esp-idf/examples/bluetooth/common/ble_uart/PORTING.md

BLE UART Porting & API Guide

This document lives in examples/bluetooth/common/ble_uart/ next to the ble_uart component sources (ble_uart.h, backend .c files).

Reference application: use the examples/bluetooth/ble_uart_service example as the working template. Its root CMakeLists.txt appends this directory to EXTRA_COMPONENT_DIRS so main can REQUIRES ble_uart; main/main.c initializes NVS and a MAC-derived GAP name, calls ble_uart_install() / ble_uart_open() with the default encrypted UART-over-BLE echo path, and the tree ships sdkconfig.defaults plus the Bluedroid overlay (sdkconfig.bluedroid). Clone or diff that project when adapting to a new target or host stack.

A complete guide to integrating ble_uart into any ESP-IDF project. Either EXTRA_COMPONENT_DIRS pointing at this component or a few copied source files plus the glue steps below are enough to bring an encrypted BLE serial peripheral up in a fresh project — the same ble_uart.h API works on top of either NimBLE or Bluedroid; pick the host with a Kconfig knob.

This guide uses NimBLE as the running example because it is the default on every ESP32 family target. The Bluedroid path is identical from the application's point of view; the only differences are the sdkconfig knobs called out in §4.3 and a few stack-specific notes flagged inline.

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1. What ble_uart Provides

Capability	Description
Widely used BLE UART-over-GATT (RX/TX)	Interoperates with every generic BLE-serial tool (mobile GATT clients, Web Bluetooth, custom scripts)
LE Secure Connections + Bonding pairing	Single switch; when enabled, a fresh 6-digit passkey is printed to UART
Auto-reconnect	After a bonded central disconnects, advertising restarts immediately and the LTK is reused — no passkey prompt
Raw byte pass-through	RX is delivered via a callback; TX is exposed as ble_uart_tx
Auto-fragmentation	TX is sliced according to the negotiated ATT MTU
Fully wrapped	The user's app_main only calls two functions: install + open

ble_uart is agnostic of any application-layer protocol (no JSON, no line framing). It only delivers bytes — what you do with those bytes is entirely up to you.

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2. Prerequisites

Requirement	Notes
ESP-IDF v5.0+	v5.x or v6.x recommended
BT controller	Must support BLE (ESP32 / C2 / C3 / C5 / C6 / C61 / H2 / S3 / …)
Host stack	Exactly one of CONFIG_BT_NIMBLE_ENABLED=y (default, smaller) or CONFIG_BT_BLUEDROID_ENABLED=y in sdkconfig (covered in detail below)
Flash size	At least 2 MB (the default partition table is plenty)

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3. File Inventory

Canonical sources live under $IDF_PATH/examples/bluetooth/common/ble_uart/ (component name ble_uart): ble_uart.h, ble_uart_nimble.c, ble_uart_bluedroid.c, CMakeLists.txt, and Kconfig (prefix + RX scratch; menuconfig → Component configuration → BLE UART library). When reusing outside this tree, copy the whole common/ble_uart/ directory or at least merge Kconfig into your component so the same CONFIG_BLE_UART_* symbols exist.

Option A — depend on the in-tree component (no copy): add the component directory to EXTRA_COMPONENT_DIRS in the project root CMakeLists.txt before include($ENV{IDF_PATH}/tools/cmake/project.cmake) / project(), then use REQUIRES ble_uart from main/CMakeLists.txt (see examples/bluetooth/ble_uart_service/CMakeLists.txt). This ensures the ble_uart target exists when CMake expands main's requirements.

Kconfig options appear under menuconfig → Component configuration → BLE UART library.

A main/idf_component.yml path dependency alone is not sufficient if main/CMakeLists.txt lists REQUIRES ble_uart: the early requirement scan runs before the component manager injects that dependency, so CMake fails with unknown component ble_uart. Prefer EXTRA_COMPONENT_DIRS (as in the reference example) or copy the sources into a normal project component.

Option B — copy into your project: pick the backend you want and copy that pair plus the public header (or copy both backends; each .c gates on its Kconfig symbol):

your_project/main/
├── ble_uart.h              ← copy from .../common/ble_uart/
├── ble_uart_nimble.c       ← if you'll set CONFIG_BT_NIMBLE_ENABLED=y
└── ble_uart_bluedroid.c    ← if you'll set CONFIG_BT_BLUEDROID_ENABLED=y

Optional: copy Kconfig from common/ble_uart/ into your component (or merge its symbols into your own Kconfig) if you want BLE_UART_* in menuconfig; otherwise hard-code the device name and rely on the 1024-byte fallback for RX scratch.

