ESP-IDF Bluetooth LE (BLE) Beginner Tutorial

This tutorial covers the basics of how to use Bluetooth Low Energy (BLE) in your ESP-IDF project. It's based on the official documentation by Espressif Systems.

This is a practical, step-by-step tutorial, focusing on building a conceptual understanding of BLE and its basic code implementation, rather than every single feature in detail. You can follow along from the beginning to the end. After completing this, you will be able to understand the official documentation and the official example.

You can download the full final code here and the full project here, but I recommend you to write the code yourself by following this tutorial instead.

Let's start!

The Setup

I am assuming you have created an empty ESP-IDF project using the sample_project template and verified building and flashing is working properly. If you haven't already, you can check out the following chapters from my ESP-IDF course:

• Installation
• Your First Project

Enabling BLE in ESP-IDF

Before you can use BLE, you have to enable it using Menuconfig. There are two ways to do this.

Option 1: Using the VS Code Command Palette

Warning: Only use this option if you are developing using the ESP-IDF VS Code extension.

1. Click Ctrl + Shift + P to open the command palette.
2. Search for and select ESP-IDF: SDK Configuration Editor (Menuconfig).
3. Navigate to Component Config -> Bluetooth.
4. Check the Bluetooth box and select NimBLE - BLE only from the Host selector.

Option 2: Using the ESP-IDF terminal

1. Open the project configuration menu by running idf.py menuconfig
2. Navigate to Component config -> Bluetooth.
3. Enable Bluetooth by setting Bluetooth to [x] (enabled).
4. Select the host stack by navigating to Host and choosing NimBLE - BLE only.
5. Press S to save, then Q to quit.

Updating CMakeLists.txt

You should add PRIV_REQUIRES bt nvs_flash esp_driver_gpio in your CMakeLists.txt inside idf_component_register(), since our program requires them.

Including necessary headers

This program requires a lot of headers to be included. Update your main.c like this:

/* Includes */
/* STD APIs */
#include <assert.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

/* ESP APIs */
#include "driver/gpio.h"
#include "esp_log.h"
#include "nvs_flash.h"
#include "sdkconfig.h"

/* FreeRTOS APIs */
#include <freertos/FreeRTOS.h>
#include <freertos/task.h>

/* NimBLE stack APIs */
#include "host/ble_hs.h"
#include "host/ble_uuid.h"
#include "host/util/util.h"
#include "nimble/ble.h"
#include "nimble/nimble_port.h"
#include "nimble/nimble_port_freertos.h"
#include "services/gap/ble_svc_gap.h"
#include "services/gatt/ble_svc_gatt.h"

/* Defines */
#define TAG "NimBLE_LED"
#define DEVICE_NAME "NimBLE_LED"

void app_main(void) {
    // Empty for now
}

You can change the TAG and DEVICE_NAME if required. TAG is just a label which we use for logging (we will use it later in the tutorial). DEVICE_NAME is how our ESP32 appears to the other devices (like our phone). Later, we are going to connect your phone to your ESP32. In your phone's "available devices" list, you will see this DEVICE_NAME.

Tips for beginners

We use #define to define macros. Macros are of two types - object-like macros and function-like macros. Here, TAG and DEVICE_NAME are object like macros. For now, let's only consider object-like macros (the ones we are using here).

How it works is that, we define DEVICE_NAME to be "NimBLE_LED" and use DEVICE_NAME like a variable in our code (e.g., we can call a function like this: setDeviceName(DEVICE_NAME)). When we compile our code, before the compilation step begins, the preprocessor literally replaces all instances of DEVICE_NAME with "NimBLE_LED", so the previous function call becomes setDeviceName("NimBLE_LED"). So, it is functionally identical to we directly using the string everywhere.

The main reason why we use it is because the exact same string "NimBLE_LED" will be used in multiple places in our code. If we use #define, we can be 100% sure that it will be identical everywhere. Also, if we want to change the name of our device, we just have to update DEVICE_NAME instead of eveywhere.

