Using Bluetooth Serial Port Profile Print


Bluetooth is widely used when it is necessary to provide a wireless access to the devices located in short distances. In this application note we will show how to organize a wireless control channel to the NXP i.MX RT1024 EVK running Linux using a USB Bluetooth adapter. In practical embedded applications such a wireless channel may be used to implement a command/response protocol to control your i.MX RT1024 based device via Bluetooth from the host machine (computer, notebook, smart-phone, etc.).

Hardware Platform

The hardware platform used in this application note is the NXP i.MX RT1024 EVK board with a USB Bluetooth adapter plugged into the J9 USB interface connector. The adapter used by Emcraft to perform the tests documented below was RFCOMM Protocol TDI Bluetooth Device. The generic Linux kernel device driver for the USB transport HCI layer (CONFIG_BT_HCIBTUSB) is used in this configuration so other USB Bluetooth adapters should work as described below too.

Software Platform

The goal is to have the i.MX RT1024 act as a device with the Serial Port Profile (SPP) in the Bluetooth network. The i.MX RT1024 will emulate the serial port over the Bluetooth transport and therefore the host will see the i.MX RT1024 as an ordinary serial RS-232 port.

The Bluetooth Serial Port Profile interface is implemented with the BT_RFCOMM_TTY kernel driver. The sdptool and rfcomm utilities, ported from the bluez-utils package to the Linux NXP i.MX RT1024 EVK BSP, are used for configuration. The sdptool utility is used to create the "Serial Port" profile, exported over Bluetooth. The rfcomm utility is started in the "listen" mode and, on the "Serial Port" profile activation, rfcomm emulates a virtual COM port on the /dev/rfcommX device file, looking from the i.MX RT1024 side.

The functionality described below is available from the rootfs.uImage project provided by Emcraft for NXP i.MX RT1024 EVK.

Test Setup

We will use the following terminology below:

  • Target: NXP i.MX RT1024 EVK board with the Bluetooth adapter plugged into the USB (J9) port.
    The Bluetooth <Target address> in the examples below is 04:7F:0E:31:B7:94.
  • Host: Any computer with a Bluetooth interface, running Linux with the Bluetooth tools (bluez-utils) installed.
    The Bluetooth <Host address> in the examples below is BC:77:37:5C:32:57.

Test Connectivity

Power-on the NXP i.MX RT1024 EVK board and wait for the Linux to boot on the Target. Run the Bluetooth daemons in the background:

init started: BusyBox v1.24.2 (2023-07-31 09:38:50 UTC)
.../ # hcid -n&
[1] 94 hcid -n
/ # hcid[94]: Bluetooth HCI daemon
/ # sdpd -n&
[2] 95 sdpd -n
/ #

Plug-in the Bluetooth adapter to the USB (J9) interface of the NXP i.MX RT1024 EVK board. Observe the messages like these in the Target console:

usb 1-1: new full-speed USB device number 2 using ci_hdrc
usb 1-1: Duplicate descriptor for config 1 interface 1 altsetting 5, skipping
hcid[94]: HCI dev 0 registered
Bluetooth: hci0: CSR: Unbranded CSR clone detected; adding workarounds and force-suspending once...
Bluetooth: hci0: CSR: Failed to suspend the device for our Barrot 8041a02 receive-issue workaround
hcid[94]: HCI dev 0 up
hcid[99]: Can't set encrypt on hci0: Invalid request code (56)
hcid[94]: Starting security manager 0

Get the <Target address> Bluetooth address of the adapter you have just plugged-in:

/ # hcitool dev
hci0               04:7F:0E:31:B7:94
/ #

Add the Serial Port profile to the list of Bluetooth profiles. In the example below the Bluetooth channel number to access this profile is 1:

/ # sdptool add --channel=1 SP
Serial Port service registered

Start the background listening for raw connections on the channel 1:

/ # rfcomm --raw listen /dev/rfcomm0 1 &
[3] 110 rfcomm --raw listen /dev/rfcomm0 1
/ # Waiting for connection on channel 1

On the Host, open a raw Bluetooth connection to the Target (<Target Address> Bluetooth Device, channel 1):

$ sudo rfcomm --raw connect 0 04:7F:0E:31:B7:94 1
Connected /dev/rfcomm0 to 04:7F:0E:31:B7:94 on channel 1
Press CTRL-C for hangup

In the Target console observe an indication of the connection from the Host:

Connection from BC:77:37:5C:32:57 to /dev/rfcomm0
Press CTRL-C for hangup

Send a text string (command) to the Target over the Bluetooth serial port from the Host:

$ echo "Hello from Host over the BT Serial" | sudo tee /dev/rfcomm0
Hello from Host over the BT Serial

On the Target receive the text string (command) just sent from Host:

/ # cat /dev/rfcomm0
Hello from Host over the BT Serial

Send a text string (response) from the Target to Host over the Bluetooth serial port:

/ # echo "Hello from Target over the BT Serial" > /dev/rfcomm0

Receive the text string (response) just sent from the Target over the Bluetooth serial port:

$ sudo cat /dev/rfcomm0
Hello from Target over the BT Serial

Disconnect the Host from the Bluetooth serial line by pressing Ctrl-C in the rfcomm --raw connect ... terminal window:


Observe the rfcomm termination on the Target side:

/ # Disconnected [3]

Done                             rfcomm --raw listen /dev/rfcomm0 1/
/ #

Please note that the rfcomm --raw listen ... command must be re-run on the Target to reopen the Bluetooth serial link again.