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4. Step-by-Step Integration

Assume you already have an ESP-IDF project (my_project/).

4.1 Copy the files

cd my_project/main
BLE_UART_SRC="$IDF_PATH/examples/bluetooth/common/ble_uart"
# Stack-agnostic public header — always.
cp "$BLE_UART_SRC/ble_uart.h" .
# Pick one (or copy both — the inactive one compiles to nothing).
cp "$BLE_UART_SRC/ble_uart_nimble.c" .
cp "$BLE_UART_SRC/ble_uart_bluedroid.c" .

4.2 Edit main/CMakeLists.txt

# List both backends; each .c file is gated on its matching Kconfig
# symbol, so only the active one contributes code.
idf_component_register(SRCS "main.c"
                            "ble_uart_nimble.c"
                            "ble_uart_bluedroid.c"
                       INCLUDE_DIRS "."
                       REQUIRES bt nvs_flash)

4.3 Edit sdkconfig.defaults (the 7 critical lines)

NimBLE backend (default, smaller footprint):

# Enable NimBLE
CONFIG_BT_ENABLED=y
CONFIG_BTDM_CTRL_MODE_BLE_ONLY=y     # only needed on classic ESP32; C3/S3/C6/... will warn "unknown" — safe to ignore
CONFIG_BT_BLUEDROID_ENABLED=n
CONFIG_BT_NIMBLE_ENABLED=y

# Encryption + persistent bonds
CONFIG_BT_NIMBLE_SM_SC=y             # LE Secure Connections
CONFIG_BT_NIMBLE_NVS_PERSIST=y       # persist LTKs in NVS — passkey-free reconnects

CONFIG_BT_NIMBLE_ATT_PREFERRED_MTU is optional; the default (256) is fine. Bumping it to 512 lets TX push larger chunks per notification, but the central must support it.

Bluedroid backend (drop-in alternative):

Use the examples/bluetooth/ble_uart_service/sdkconfig.bluedroid file as the authoritative Kconfig overlay: it enables the host stack, SMP, GATTS (service-table API), and the BLE-only advertising knobs that ble_uart_bluedroid.c expects. Either merge those lines into your own sdkconfig.defaults, or pass them as a second defaults file:

idf.py -D SDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.bluedroid" reconfigure

(Paths are relative to the example project root; copy sdkconfig.bluedroid into your tree if you are not starting from ble_uart_service.)

A minimal inline sketch (may drift from IDF defaults — diff against sdkconfig.bluedroid after each IDF upgrade):

CONFIG_BT_ENABLED=y
CONFIG_BT_NIMBLE_ENABLED=n
CONFIG_BT_BLUEDROID_ENABLED=y

# LE Secure Connections + bonding (Bluedroid persists LTKs by default)
CONFIG_BT_BLE_SMP_ENABLE=y

# Optional: bigger MTU (when supported by your IDF target / menuconfig)
# CONFIG_BT_GATT_MAX_MTU_SIZE=512

# BLE-only feature set (saves flash on classic-BT-capable parts)
CONFIG_BT_BLE_42_FEATURES_SUPPORTED=y
CONFIG_BT_BLE_42_ADV_EN=y

4.4 Write app_main (template)

Minimal working template:

#include "esp_log.h"
#include "esp_mac.h"
#include "nvs_flash.h"

#include "ble_uart.h"

static const char *TAG = "app";

/* What to do with received bytes — up to you */
static void ble_uart_on_rx(const uint8_t *data, size_t len)
{
    ESP_LOGI(TAG, "rx %u bytes", (unsigned)len);
    /* echo it back as a demo */
    ble_uart_tx(data, len);
}

void app_main(void)
{
    /* 1. NVS: NimBLE uses it for PHY calibration and bond storage */
    esp_err_t err = nvs_flash_init();
    if (err == ESP_ERR_NVS_NO_FREE_PAGES || err == ESP_ERR_NVS_NEW_VERSION_FOUND) {
        ESP_ERROR_CHECK(nvs_flash_erase());
        err = nvs_flash_init();
    }
    ESP_ERROR_CHECK(err);

    /* 2. Bring up BLE UART */
    ESP_ERROR_CHECK(ble_uart_install(&(ble_uart_config_t){
        .encrypted      = true,
        .device_name    = "MyDevice",
        .ble_uart_on_rx = ble_uart_on_rx,
    }));

