burn the bootloader:

isp-006

after few seconds, the IDE should confirm that the bootloader was programmed on the chip:

isp-011

Shield

On some webstores I found an Arduino shield which makes it simple to program ATmega chips. It’s named AVR ISP Shield and manufactured by a company called “OPEN-SMART”:

isp-012

This shield has a ZIF (Zero Insertion Force) socket where the chip goes, some pins to connect it to external boards and a buzzer that is used to confirm the burn process with two beeps.

On the Internet I found an archive with the documentation about this shield and the “official” sketch. You can also use the sketch shipped with the IDE, it won’t only activate the buzzer when the burning process ends successfully.

If you need to program several ATmega328p chips (for example if you’re building your Arduino-compatible boards) the use of this shield makes the programming process much easier and faster!

 

DIY Arduino PCB Pryamiduino

pryamdiuino-r1-and-apple-remote

A how-to on designing a DIY Arduino PCB Pryamiduino from Bald Engineer:

Continuing the DIY Arduino tutorial series, this AddOhms episode shows how to create a PCB in KiCad. I make a joke that the original design was a rectangle, which I found boring and pointless. So instead, I designed a triangle to give the board 3 points. Get it? Puns! I am calling it the Pryamiduino. To be honest, I found not having a constraint to be a problem. By forcing a specific board size and shape, many decisions were more manageable.

More details at Bald Engineer’s blog.

Check out the video after the break.

ESP32 (32) – BLE, iBeacon

In my previous article I explained the Bluetooth Low Energy technology and the advertising process.

You learned that a BLE device can leverage the advertising packets to send data; in this case the device is called broadcaster and the devices which receive data are called observers.

The payload of an advertising packet has the following structure:

ibeacon-002

ADV ADDR is the device MAC address (this is the address displayed by the program developed in the previous article) and ADV DATA is a field, with a max length of 31 bytes, that contains one or more structures, each with 3 elements:

  • AD length is the total length (in bytes) of each data structure
  • AD type is the type of data contained in the structure
  • AD data is the real data

The official website of the Bluetooth Special Interest Group lists all the available AD types.

A device, for example, can transmit its local name using the AD type 0x09:

ibeacon-003

In scan mode, the Bluetooth driver returns to the program the received data (ADV DATA) in the scan_result->scan_rst.ble_adv array. This array contains uint8_t values and it’s size is scan_result->scan_rst.adv_data_len.

The Bluedroid library contains a method, esp_ble_resolve_adv_data(), which allows to get the value for a specific AD type passing the raw data. The header file esp_gap_ble_api.h contains definitions for the most common AD types:

ibeacon-004

In my Github repository you can find an updated version of the scan program. Thanks to what explained above, now the program can also display – if available – the name of the device:

ibeacon-009

iBeacon

A particular family of broadcaster devices are the iBeacons. These devices have been designed by Apple to allow interaction with IOS devices (iPhone …) based on location awareness. Let’s make an example: an iPhone can “notice” that it is close to a particular iBeacon, associated with a room in a museum, and therefore offer the user a brief guide to the exhibited works.

ibeacon-001

iBeacon specifications are available on Apple’s developer portal. iBeacons work transmitting advertising packets with  specific payload (ADV DATA):

ibeacon-006

The first structure has AD type = flags (0x01). Each bit has a different meaning, usually iBeacons use 0x0A value for AD data.

The second structure has type = 0xFF, that is Manufacturer Specific Data. The Bluetooth standard allows the different manufacturers to use this ID to transmit custom data. The total data length is 25 bytes (0x1A – 0x01 that is the length of the AD type field).

Apple specifications further subdivide the AD data field in several elements:

ibeacon-008

The first field is the manufacturer/company; iBeacons normally use the code 0x004C, assigned to Apple Inc. The next two fields define the iBeacon type and have a fixed value (0x02 e 0x15). The UUID field, together with the Major and Minor ones (optional, they can have a value of 0) uniquely identifies each iBeacon.  (insieme con i campi Major e Minor (facoltativi, possono essere impostati a 0) identificano univocamente il singolo iBeacon. Finally, the TX power field contains a measurement, one meter away from the iBeacon, of the received power and is useful for  precisely estimate the distance between the phone and the iBeacon itself.

esp32

I developed a program for the esp32 chip which turns a relay on if it detects a specific iBeacon. Via menuconfig you can configure the UUID of the iBeacon which triggers the led, the pin the led is connected to and the timeout – in seconds – after which the program turns the led off if the iBeacon is not detected anymore. You can moreover set a power threshold to control the distance at which the iBeacon is detected.