Initialize NVS Flash (Non-Volatile Storage)

The BLE stack requires NVS flash to store configurations, so it should be initialized using nvs_flash_init(). You don't need to overthink about it at this point. Just call it in the main function:

void app_main(void) {
    nvs_flash_init();
}

This function returns a return code. If the return code is 0, it means no errors. Any non-zero value means an error, and each value corresponds to a particular error. To make our code bulletproof, let's add some error handling:

void app_main(void) {
  esp_err_t ret; // To store the return code

  ret = nvs_flash_init();
  if (ret == ESP_ERR_NVS_NO_FREE_PAGES ||
      ret == ESP_ERR_NVS_NEW_VERSION_FOUND) {
    ESP_ERROR_CHECK(nvs_flash_erase());
    ret = nvs_flash_init();
  }
  if (ret != ESP_OK) {
    ESP_LOGE(TAG, "failed to initialize nvs flash, error code: %d ", ret);
    return;
  }
}

Tips for beginners

Here, esp_err_t is just an int, not some mysterious type. It's provided by esp_err.h which we included. esp_err_t stands for "ESP Error Type". That alias was defined for code readability.

Similarly, ESP_ERR_NVS_NO_FREE_PAGES, ESP_ERR_NVS_NEW_VERSION_FOUND, etc. are also ints, which are defined in nvs.h. They are object-like macros defined using #define (just like how defined TAG and DEVICE_NAME), which means they will be replaced with their actual values (4365 and 4368 in this case) in the preprocessing step.

ESP_LOGE ("E" stands for "Error") is a function-like macro used to log errors. They are similar to object-like macros, but instead of replacing with an object, it replace with a function. ESP_LOGI is used for logging "Info".

Initialize GPIO2 output pin

Here, we are using GPIO2 because most ESP32s have an inbuilt LED for it.

void app_main() {
    // ...
    gpio_reset_pin(GPIO_NUM_2);
    gpio_set_direction(GPIO_NUM_2, GPIO_MODE_OUTPUT);
}

Initialize NimBLE stack

Call nimble_port_init() to initialize NimBLE, the software stack we are using to manage BLE.

void app_main(void) {
  // ...
  ret = nimble_port_init();
  if (ret != ESP_OK) {
    ESP_LOGE(TAG, "failed to initialize nimble stack, error code: %d ", ret);
    return;
  }
}

Here, we are reusing the ret variable we created earlier.

Initialize GAP

GAP stands for Generic Access Profile. Initializing it makes our ESP32 discoverable to other devices (e.g., our phone).

void app_main(void) {
  // ...
  BaseType_t rc = 0; // Still just an int; "rc" stands for "return code"

  ble_svc_gap_init();

  rc = ble_svc_gap_device_name_set(DEVICE_NAME);
  if (rc != 0) {
    ESP_LOGE(TAG, "failed to set device name to %s, error code: %d",
             DEVICE_NAME, rc);
    return;
  }
}

DEVICE_NAME is defined in common.h. You can change it if you want.

Initialize the GATT profile

This is the hard part. GATT stands for Generic Attribute. It defines how two connected devices package, format, and send data using a structured hierarchy of services and characteristics.

A GATT profile has one or more services, which have one or more characteristics within them, which hold data and descriptors. Each service and characteristic has a UUID. Here is a diagram showing the GATT hierarchy:

+---------------------------+
|          PROFILE          |
+---------------------------+
|                           |
|  +---------------------+  |
|  | SERVICE             |  |
|  +---------------------+  |
|  |                     |  |
|  | CHARACTERISTIC      |  |
|  |  - PROPERTIES       |  |
|  |  - VALUE            |  |
|  |  - DESCRIPTOR       |  |
|  |                     |  |
|  +---------------------+  |
|  |                     |  |
|  |  CHARACTERISTIC     |  |
|  |  - PROPERTIES       |  |
|  |  - VALUE            |  |
|  |  - DESCRIPTOR       |  |
|  |                     |  |
|  +---------------------+  |
|                           |
|  +---------------------+  |
|  | SERVICE             |  |
|  +---------------------+  |
|  |                     |  |
|  | CHARACTERISTIC      |  |  
|  |  - PROPERTIES       |  |
|  |  - VALUE            |  |
|  |  - DESCRIPTOR       |  |
|  |                     |  |
|  +---------------------+  |
|                           |
+---------------------------+

For example, you can create a "Heart Rate" service that has a "Heart Rate Measurement" characteristic, which holds the value.

Before defining our services and characteristics, let's initialize GATT:

void app_main(void) {
  // ...
  ble_svc_gatt_init();
}

Define GATT services table

Here is where we define which services and characteristics our ESP32 have which can be tweaked from our phone. Define these before (outside) the app_main function, but after the #includes:

static const ble_uuid16_t auto_io_svc_uuid = BLE_UUID16_INIT(0x1815);
static const ble_uuid16_t led_chr_uuid = BLE_UUID16_INIT(0x2A56);

static uint16_t led_chr_val_handle;
static int led_chr_access(uint16_t conn_handle, uint16_t attr_handle,
                          struct ble_gatt_access_ctxt *ctxt, void *arg);


/* GATT services table */
static const struct ble_gatt_svc_def gatt_svr_svcs[] = {
    /* Automation IO service */
    {.type = BLE_GATT_SVC_TYPE_PRIMARY,
     .uuid = &auto_io_svc_uuid.u,
     .characteristics =
         (struct ble_gatt_chr_def[]){
             {/* LED characteristic */
              .uuid = &led_chr_uuid.u,
              .access_cb = led_chr_access,
              .flags = BLE_GATT_CHR_F_WRITE,
              .val_handle = &led_chr_val_handle},
             {
                 0, /* No more characteristics in this service. */
             }}},

    {
        0, /* No more services. */
    },
};

You might wonder why we used 0x1815 as the UUID of our Automation IO service and 0x2A56 as the UUID of our LED characteristic. These short UUIDs are standardized by the Bluetooth SIG (Special Interest Group). Just like how the UNICODE value U+1F60A corresponds to the smiling face emoji (😊), 0x1815 corresponds to "Automation IO" and 0x2A56 corresponds to "Digital", meaning that any device that connects to our ESP32 will decide that this service is for pin control and I/O automation when it sees the UUID 0x1815. There are even more such standardized UUIDs with their own specific meaning. You can check the Bluetooth SIG Official List for all assigned 16-bit and 32-bit UUIDs.

Later in this tutorial, we are going to connect your phone to your ESP32, and you will see our service listed as "Automation IO". So keep this in mind.

You can also use your own UUIDs instead of these standardized UUIDs, but make sure to use a 128bit UUID instead to make sure you don't accidentally use a standardized UUID, which can be misleading.

Here, {0} acts as the null terminator. It is required to denote the end of the array.

Now we have defined the services tables. Let's define the led_chr_access callback.

Defining the Access Callback

The access callback function handles incoming requests from connected clients. Since we configured our LED characteristic with BLE_GATT_CHR_F_WRITE, NimBLE will invoke this function whenever a client (such as your phone) sends a write command.

Copy the following code below the GATT services table, but above the main function:

static int led_chr_access(uint16_t conn_handle, uint16_t attr_handle,
                          struct ble_gatt_access_ctxt *ctxt, void *arg) {
    if (ctxt->op == BLE_GATT_ACCESS_OP_WRITE_CHR) {
        // Extract the incoming byte payload
        uint8_t val = ctxt->om->om_data[0];

        if (val == 1) {
            gpio_set_level(GPIO_NUM_2, 1);
            ESP_LOGI(TAG, "LED turned ON");
        } else if (val == 0) {
            gpio_set_level(GPIO_NUM_2, 0);
            ESP_LOGI(TAG, "LED turned OFF");
        } else {
            ESP_LOGW(TAG, "Invalid payload value received: %d", val);
        }
        return 0;
    }

    return BLE_ATT_ERR_UNLIKELY;
}

Here, ctxt->op indicates what operation triggered the callback. We check if it equals BLE_GATT_ACCESS_OP_WRITE_CHR. ctxt->om points to an os_mbuf buffer structure containing the received payload data. ctxt->om->om_data[0] reads the first byte. gpio_set_level() sets the voltage of GPIO pin 2, which corresponds to the built-in LED in most ESP32s. If you don't have it, you can change the pin or connect an external LED.