    /* 3. Take off */
    ESP_ERROR_CHECK(ble_uart_open());
}

4.5 Build & flash

idf.py set-target esp32s3   # or whichever target you use
idf.py build flash monitor

Once flashed, the UART monitor should show (NimBLE backend):

I (xxx) ble_uart: registered service 6e400001-... handle=14
I (xxx) ble_uart: registered chr 6e400002-... def=15 val=16
I (xxx) ble_uart: registered chr 6e400003-... def=17 val=18
I (xxx) ble_uart: addr=...
I (xxx) ble_uart: BLE host task started
I (xxx) ble_uart: advertising as 'MyDevice'

…or with the Bluedroid backend:

I (xxx) ble_uart: gatts reg status=0 app_id=85 gatts_if=3
I (xxx) ble_uart: registered service svc_handle=40 rx=42 tx=44 cccd=45
I (xxx) ble_uart: advertising started

A phone GATT client app discovers MyDevice; connect, enter the passkey, subscribe to TX, write to RX, and you will see the echo come back.

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5. API Reference

5.1 Configuration struct

typedef struct {
    bool        encrypted;          /* Master switch for SC + Bonding + MITM */
    const char *device_name;        /* GAP device name; NULL uses the NimBLE default */
    ble_uart_rx_cb_t ble_uart_on_rx;/* RX byte callback */
} ble_uart_config_t;

Field	Type	Required	Default / meaning
encrypted	bool	yes	true = SC + Bonding + MITM + DisplayOnly + encrypted GATT chars; false = fully plaintext (sniffable, lab use only)
device_name	const char *	recommended	Any string. Mind the 31-byte primary advertising packet limit: flags(3) + tx_pwr(3) + name(2 + length) + 128-bit UUID(18) → keep the name ≤ 8 bytes
ble_uart_on_rx	callback	optional	NULL discards every received byte

5.2 RX callback signature

typedef void (*ble_uart_rx_cb_t)(const uint8_t *data, size_t len);

static void my_handler(const uint8_t *data, size_t len)
{
    /* `data` is reused after the callback returns; memcpy into your own
     * buffer if you need to keep it. */
}

Caveats:

• The callback runs in the NimBLE host task context — do not block; offload heavy work to your own task.
• A single callback may carry only part of an upper-layer frame (the central slices on ATT MTU). Framing logic (line / TLV / length-prefixed) is your responsibility.
• The data carries no ctx argument. If your callback needs state, use a file-scope static or a global.

5.3 Lifecycle functions

int  ble_uart_install(const ble_uart_config_t *cfg);
int  ble_uart_open(void);
int  ble_uart_close(void);
int  ble_uart_uninstall(void);

Function	What it does (NimBLE)	What it does (Bluedroid)	When to call	Blocking?
install	nimble_port_init + ble_hs_cfg + SM + SIG services + UART GATT	controller_init/enable + bluedroid_init/enable + SM + esp_ble_gatts_create_attr_tab (waits ≤500 ms for the attr-table event)	After nvs_flash_init(), before open	No, ~50 ms (NimBLE) / ~150 ms (Bluedroid)
open	Bond store + spawn host task + start advertising once synced	Configure adv data + scan rsp + start advertising	After install	No, host runs in the background
close	Stop adv → graceful disconnect (LL_TERMINATE_IND, waits ≤500 ms for the disconnect event) → nimble_port_stop()	Stop adv → graceful disconnect (esp_ble_gap_disconnect, waits ≤500 ms)	After open, before uninstall	Yes, up to ~500 ms while waiting for the peer disconnect
uninstall	Calls close if still open, then nimble_port_deinit() and resets module state	Calls close if still open, then bluedroid_disable+deinit + controller_disable+deinit	After close (or directly — uninstall cascades into close on its own)	Yes, follows the same wait window as close

Call order:

nvs_flash_init
    └── ble_uart_install
            └── ble_uart_open      ← BLE is live
                    └── ble_uart_close
                            └── ble_uart_uninstall   ← clean state, can install again

Each call returns BLE_HS_EALREADY if the corresponding state is already true (e.g. open called twice, or close called when the radio is already down). It is therefore safe to call close / uninstall defensively at shutdown without checking the current state yourself.

Do NOT call close / uninstall from inside ble_uart_on_rx — that callback runs on the NimBLE host task, and close blocks on nimble_port_stop() which expects the host task to exit. Self-stop deadlocks. Forward the request to a normal FreeRTOS task instead.

