HCI UART

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Overview

Expose Bluetooth controller support over UART to another device/CPU using the H:4 HCI transport protocol (requires HW flow control from the UART).

Requirements

  • A board with Bluetooth LE support

Default UART settings

By default the controller builds use the following settings:

  • Baudrate: 1Mbit/s

  • 8 bits, no parity, 1 stop bit

  • Hardware Flow Control (RTS/CTS) enabled

Building and Running

This sample can be found under samples/bluetooth/hci_uart in the Zephyr tree, and it is built as a standard Zephyr application.

Using the controller with emulators and BlueZ

The instructions below show how to use a Nordic nRF5x device as a Zephyr BLE controller and expose it to Linux’s BlueZ. This can be very useful for testing the Zephyr Link Layer with the BlueZ Host. The Zephyr Bluetooth LE controller can also provide a modern Bluetooth LE 5.0 controller to a Linux-based machine for native BLE support or QEMU-based development.

First, make sure you have a recent BlueZ version installed by following the instructions in the Using BlueZ with Zephyr section.

Now build and flash the sample for the Nordic nRF5x board of your choice. All of the Nordic Development Kits come with a Segger IC that provides a debugger interface and a CDC ACM serial port bridge. More information can be found in Nordic nRF5x Segger J-Link.

For example, to build for the nRF52832 Development Kit:

# From the root of the zephyr repository
west build -b nrf52dk/nrf52832 samples/bluetooth/hci_uart
west flash

Using the controller with QEMU or native_sim

In order to use the HCI UART controller with QEMU or native_sim you will need to attach it to the Linux Host first. To do so simply build the sample and connect the UART to the Linux machine, and then attach it with this command:

sudo btattach -B /dev/ttyACM0 -S 1000000 -R

Note

Depending on the serial port you are using you will need to modify the /dev/ttyACM0 string to point to the serial device your controller is connected to.

Note

If using the BBC micro:bit you will need to modify the baudrate argument from 1000000 to 115200.

Note

The -R flag passed to btattach instructs the kernel to avoid interacting with the controller and instead just be aware of it in order to proxy it to QEMU later.

If you are running btmon you should see a brief log showing how the Linux kernel identifies the attached controller.

Once the controller is attached follow the instructions in the Running on QEMU or native_sim section to use QEMU with it.

Using the controller with BlueZ

In order to use the HCI UART controller with BlueZ you will need to attach it to the Linux Host first. To do so simply build the sample and connect the UART to the Linux machine, and then attach it with this command:

sudo btattach -B /dev/ttyACM0 -S 1000000

Note

Depending on the serial port you are using you will need to modify the /dev/ttyACM0 string to point to the serial device your controller is connected to.

Note

If using the BBC micro:bit you will need to modify the baudrate argument from 1000000 to 115200.

If you are running btmon you should see a comprehensive log showing how BlueZ loads and initializes the attached controller.

Once the controller is attached follow the instructions in the Using Zephyr-based Controllers with BlueZ section to use BlueZ with it.

Debugging the controller

The sample can be debugged using RTT since the UART is otherwise used by this application. To enable debug over RTT the debug configuration file can be used.

west build samples/bluetooth/hci_uart -- -DEXTRA_CONF_FILE='debug.conf'

Then attach RTT as described here: Using Segger J-Link

Support for the Direction Finding

The sample can be built with the support for the Bluetooth LE Direction Finding. To enable this feature build this sample for specific board variants that provide required hardware configuration for the Radio.

west build samples/bluetooth/hci_uart -b nrf52833dk/nrf52833@df -- -DCONFIG_BT_CTLR_DF=y

You can use following targets:

  • nrf5340dk/nrf5340/cpunet@df

  • nrf52833dk/nrf52833@df

Check the Direction Finding Periodic Advertising Locator and the Direction Finding Periodic Advertising Beacon for more details.

Using a USB CDC ACM UART

The sample can be configured to use a USB UART instead. See samples/bluetooth/hci_uart/boards/nrf52840dongle_nrf52840.conf.

Using the controller with the Zephyr host

This describes how to hook up a board running this sample to a board running an application that uses the Zephyr host.

On the controller side, the zephyr,bt-c2h-uart DTS property (in the chosen block) is used to select which uart device to use. For example if we want to keep the console logs, we can keep console on uart0 and the HCI on uart1 like so:

/ {
   chosen {
      zephyr,console = &uart0;
      zephyr,shell-uart = &uart0;
      zephyr,bt-c2h-uart = &uart1;
   };
};

On the host application, some config options need to be used to select the H4 driver instead of the built-in controller:

CONFIG_BT_HCI=y
CONFIG_BT_CTLR=n

Similarly, the zephyr,bt-hci DTS property selects which HCI instance to use. The UART needs to have as its child node a HCI UART node:

/ {
   chosen {
      zephyr,console = &uart0;
      zephyr,shell-uart = &uart0;
      zephyr,bt-hci = &bt_hci_uart;
   };
};

&uart1 {
   status = "okay";
   bt_hci_uart: bt_hci_uart {
      compatible = "zephyr,bt-hci-uart";
      status = "okay";
   };
};

See also

HCI RAW channel
Bluetooth APIs
UART Interface