To parse the received packet and get the UUID value, in my program I used the method described in this article (parsing using a struct).

The program verifies if the received packet (event ESP_GAP_SEARCH_INQ_RES_EVT) was sent by an iBeacon checking that the packet length is 30 bytes and that its header contains the values listed above:

// iBeacon fixed header
ibeacon_header_t ibeacon_fixed_header = {
  .flags = {0x02, 0x01, 0x06},
  .length = 0x1A,
  .type = 0xFF,
  .company_id = 0x004C,
  .beacon_type = 0x1502
};

It compares the fixed header with the received one using memcmp, function that compares two blocks in memory:

if(memcmp(adv_data, ibeacon_fixed_header, sizeof(ibeacon_fixed_header)))
  result = true;

The source program is available in my Github repository, here’s a video that shows how it works:

Debug SONOFF AC relay with a thermal camera

pSONOFF-FLIR-Banner-600

James Lewis shares his experience in debugging SONOFF AC relay with a thermal camera:

The clever solution seemed to be clever, at least for a few minutes. Suddenly the light turned off. I thought maybe there was a timeout for the manual button. Annoying, but workable. The lamp remained off for about another 2 minutes when I started to smell that unmistakeable burning plastic odor. Touching the case of the SONOFF identified the culprit immediately.
Great. So I have an AC mains switch that isn’t working, but I do not want to go poking my multimeter into it. What do I do?
Turns out, that SONOFF module was defective. I wanted to debug it, but I did not want to measure anything while connected to AC. Here’s how I used a thermal camera to debug my SONOFF.

See the full post on Bald Engineer blog.

ESP32 (31) – BLE, GAP

In my previous tutorials you learned how to use the wifi interface of the esp32 chip. Starting from this post, I’m going to explain you the second wireless technology the esp32 chip supports: bluetooth.

In particular, my tutorial will be about Bluetooth Low Energy (BLE), sometimes called also Bluetooth 4.0 or Bluetooth Smart:

ble-001

Bluetooth Low Energy

BLE is a technology to build personal wireless area networks (WPAN); that is it allows to put in communication different devices (computers, smartphones, smartwatches…) “close” to each other (a theoretical maximum distance of 100m). As the name suggests, version 4.0 of the Bluetooth standard was designed to reduce the power consumption of the devices connected to the network.

Devices are divided into two families:

  • central
  • peripheral

the first ones (central) are devices like PCs, tablets or smartphones with good processing power and memory. The second ones (peripheral) are instead sensors, tags… with less hardware resources and power. A central device can be connected to more peripheral devices at the same time, while it’s not true the opposite:

ble-002

BLE devices periodically report their presence by transmitting advertising packets. The advertising packet can contain up to 31 bytes of data and the transmission frequency can be chosen by the single device: reducing this frequency can indeed reduce energy consumption.

If a BLE device, after having received an avertising package, wants to obtain more information from the device that transmitted it, it can request a second packet of information (always for a maximum of 31 bytes), the scan response packet. The transmission of this second data package is optional:

ble-003

A BLE device can take advantage of advertising packages to send data in broadcast mode. In this case, this device is called a broadcaster, while the devices that receive the data are called observers.

What explained above is defined within a BLE specification called Generic Access Profile (GAP).

esp32

In this first tutorial you’ll learn how to develop a program that will periodically scan the air looking for BLE devices, that is a program which receives advertising packets and displays the data received in the serial console.

Before compiling a program which uses the Bluetooth controller, make sure (using menuconfig) that the controller is enabled (Component config -> Bluetooth):

ble-004

Start your program with the required header files:

#include "esp_bt.h"
#include "esp_bt_main.h"
#include "esp_gap_ble_api.h"

You also need to initialize the NVS partition, used by the Bluetooth driver:

ESP_ERROR_CHECK(nvs_flash_init());

the Bluetooth controller of the esp32 chip supports both the classic and the low energy mode. If one of this two modes is not required in your program, you can release the memory the framework normally allocates to manage it using the esp_bt_controller_mem_release() command. In this example you’re not going to use the classic mode, so:

ESP_ERROR_CHECK(esp_bt_controller_mem_release(ESP_BT_MODE_CLASSIC_BT));

Now you can configure (using the default settings) the controller in BLE mode:

esp_bt_controller_config_t bt_cfg = BT_CONTROLLER_INIT_CONFIG_DEFAULT();
esp_bt_controller_init(&bt_cfg);
esp_bt_controller_enable(ESP_BT_MODE_BLE);

The esp-idf framework esp-idf includes the Bluedroid bluetooth stack. This library was developed by Broadcom and used by Android since version 4.2 Bluedroid is initialized and enabled with the following commands:

esp_bluedroid_init();
esp_bluedroid_enable();

Now you’re ready to start scanning…

GAP, events

In a similar way to what you learned about the wifi driver, the bluetooth driver also runs in a thread separate from our program and communicates with it via events. In order to receive such events, you have to implement a callback function. Whenever the bluetooth driver has to notify an event, it will call that function.