5.4 TX interface

int ble_uart_tx(const uint8_t *data, size_t len);

For formatted output, format into your own buffer with snprintf first and pass it to ble_uart_tx:

char line[64];
int n = snprintf(line, sizeof(line), "temp=%d.%d\n", t / 10, t % 10);
ble_uart_tx((const uint8_t *)line, (size_t)n);

Return values:

Return	Meaning
0	Success (notification handed to the stack)
BLE_HS_ENOTCONN	No central connected; this is normal — typically ignore
BLE_HS_EINVAL	data == NULL or len == 0
BLE_HS_ENOMEM	Stack mbuf pool exhausted
other	Internal stack error — see ble_hs.h

Calling context: any FreeRTOS task at any priority. Not callable from an ISR — push the data to a queue from the ISR and let a task call ble_uart_tx.

Auto-fragmentation: regardless of buffer size, the implementation splits the payload into successive notifications of (MTU - 3) bytes. The central receives them in transmission order.

5.5 Status queries

bool ble_uart_is_connected(void);
bool ble_uart_is_subscribed(void);

• is_connected(): a central is connected (it may not be paired yet).
• is_subscribed(): the central has subscribed to TX notifications (note: bonded reconnects often skip CCCD writes).

You usually don't need to query these up-front — ble_uart_tx returns ENOTCONN to tell you.

5.6 Service UUID constant

extern const ble_uart_uuid128_t ble_uart_service_uuid;

Always 6e400001-b5a3-f393-e0a9-e50e24dcca9e (the de-facto BLE UART service UUID). It is already inserted into the scan response, so the application normally does not touch it. You only need it if you take over advertising yourself (see 6.3).

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6. Advanced Usage

6.1 Different RX framing strategies

A. Split on \n (suits ASCII protocols / JSON)

static uint8_t  s_buf[1024];
static size_t   s_len;

static void on_rx(const uint8_t *d, size_t n)
{
    for (size_t i = 0; i < n; i++) {
        if (d[i] == '\n') { handle_line(s_buf, s_len); s_len = 0; }
        else if (s_len < sizeof s_buf) s_buf[s_len++] = d[i];
    }
}

B. Length-prefixed binary frames

static void on_rx(const uint8_t *d, size_t n)
{
    static uint16_t need = 0;
    static uint8_t  frame[256];
    static size_t   got = 0;

    for (size_t i = 0; i < n; i++) {
        if (need == 0) { need = d[i]; got = 0; continue; }
        frame[got++] = d[i];
        if (got == need) { handle_frame(frame, got); need = 0; }
    }
}

C. Forward straight to UART

static void on_rx(const uint8_t *d, size_t n)
{
    uart_write_bytes(UART_NUM_1, (const char *)d, n);
}

6.2 Disabling encryption (lab scenarios)

ble_uart_install(&(ble_uart_config_t){
    .encrypted = false,        /* ← turn it off */
    .device_name = "OpenDev",
    .ble_uart_on_rx = ...,
});

Effect:

• GATT characteristics drop the _ENC | _AUTHEN flags.
• Any central can read/write — no pairing required.
• No passkey prompt.
• Data is sniffable by any nearby BLE sniffer or compromised radio in range.

Do not ship this in production firmware.

6.3 Coexisting with other GATT services

The snippet below is for the NimBLE backend. With Bluedroid, register additional profiles via esp_ble_gatts_app_register() before calling ble_uart_open() — the gating rule is the same: extra services must be in place before advertising starts.

ble_uart registers its own service; you can call ble_gatts_add_svcs() multiple times and NimBLE will build all of them into the GATT table. Caveat: this must happen before ble_uart_open(), otherwise the host task is already running and the GATT table is locked.

ble_uart_install(&cfg);

/* Register your extra services before open() */
ble_svc_dis_init();              /* Device Information Service */
my_battery_service_init();       /* your own battery service */

ble_uart_open();

If your service must appear in the advertising packet, you have to bypass ble_uart's internal advertising logic — override ble_hs_cfg.sync_cb with your own implementation after ble_uart_install, then call ble_uart_open(). Note that ble_uart's internal start_advertising will not run, so you must call ble_gap_adv_start yourself. In that case, just fork ble_uart_nimble.c (or the matching ble_uart_bluedroid.c).

