BTool is a PC Application that allows a user to form a connection between two BLE devices. BTool works by communicating with the CC26x2R LaunchPad, acting as a network processor, by means of HCI vendor specific commands. This requires the CC2642R to be running the Host Test sample application.
More information on the network processor configuration can be found in the Bluetooth low energy Software Developer’s Guide. More information on the Host Test sample application can be found in the Host Test overview. More information on the HCI interface, as well as details on the HCI vendor specific commands that are used by the CC26x2R LaunchPad can be found in the TI BLE Vendor Specific HCI Guide.
For this section, a PC running Windows 7 has been used, but the procedures are essentially the same for other Windows versions.
Starting the Application¶
Once a CC2642R has been connected to the PC via USB, upon opening BTool, you should be able to set the Serial Port Settings. Set the “Port” value to the COM Port number associated with “XDS110 Class Application/User.” For the other settings, use the default values as shown in Fig. 1.
When BTool has a serial connection to the LaunchPad, you should see the screen presented in Fig. 2. This screen indicates that you now have a serial port connection to the CC26x2R LaunchPad. The screen is divided up into a few sections. The left sidebar contains information on the status of the connected device. The left side of the sub-window contains a log of all messages sent from BTool to the CC2642R LaunchPad and received by BTool from the CC26x2R LaunchPad. The right side of the sub-window contains a GUI for control of the CC26x2R LaunchPad.
Creating a Connection Between the CC26x2R LaunchPad and a Peripheral Device¶
At this point, the CC26x2R LaunchPad (central device) is ready to discover other BLE devices that are advertising. This section will use a CC2642R running the Simple Peripheral sample application as the peripheral device that is being connected to. More information on running this project can be found in the Bluetooth low energy Software Developer’s Guide and the Simple Peripheral sample application page. The Simple Peripheral device will immediately begin advertising upon startup.
Scanning for Devices¶
In BTool, press the “Scan” button under the “Discover/Connect” tab, as shown in Fig. 3. The device running Host Test will begin searching for other BLE devices.
As devices are found, the log on the left side of the screen will display the devices that are discovered. After 10 seconds, the device discovery process will complete, and the device running Host Test will stop scanning. A summary of all of the scanned devices will be displayed in the log window. In the example in Fig. 4, 19 peripheral devices were discovered while scanning. If you do not want to wait through the full 10 seconds of scanning, the “Cancel” button can be pressed to stop the device discovery process. The address of any scanned devices will appear in the “Slave BDA” section of the “Link Control” section in the bottom right corner of the sub-window.
Selecting Connection Parameters¶
Before establishing a connection, you can set up the desired connection parameters. The default values of 100ms connection interval, 0 slave latency, and 20s supervision timeout should serve as a good starting point; however for different applications you may want to experiment with other values.
Once the desired values have been set, be sure to click the “Set” button; otherwise the settings will not be saved. Note that the connection parameters must be set before a connection is established; changing the values and clicking the “Set” button while a connection is active will not change the settings of an active connection. The connection must be terminated and re-established to use the new parameters. (The Bluetooth specification does support connection parameter updates while a connection is active; however this must be done using either an L2CAP connection parameter update request, or using a direct HCI command. More information can be found in the Specification of the Bluetooth System).
Establishing a Connection¶
To establish a connection with the CC26x2R LaunchPad that is running Simple Peripheral, select the address of the device to connect with, and click the “Establish” button as shown in Fig. 6.
If the device running Simple Peripheral is still in discoverable mode, a connection should be established. Once a connection is established, the message window will return a “GAP_EstablishLink” event message with a “Status” value of “0x00 (Success)” as shown in Fig. 7.
In BTool, you can see your connected peripheral device in the Device Information field along with the address of the slave device under “Connection Info”, as shown in Fig. 8.
Simple Peripheral Profiles¶
The Simple Peripheral application contains several GATT service profiles (more information can be found in the Simple Peripheral Sample Application Guide). GATT services contain data values known as “characteristic values.” All application data that is being sent or received in Bluetooth low energy must be contained within characteristic values. This section details a step-by-step process that demonstrates several processes for reading, writing, discovering, and notifying GATT characteristic values using BTool.
In a Bluetooth low energy system, upon connection, the the central device (client) performs a service discovery on the peripheral device (server) to build up an attribute table. This attribute table will provide handles (internal addresses of the characteristics) which can be used by the client to access the data located in the server. The device discovery is typically an automated process that can be started with a single command. In BTool, this can be done by right clicking on “Handle” (under “Connection Info” on the left side of the screen) and then selecting “Discover UUIDs”.