Registering Services and Characteristics

Now that our GATT table and access callback are defined, we must register them with the NimBLE stack. Paste the following code in app_main():

void app_main(void) {
    // ...
    rc = ble_gatts_count_cfg(gatt_svr_svcs);
    if (rc != 0) {
        ESP_LOGE(TAG, "failed to count gatt services, error code: %d", rc);
        return;
    }

    rc = ble_gatts_add_svcs(gatt_svr_svcs);
    if (rc != 0) {
        ESP_LOGE(TAG, "failed to add gatt services, error code: %d", rc);
        return;
    }
}

Advertising

To enable smartphones or central devices to discover your ESP32, you must start advertising. Advertising is like sending the signals so that other devices (like your phone) can find your ESP32.

Define an on_sync callback function that handles advertising. You don't really need to worry about this function right now, just copy it into your file:

static void gatt_svr_on_sync(void) {
    struct ble_hs_adv_fields fields;
    memset(&fields, 0, sizeof(fields));

    // Make device discoverable and broadcast the name
    fields.flags = BLE_HS_ADV_F_DISC_GEN | BLE_HS_ADV_F_BREDR_UNSUP;
    fields.name = (uint8_t *)DEVICE_NAME;
    fields.name_len = strlen(DEVICE_NAME);
    fields.name_is_complete = 1;

    int rc = ble_gap_adv_set_fields(&fields);
    if (rc != 0) {
        ESP_LOGE(TAG, "failed to set advertising data, error code: %d", rc);
        return;
    }

    // Configure advertising behavior
    struct ble_gap_adv_params adv_params;
    memset(&adv_params, 0, sizeof(adv_params));
    adv_params.conn_mode = BLE_GAP_CONN_MODE_UND; // Undirected connectable
    adv_params.disc_mode = BLE_GAP_DISC_MODE_GEN; // General discoverable

    rc = ble_gap_adv_start(BLE_OWN_ADDR_PUBLIC, NULL, BLE_HS_FOREVER,
                           &adv_params, NULL, NULL);
    if (rc != 0) {
        ESP_LOGE(TAG, "failed to start advertising, error code: %d", rc);
        return;
    }
    ESP_LOGI(TAG, "Advertising started successfully");
}

Now, assign the sync callback in the main function so that it runs at the correct time:

void app_main(void) {
    // ...
    ble_hs_cfg.sync_cb = gatt_svr_on_sync;
}

Running the NimBLE Task

NimBLE operates asynchronously within its own FreeRTOS task context. Define a dedicated host task function:

void nimble_host_task(void *param) {
    nimble_port_run(); // Runs continuously to handle stack events
    nimble_port_freertos_deinit();
}

Start the task from the main function:

void app_main(void) {
    // ...
    nimble_port_freertos_init(nimble_host_task);
}

Now you have done the coding part. Let's test your device!

Testing Your ESP32 BLE Server

1. Build and flash the code to your ESP32.
2. Install nRF Connect on your mobile phone.
3. Enable Bluetooth on your phone.
4. Open nRF Connect and scan for nearby devices.
5. Select your ESP32 device using the provided name (DEVICE_NAME) and tap Connect.
6. Locate the Automation IO service (0x1815).
7. Select the Digital characteristic (0x2A56) and tap the Write action.
8. Send a payload of 01 (type = BYTE) to turn GPIO2 High (LED ON), or send 00 to switch it Low (LED OFF).

If your LED responded, congrats! You just successfuly created a bluetooth-controlled IoT device and controlled it using your phone!

What's Next

Now you have built the foundations of BLE. To learn more about BLE, check out the official documentation and the official example. Since you now know the basic concepts, the docs will feel much easier to digest.

If you found this tutorial helpful, please let me know!