esp-nus — High-throughput BLE 5 UART bridge

In the world of wireless prototyping, the “wireless serial cable” is a classic utility. The esp-nus project provides a high-performance implementation of this concept for the ESP32-C6, leveraging the Nordic UART Service (NUS) to bridge hardware UART signals over Bluetooth Low Energy. While many BLE-UART bridges suffer from latency or low data rates, this project is specifically engineered to maximize throughput by utilizing the advanced features of the BLE 5 stack.

High-Throughput Optimization

What sets this project apart is its focus on efficiency and speed. Most standard BLE implementations default to conservative parameters to save power, but esp-nus is tuned for raw performance. It achieves this through several key mechanisms:

• LE 2M PHY: By using the 2Mbps physical layer, the project effectively doubles the raw air-rate compared to standard 1M PHY.
• Data Length Extension (DLE): The system requests 251-byte link-layer payloads. This ensures that large data packets aren’t fragmented at the air-interface level, reducing overhead.
• Optimal MTU Sizing: While it supports a preferred MTU of up to 512 bytes, it is optimized for an MTU of 247. This specific size allows a 244-byte payload to fit perfectly into a single link-layer PDU when combined with L2CAP headers, avoiding on-air fragmentation entirely.
• Aggressive Connection Parameters: It requests a 7.5 ms connection interval, the minimum allowed by the BLE specification, to ensure the lowest possible latency for serial data.

Architecture and Flow Control

The bridge is built on the ESP-IDF framework using the NimBLE stack. One of the most critical challenges in high-speed UART-to-BLE bridging is handling the mismatch between the continuous stream of UART data and the packetized nature of BLE.

To solve this, esp-nus implements flow-controlled notifications using a credit pool (NUS_TX_CREDITS). This mechanism keeps multiple notifications pipelined to the Bluetooth controller without exhausting the memory pool. When the controller confirms a transmission via the BLE_GAP_EVENT_NOTIFY_TX event, credits are returned. This creates a natural backpressure system: if the BLE link cannot keep up, the UART source is throttled, preventing data loss or buffer overflows.

NUS GATT Layout

The project implements the standard Nordic UART Service, making it compatible with a wide range of mobile apps and desktop tools that support the NUS protocol:

Role	UUID	Properties
Service	6E400001-B5A3-F393-E0A9-E50E24DCCA9E	Primary
RX (Central → Device)	6E400002-…	Write, Write No Response
TX (Device → Central)	6E400003-…	Notify

Hardware and Configuration

While developed and tested on the ESP32-C6-DevKitC, the project is highly configurable via the ESP-IDF menuconfig system. Users can adjust the advertised device name, UART port numbers, and baud rates (defaulting to 921600 to match BLE throughput).

For high-speed operation, hardware flow control (RTS/CTS) is highly recommended and supported by the driver to avoid RX overruns. The project also utilizes the ESP32-C6’s ability to move the console to USB-Serial-JTAG, which frees up the primary hardware UART (UART0) specifically for the bridge application.