Reading a Characteristic Value by UUID¶
A characteristic value is essentially where the data is stored, which could be, for example, temperature data or battery level. It’s the stored data in a server that a client wants to access. A characteristic is a discrete value that has the following three properties associated with it:
- A handle (address)
- A type (UUID)
- A set of permissions
The simplest way to read its value is to use the “Read Characteristic by UUID” sub-procedure. To do this, you will first need to click the “Read Write” option in BTool under “Select Device Operation” in the upper left corner. Select the option “Read Using Characteristic UUID” under the”sub-procedure” option in the “Characteristic Read” section at the top of the screen. Enter the UUID (note that the LSB is entered first, and the MSB is entered last) in the “Characteristic UUID” box, and click the “Read” button. An attribute protocol Read by Type Request packet gets sent over the air from the Host Test device to the Simple Peripheral device, and an attribute protocol Read by Type Response packet gets sent back from the Simple Peripheral device to the Host Test device. The characteristic value is displayed in the “Value” box, and “Success” is displayed in the “Status” box. In addition, the message window will display information on the Read by Type Response packet that was received by the Host Test device. The message includes not only the characteristic’s data value, but also the handle of the characteristic value.
Reading a Characteristic Value by Handle¶
It is also possible to read the characteristic value if the handle is known. This is done by selecting the option “Read Characteristic Value/Descriptor” under the “Sub-Procedure” option in the “Characteristic Read” section. Enter the handle into the “Characteristic Value Handle” box and click the “Read” button to execute the read.
An attribute protocol Read Request packet gets sent over the air from the Host Test device to the Simple Peripheral device, and an attribute protocol Read Response packet gets sent back from the Simple Peripheral device to the Host Test device. The new value is displayed in the “Value” box, and “Success” is displayed in the “Status” box. This value should match the value that was written in the previous step for the same characteristic.
Writing a Characteristic Value¶
If a characteristic has write permission, it is possible for a client to write a value to the server. This is done in the “Characteristic Write” section by entering the handle in the “Characteristic Value Handle” box and the value in the “Value” section (the format can be set to either “Decimal” or “Hex”). The write operation is performed when the “Write Value” button is clicked.
An attribute protocol Write Request packet gets sent over the air from the Host Test device to the Simple Peripheral, and an attribute protocol Write Response packet gets sent back from the Simple Peripheral device to the Host Test device. The status box will display “Success”, indicating that the write was successful.
Discovering a Characteristic by UUID¶
By discovering a characteristic by UUID, not only will the handle of the UUID be obtained, but the properties of the characteristic (handle and permission) will as well. The operation is performed by selecting the option “Discover Characteristic by UUID” under the “Sub-Procedure” option in the “Characteristic Read” section at the top of the screen. The discovery is performed when the UUID is entered into the “Characteristic UUID” box, and the “Read” button is clicked.
A series of attribute protocol Read by Type Request packets get sent over the air over the air from the Host Test device to the Simple Peripheral device, and for each request an attribute protocol Read by Type Response packet gets sent back from the Simple Peripheral device to the Host Test device. Essentially, the the Host Test device is reading every attribute on the Simple Peripheral device with a UUID of 0x2803 (this is the UUID for a characteristic declaration), and checking the “Characteristic Value UUID” portion of each declaration to see if it matches the UUID type you’ve entered. The procedure is complete once every characteristic declaration has been read.
As per the Bluetooth specification, the first byte presents the properties of the characteristic. The second and third bytes present the handle of the characteristic value. The fourth and fifth bytes present the UUID of the characteristic.
Reading Multiple Characteristic Values¶
It is also possible to read multiple characteristic values with one request, as long as the handle of each value is known. To read several values from different characteristics, select the option “Read Multiple Characteristic Values” under the “Sub-Procedure option” in the “Characteristic Read” section at the top of the screen. Enter the handle with the semicolon separation (ie. “0x0022;0x0025”) in the “Characteristic Value Handle” box, and click the “Run” button.
An attribute protocol Read Multiple Request packet gets sent over the air from the host Test device to the Simple Peripheral device, and an attribute protocol Read Multiple Response packet gets sent back from the Simple Peripheral device to the Host Test device. The values of the two characteristics are displayed in the “Value” box, and “Success” is displayed in the “Status” box.
The response will not parse the separate values. This means that the size of each value being read must be fixed, and must be known by the client. In the example here, this is not an issue since there are only two bytes in the response; however, care must be taken when using this command.
In BLE, it is possible for a GATT server device to “push” characteristic value data out to a client device without being prompted with a read request. This process is called “Characteristic Value Notification”. Notifications are useful in that they allow a device in a BLE connection to send out as much or as little data as required at any point in time. In addition, since no request from the client is required, the overhead is reduced and the data is transmitted more efficiently.