6.4 Configuring the device-name prefix via Kconfig

If you use the shared ble_uart component, options are already in menuconfig → Component configuration → BLE UART library. If you copied only the .c / .h files into main/, copy Kconfig from common/ble_uart/ as well (or merge its symbols into your own Kconfig.projbuild), then:

char name[24];
snprintf(name, sizeof(name), "%s-%02X%02X",
         CONFIG_BLE_UART_DEVICE_NAME_PREFIX, mac[4], mac[5]);

ble_uart_install(&(ble_uart_config_t){
    .encrypted = true,
    .device_name = name,
    .ble_uart_on_rx = on_rx,
});

Edit the default through menuconfig → Component configuration → BLE UART library → BLE device name prefix.

6.5 Pushing data proactively

You can call TX from any task:

/* A periodic sensor-reporting task */
static void sensor_task(void *arg)
{
    char line[64];
    while (1) {
        int t = read_temperature();
        int n = snprintf(line, sizeof(line), "temp=%d.%d\n", t / 10, t % 10);
        ble_uart_tx((const uint8_t *)line, (size_t)n);
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

/* Spawn it from app_main */
xTaskCreate(sensor_task, "sensor", 3072, NULL, 5, NULL);

When nobody is subscribed, ble_uart_tx returns BLE_HS_ENOTCONN — just ignore it.

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7. Calling Context & Thread Safety

Function	Calling context	Thread-safe
ble_uart_install	Any task; once per uninstall cycle	One-shot until uninstall
ble_uart_open	Any task; after install	One-shot until close
ble_uart_close	Any task except the BLE host task (NimBLE host task / Bluedroid BTC task)	Idempotent; second call returns EALREADY
ble_uart_uninstall	Any task except the BLE host task	Idempotent; cascades into close if needed
ble_uart_tx	Any FreeRTOS task	Yes — multi-task concurrent
ble_uart_is_connected / is_subscribed	Any context	Yes (bool read; best-effort snapshot)
ble_uart_on_rx callback	BLE host task (NimBLE host task / Bluedroid BTC task)	Your code must not block, must not call close / uninstall
Calling any ble_uart API from an ISR	not allowed	Neither host stack supports it

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8. Memory / Performance

Item	Footprint
Code segment (ble_uart_nimble.c.o)	~14 KB (with -Os)
Code segment (ble_uart_bluedroid.c.o)	~22 KB (with -Os; larger because long-write reassembly is open-coded)
Static RAM (globals + RX buffer)	~1.1 KB (the bulk is CONFIG_BLE_UART_RX_SCRATCH_SIZE, default 1024 B)
Host task stack (NimBLE host / Bluedroid BTC)	4 KB (default)
Controller task stack	~3 KB (default)
Bond store (NVS)	~80 bytes per bonded peer
ATT MTU	Negotiated; whatever you set in sdkconfig (247 / 256 / 512)

Measured throughput (ESP32-S3, iPhone 14 Pro central, MTU 247):

• TX (notify): ~25 KB/s
• RX (write): ~20 KB/s

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9. FAQ

Symptom	Cause / fix
nimble_port_init rc=...	NVS not initialised, or BT controller not enabled
Compile error: host/ble_hs.h not found	REQUIRES bt is missing from CMakeLists
Device not discoverable	Device name exceeds the advertising packet limit (drop the tx_pwr field or shorten the name)
Pairing fails	Central uses "Just Works" but we require MITM (encrypted=true). Use a central that supports passkey entry
enc_change status=13 encrypted=1 bonded=1	13 = BLE_HS_ETIMEOUT. Bonded-reconnect race; the link is actually encrypted — safe to ignore
Notifications missing after a reconnect	Bonded centrals often skip the CCCD write; our TX path doesn't gate on subscription state, so notifications still go out — make sure the central side has its callback registered
Second connection rejected	MAX_CONNECTIONS = 1 by default. For multi-connection support, bump the sdkconfig value and turn s_conn_handle (NimBLE backend) / s_conn_id (Bluedroid backend) into an array
Flash fills up	Bond entries accumulate. Periodically run idf.py erase-flash, or call ble_store_clear() in code

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10. Differences from This Example

If you build directly on top of this example:

You already have	No further work needed
main.c echo template	Replace with your own on_rx body
sdkconfig.defaults	Reuse as-is
sdkconfig.bluedroid	Only if you switch to Bluedroid host — reuse as-is (see §4.3); omit for default NimBLE
Root CMakeLists.txt (EXTRA_COMPONENT_DIRS → ../common/ble_uart)	Reuse as-is (or follow §3 option B)
CMakeLists.txt (root + main)	Reuse as-is

If you start from an empty project:

What you need to do	Source
Add EXTRA_COMPONENT_DIRS for examples/bluetooth/common/ble_uart in root CMakeLists.txt, or copy ble_uart.h + at least one of ble_uart_nimble.c / ble_uart_bluedroid.c into main/	§3 of this guide
Copy the key lines of sdkconfig.defaults	§4.3 of this guide
Add REQUIRES ble_uart nvs_flash (after EXTRA_COMPONENT_DIRS) or SRC + REQUIRES bt nvs_flash (copied sources) to main/CMakeLists.txt	§4.2 of this guide
Write install + open in app_main	§4.4 of this guide

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11. API Cheat Sheet (print and pin to the wall)

#include "ble_uart.h"

/* === Types === */
typedef void (*ble_uart_rx_cb_t)(const uint8_t *data, size_t len);

typedef struct {
    bool encrypted;
    const char *device_name;
    ble_uart_rx_cb_t ble_uart_on_rx;
} ble_uart_config_t;

/* === Lifecycle === */
int  ble_uart_install(const ble_uart_config_t *cfg);   /* host + GATT */
int  ble_uart_open(void);                              /* start advertising (NimBLE: spawn host task) */
int  ble_uart_close(void);                             /* stop adv / disconnect / quiesce host */
int  ble_uart_uninstall(void);                         /* tear down host + reset state */

/* === Send (callable from any task) === */
int  ble_uart_tx(const uint8_t *data, size_t len);

/* === Receive === */
/* Via the cfg.ble_uart_on_rx callback, signature:
 * void cb(const uint8_t *data, size_t len);  */

/* === Status === */
bool ble_uart_is_connected(void);
bool ble_uart_is_subscribed(void);

/* === Service UUID (for advertising; usually no need to touch) === */
extern const ble_uart_uuid128_t ble_uart_service_uuid;

---

12. Minimal Project Template (ready to flash)

A complete flashable project takes a handful of files. The inactive backend .c compiles to nothing if you ship both.

Using the shared component (fewer copies):

my_ble_uart_project/
├── CMakeLists.txt              ← EXTRA_COMPONENT_DIRS → …/common/ble_uart (before project())
├── sdkconfig.defaults
└── main/
    ├── CMakeLists.txt          ← REQUIRES ble_uart nvs_flash; SRCS main.c only
    └── main.c

Copying sources into main/ (classic layout):

my_ble_uart_project/
├── CMakeLists.txt
├── sdkconfig.defaults
└── main/
    ├── CMakeLists.txt
    ├── ble_uart.h              ← from $IDF_PATH/examples/bluetooth/common/ble_uart/
    ├── ble_uart_nimble.c
    ├── ble_uart_bluedroid.c    ← optional second backend
    └── main.c

Root CMakeLists.txt (shared ble_uart via EXTRA_COMPONENT_DIRS):

cmake_minimum_required(VERSION 3.16)
list(APPEND EXTRA_COMPONENT_DIRS "${CMAKE_CURRENT_LIST_DIR}/../path/to/common/ble_uart")
include($ENV{IDF_PATH}/tools/cmake/project.cmake)
project(my_ble_uart)

main/CMakeLists.txt (shared component — no .c copies in main/):

idf_component_register(SRCS "main.c"
                       INCLUDE_DIRS "."
                       REQUIRES ble_uart nvs_flash)

main/CMakeLists.txt (classic copy layout — both backends in main/):

idf_component_register(SRCS "main.c"
                            "ble_uart_nimble.c"
                            "ble_uart_bluedroid.c"
                       INCLUDE_DIRS "."
                       REQUIRES bt nvs_flash)

sdkconfig.defaults (7 lines, NimBLE backend):

CONFIG_BT_ENABLED=y
CONFIG_BTDM_CTRL_MODE_BLE_ONLY=y
CONFIG_BT_BLUEDROID_ENABLED=n
CONFIG_BT_NIMBLE_ENABLED=y
CONFIG_BT_NIMBLE_SM_SC=y
CONFIG_BT_NIMBLE_NVS_PERSIST=y
CONFIG_BT_NIMBLE_ATT_PREFERRED_MTU=512

Bluedroid host instead of NimBLE: copy examples/bluetooth/ble_uart_service/sdkconfig.bluedroid next to your sdkconfig.defaults and pass -D SDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.bluedroid" (see §4.3).

main/main.c — copy the §4.4 template verbatim.

Flash:

idf.py set-target esp32s3
idf.py build flash monitor

Done.