The prototype of the callback function is:

static void esp_gap_cb(esp_gap_ble_cb_event_t event, esp_ble_gap_cb_param_t *param);

You tell the driver which callback function has to use with the esp_ble_gap_register_callback() method:

ESP_ERROR_CHECK(esp_ble_gap_register_callback(esp_gap_cb));

The Bluetooth driver handles several events, there are the ones related to the scan process:

ble-005

Before being able to start the scan process, you have to configure the scan parameters. The configuration is performed using the esp_ble_scan_params_t struct. It’s very important that the variable with the scan parameters is available during all the scan process; it’s therefore necessary to define it globally:

static esp_ble_scan_params_t ble_scan_params = {
  .scan_type              = BLE_SCAN_TYPE_ACTIVE,
  .own_addr_type          = BLE_ADDR_TYPE_PUBLIC,
  .scan_filter_policy     = BLE_SCAN_FILTER_ALLOW_ALL,
  .scan_interval          = 0x50,
  .scan_window            = 0x30
};

With the esp_ble_gap_set_scan_params() method you configure the scan process passing the struct defined above to the driver:

esp_ble_gap_set_scan_params(&ble_scan_params);

When the driver has finished the configuration, it calls the callback function with the event ESP_GAP_BLE_SCAN_PARAM_SET_COMPLETE_EVT. Depending on the event raised, the callback function also receives some parameters. The framework’s Programming Guide explains – for each event – the related parameters. For this event, it’s available the variable scan_param_cmpl that contains only the status parameter.

In the callback function you can use the switch statement to identify each event:

switch (event) {
  case ESP_GAP_BLE_SCAN_PARAM_SET_COMPLETE_EVT:
  [...]
  break;

and check if the configuration was successful with:

if(param->scan_param_cmpl.status == ESP_BT_STATUS_SUCCESS)

If so, you can start the scan process:

esp_ble_gap_start_scanning(10);

The parameter is the scan duration (in seconds).

Once the scan process has started, the driver raises the ESP_GAP_BLE_SCAN_START_COMPLETE_EVT event. For this event too it’s possible to verify the correct execution by reading the status parameter (pay attention: the name of the variable which contains the parameter changes!):

case ESP_GAP_BLE_SCAN_START_COMPLETE_EVT:
  if(param->scan_start_cmpl.status == ESP_BT_STATUS_SUCCESS)
    printf("Scan started\n\n");
  else 
    printf("Unable to start scan process");
  break;

GAP, scan process

During the scan process, for each advertising packet the chip receives the event ESP_GAP_BLE_SCAN_RESULT_EVT is raised.

This event contains some subevents. You can identify which subevent was raised reading the scan_rst.search_evt parameters. Two subevents are in particular interesting:

ble-006

the first tells you that a device was detected, while the second one that the scan process completed.

For each detected device, various information is available. For now let’s print its address in the console:

case ESP_GAP_BLE_SCAN_RESULT_EVT:
  if(param->scan_rst.search_evt == ESP_GAP_SEARCH_INQ_RES_EVT) {
    printf("Device found: ADDR=");
    for(int i = 0; i < ESP_BD_ADDR_LEN; i++) {
      printf("%02X", param->scan_rst.bda[i]);
      if(i != ESP_BD_ADDR_LEN -1) printf(":");
    }

The address is an uint8_t array, whose size is defined by the ESP_BD_ADDR_LEN constant. The address is normally displayed in hex form, with the bytes separated by :

ble-007

Device list management

As explained above, the ESP_GAP_BLE_SCAN_RESULT_EVT event is raised everytime a device sends an advertising packet. This means that a single device will be detected multiple times during the scan process.

It’s therefore necessary to maintain a list of the known devices. In my Github repository you can find a test program that scans the network and prints all the detected devices.

You can verify if it works correctly comparing what the program detects with the BLE devices listed by a smartphone… for Android for example you can use the very good nRF Connect application by Nordic.

Here’s what my program detected:

ble-008

and here’s the nRF Connect’s screenshot:

ble-009