In order to enable notifications, the client device must write a value of 0x0001 to the Client Characteristic Configuration Descriptor (CCCD) for the particular characteristic. The handle for the CCCD immediately follows the characteristic value’s handle. Therefore, a value of 0x0001 must be written to the “handle + 1”.
Using BLE Security¶
BTool also includes the ability to make use of security features in BLE, including encryption, authentication, and bonding.
Encrypting the Connection¶
To encrypt the link, the pairing process must be initiated. click on the “Pairing Bonding” operation in BTool. In the “Initiate Pairing” section at the top of the screen, check the boxes labeled “Bonding Enabled” and “Authentication (MITM) Enabled”, and click the “Send Pairing Request” button as shown in Fig. 9. This will send the pairing request to the peripheral device.
The peripheral will send a pairing response in return, which will require a six-digit passkey to be entered by the user in order to complete the process. Typically, this passkey is intended to be used by a peripheral device containing a display. By displaying the passkey on the peripheral device and requiring the user to enter it in the central device’s user interface, the link is authenticated, and thus verifying that the connection has not been hijacked using a man-in-the-middle (MITM) attack.
In the box labeled “Passkey” in the “Passkey Input” section in BTool, enter the passkey value and click the “Send Passkey” button as shown in Fig. 10. Note that if you do not send the passkey within 30 seconds after receiving the pairing response message, the pairing process will fail, and you will need to re-send the pairing request.
When pairing is successfully completed, you will see a “GAP_AuthenticationComplete” event in the log window, with a “Success” status. The BLE connection is now encrypted as shown in Fig. 11.
Using Bonding and Long-Term Keys¶
Bonding is a feature in BLE that allows a device, after initial pairing with a peer, to remember specific information about that peer device. In particular, the long-term key data that is generated during the initial pairing process can be stored locally. If the connection is then terminated and the two devices later reconnect, this data can be used to quickly re-initiate encryption without needing to go through the full pairing process and/or use a passkey. In addition, if a client device had enabled notifications of any characteristics on the server device while the two devices were bonded, the server device will remember the setting and the client will not have to re-enable them.
After pairing has been completed with the bonding enabled, the “Long-term Key (LTK) Data” will be populated with some of the data from the “GAP_AuthenticationComplete” event that was generated during the encryption process. This data is required for re-initiating encryption upon reconnect. Click the “Save Long-term Key Data to File” button to save this information to file. The data is saved in a “comma separated value” (CSV) format as simple text, and can be stored anywhere on disk. Be sure to note the location that the file is stored in.
Within the Simple Peripheral device, a similar process is going on, in that the the Simple Peripheral sample application contains a bond manager that is storing the long-term key data that it had generated during encryption. Since the Simple Peripheral sample application does not have a file system, it is simply storing the data in the nonvolatile memory of the CC2642R. More information on the bond manager can be found in the Bluetooth low energy Software Developer’s Guide. With a bond now active, you can enable notifications of a characteristic value and have that setting remembered for later. Note that if notifications were enabled before going through the pairing process, then the setting will not be stored. Therefore, you will need to re-write the value “01:00” to a CCCD. Because the devices are paired with bonding enabled, the bond manager in the Simple Peripheral sample application will store the CCCD data in nonvolatile memory.
To verify that bonding worked, you will need to disconnect and re-connect. Click on the “Discover Connect” operation and click the “Terminate” button at the bottom of the screen to disconnect from the Simple Peripheral device. The message window will show a “GAP_TerminateLink” event with “Success” status as shown in Fig. 13. In addition, the connection information in the upper-left corner of the screen will disappear.
At a later time, re-connect with the Simple Peripheral device following the procedure in Creating a Connection Between the CC26x2R LaunchPad and a Peripheral Device. Note that the value of the CCCD for a characteristic is always set back to “00:00” if a connection is terminated or if the device resets.
To re-initiate encryption and re-enable any notifications that were enabled in the previous connection, return to the “Pairing/Bonding” tab. In the “Encrypt Using Long-Term Key” section, click the “Load Long-Term Key Data From File” button, and select the file in which the data was previously stored. The data fields will get automatically populated from the data in the file. Click the “Encrypt Link” button to re-enable encryption as shown in Fig. 14.
A “GAP_BondComplete” event with “Success” status will be displayed in the log window. This indicates that the link has been re-encrypted as shown in Fig. 15. You will also now be able to receive any notifications that were enabled in the previous connection. Any changes to the CCCD value of a characteristic will be saved to nonvolatile memory and remembered for the next time that encryption is initiated using the long-term key.