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 RT1170 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 RT1170 based device via Bluetooth from the host machine (computer, notebook, smart-phone, etc.).

1. Hardware Platform

The hardware platform used in this application note is the NXP i.MX RT1170 EVK board with a USB Bluetooth adapter plugged into the USB1 or USB2 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.

2. Software Platform

The goal is to have the i.MX RT1170 act as a device with the Serial Port Profile (SPP) in the Bluetooth network. The i.MX RT1170 will emulate the serial port over the Bluetooth transport and therefore the host will see the i.MX RT1170 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 RT1170 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 RT1170 side.

The functionality described below is available from the rootfs.uImage project provided by Emcraft for NXP i.MX RT1170 EVK.

3. Test Setup

We will use the following terminology below:

• Target: NXP i.MX RT1170 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.

4. Test Connectivity

1. Power-on the NXP i.MX RT1170 EVK board and wait for the Linux to boot on the Target. Run the Bluetooth daemons in the background:

   ... / # hcid -n& [1] 97 hcid -n hcid[97]: Bluetooth HCI daemon / # sdpd -n& [2] 98 sdpd -n / #

2. Plug-in the Bluetooth adapter to the USB1 interface of the NXP i.MX RT1170 EVK board. Observe the messages like these in the Target console:

   ci_hdrc ci_hdrc.0: EHCI Host Controller ci_hdrc ci_hdrc.0: new USB bus registered, assigned bus number 1 ci_hdrc ci_hdrc.0: USB 2.0 started, EHCI 1.00 hub 1-0:1.0: USB hub found hub 1-0:1.0: 1 port detected random: crng init done 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[97]: HCI dev 0 registered Bluetooth: hci0: CSR: Setting up dongle with HCI ver=9 rev=0001; LMP ver=9 subver=0001; manufacturer=2279 Bluetooth: hci0: CSR: Unbranded CSR clone detected; adding workarounds and force-suspending once... Bluetooth: hci0: CSR: Couldn't suspend the device for our Barrot 8041a02 receive-issue workaround Bluetooth: hci0: HCI Delete Stored Link Key command is advertised, but not supported. Bluetooth: hci0: HCI Read Default Erroneous Data Reporting command is advertised, but not supported. Bluetooth: hci0: HCI Set Event Filter command not supported. hcid[97]: HCI dev 0 up hcid[102]: Can't set encrypt on hci0: Invalid request code (56) hcid[97]: Starting security manager 0

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

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

4. 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:

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

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

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

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

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

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

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

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

13. Observe the rfcomm termination on the Target side: