With a Bluetooth-enabled mobile phone, the Bluetooth in-vehicle system can realize a variety of creative applications such as automatic vehicle fault diagnosis and electronic navigation. This article introduces these application design ideas, and analyzes the composition of the Bluetooth in-vehicle system and the Bluetooth hands-free application specification.

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The vehicle system is developing in the direction of intelligence, information and network. The automobile market has become an important growth point of the electronics industry. Wireless communication technology has broad application prospects in mobile systems such as automobiles. The car Bluetooth application described in this article mainly uses the mobile phone as the wireless gateway, and the in-vehicle system is connected to the mobile phone through the Bluetooth wireless link, and then connected to the external wireless network. With this solution, users can easily use the following functions through a Bluetooth mobile phone:

Speakerphone. The user enters the car and the in-vehicle system automatically connects to the user's mobile phone. When driving, the user can use the voice control to complete the functions of dialing, answering, hanging up and volume adjustment without manual operation, and enter the full-duplex hands-free call through the in-car microphone and audio system.

Car remote control. Users can control the doors and various switches in the car with a mobile phone within 10 meters.

music download. Users can download music to the car stereo through their mobile phones.

Electronic navigation. The user can download the electronic map and other data to the car GPS navigation system through the mobile phone, and the navigation system obtains the current coordinate parameter and then transmits it back to the navigation center through the mobile phone short message.

Auto automatic fault diagnosis system. The in-vehicle system can send the fault code and other information to the repair center through the mobile phone. When the repair center sends someone to come to the repair, the corresponding accessories and repair tools can be prepared according to the fault code and other information.

The full application of Bluetooth in in-vehicle systems requires addressing hardware and software issues. The hardware implements the RF and baseband parts of Bluetooth. The operating temperature range, reliability and price are the key issues to be considered when designing the hardware. The various upper functions of the in-vehicle system are implemented by software. As the main application of Bluetooth in the in-vehicle system, the hands-free phone has matured and has been supported by many mobile phone manufacturers.


figure 1

As shown in Figure 1, the RF chip provides both data and voice wireless bi-directional transmission in the 2.4 GHz band. The baseband chip includes hardware such as frequency hopping, channel encryption and decryption, authentication, SCO voice codec, and firmware such as link management and HCI interface. The processor runs core software such as the Bluetooth upper layer protocol stack, application specifications, and voice algorithms. For convenience of description, the radio frequency, baseband and processor parts are referred to as Bluetooth subsystems. Car audio provides audio input and output to the Bluetooth subsystem through the in-car microphone and speakers. The main controller runs software such as Human Machine Interface (MMI) to centrally control and manage the Bluetooth subsystem, car audio and other in-vehicle electronic devices, and display relevant information on a graphical user interface (GUI).

Bluetooth subsystem structure analysis

The Bluetooth subsystem contains hardware and software related to implementing Bluetooth in-vehicle application specifications. The baseband chip connects to the processor via the UART and SSI interfaces: HCI commands, data, and events are transmitted over the UART interface; SCO data is transmitted over the SSI interface. As mentioned earlier, the processor runs core software such as the Bluetooth upper layer protocol stack, application specifications, and voice algorithms. Figure 2 shows the software architecture running on the processor in the Bluetooth subsystem.


figure 2

The Circuit Support Package (BSP) in Figure 2 contains all the hardware-related code to ensure good hardware independence of the upper layer software. The RTOS provides good structure and real-time performance for the software. The design should take full account of the RTOS RAM, ROM footprint, interrupt response and task switching time, and whether the scheduling algorithm meets the real-time requirements of the speech algorithm. The Bluetooth protocol stack includes upper layer protocols such as HCI HOST, L2CAP, SDP, and RFCOMM. On top of the protocol stack, several application specifications, such as hands-free application specifications and headset application specifications, are placed according to different application requirements. As new application specifications continue to emerge, the software architecture should facilitate the addition of new application specifications for future expansion. The database provides record addition, deletion, and lookup functions to manage data for local and remote Bluetooth devices, usually in Flash memory. The stored data includes the Bluetooth address of the local device, the device name, the pairing password, and the SDP record, as well as the Bluetooth address, device name, SDP record, link key, authentication, and authorization scheme of the remote device. Since the system needs to be paired with multiple remote devices, the database should have enough capacity to hold data from multiple remote devices.

The Bluetooth management layer (Bluetooth ME) calls the protocol stack and database to work in a certain process to perform access control management, connection management, and security management. A voice algorithm is also required in the in-vehicle system because the other party can hear their own echo during the hands-free call and can be eliminated using echo cancellation (AEC) technology. When hands-free calling is carried out during driving, engine noise, road noise, and wind noise will enter the in-car microphone, affecting the quality of the call. Noise suppression (NS) technology can be used to suppress in-vehicle noise to improve call clarity.

In order to increase driving safety, voice recognition (VR) technology allows the driver to manually operate the electronic device, the pre-recorded voice command, the driver can make a call, answer the phone, etc.; speech synthesis (VS) technology uses a voice signal to alert the driver The operations that need to be performed or some notable information; the voice management layer manages various voice algorithm modules, such as configuring algorithm parameters and buffer data input and output; the human interface layer (MMI Interface) is externally and the Bluetooth subsystem. Communication interface, this communication uses a special protocol, the external MMI host sends commands to the human-machine interface layer through the protocol, the human-machine interface layer interprets the received commands, and then controls the application layer to perform corresponding actions, and the application layer also The data, status and events of the Bluetooth subsystem are sent to the human interface layer, and the human interface layer sends commands to the external MMI host through the protocol. The application layer accepts all events from the human interface layer and the Bluetooth management layer, processes the received events under certain application conditions, and performs corresponding actions to control the human interface layer, Bluetooth management layer and voice management layer. Work in an orderly manner. The application layer is also responsible for handling exceptions, such as the human interface layer receiving error commands, Bluetooth link abnormal disconnection, authentication failure, etc., through appropriate error handling mechanisms to improve system reliability.

The Bluetooth Application Specification specifies some of the technical standards that Bluetooth devices should meet in order to implement an application. Including provisions for application characteristics, definition of application models, configuration of core protocol stacks, protocols defined for implementation of specific applications on core protocols, and other provisions to meet interoperability requirements, these provisions are divided into mandatory support, Conditional support and optional support. It can be seen from the above structure that multiple application specifications can be supported on the same hardware platform by properly configuring each software function module.
Bluetooth hands-free application specification

The Bluetooth hands-free application specification specifies the process of establishing a Bluetooth data and voice connection between a voice gateway and a hands-free device, and how the hands-free device remotely controls the handset based on the connection and accesses the external network for telephony functions.

The voice gateway in this specification is a bridge for hands-free devices to communicate with the outside world. A general voice gateway refers to a GSM or CDMA mobile phone. Through the voice gateway, the hands-free device can use the telephone service provided by the cellular network. After the hands-free device is connected to the voice gateway, the user can make a hands-free call. Here, the hands-free device generally refers to a Bluetooth in-vehicle system. This specification specifies the features that voice gateways and hands-free devices should support. Voice gateways and hands-free devices must support the establishment of service level connections (SLCs). All control signaling is passed on the SLC.


image 3

A typical application of the Bluetooth hands-free application specification is shown in Figure 3. At this time, the Bluetooth car system is a hands-free device, and the mobile phone acts as a voice gateway. The mobile phone in the figure has a Bluetooth function, and needs to be bound to the Bluetooth car system before the first use. After the user enters the correct password, the binding is successful. After that, the bound Bluetooth mobile phone is close to the Bluetooth car system about 10 meters, and the two can automatically establish a connection.


Figure 4

Figure 4 shows the configuration of the protocol stack. The hands-free specification is implemented by transmitting dedicated control commands on RFCOMM, which are a subset of the generic AT instruction set, with the addition of associated dedicated instructions. These AT commands can be used to display, answer, reject, hang up, transmit dual tone multi-frequency code (DTMF), remote volume control, control echo cancellation, noise suppression and speech recognition.

In addition, in order to meet the interoperability requirements, the hands-free specification also made some provisions on the connection process and security management of the Bluetooth channel. When the hands-free specification specifies that the voice gateway and the hands-free device are bound, the voice gateway should be the initiator. The voice gateway initiates device query, and the hands-free device performs query scanning. The device can query the voice gateway to find the hands-free device within the valid range. Then, the voice gateway establishes a connection with the hands-free device through a page (Page), and initiates a binding process. In this case, the voice gateway can only act as the master device and the hands-free device as the slave device. After the binding is completed, the connection request can be initiated by either the hands-free device or the voice gateway, and there is no fixed master-slave device.

The hands-free specification itself does not impose mandatory requirements on the security of Bluetooth links. For hands-free applications, authentication and encryption should be performed to ensure the security of the call content. Authentication is performed by the device side to verify the legitimacy of the other device by checking the link key. The authentication can be performed in both directions. Encryption is to process the ACL data between the two devices according to a certain algorithm to generate encrypted data. The original data can be recovered from the encrypted data only after knowing the correct decryption key. The encryption key and decryption key used for encryption are generated according to a fixed algorithm based on the link key, so the link key is the key to system security. In a Bluetooth hands-free application, the link key is generated by a voice gateway and a hands-free device through a pairing process. In this process, the user needs to enter the correct PIN code on the voice gateway side to create a link key.

In addition, the Bluetooth in-vehicle system can provide multiple different application services. When other Bluetooth devices access these services, the Bluetooth in-vehicle system also needs to control access to its different services. This control is implemented by the authorization process. For voice gateways, access to hands-free applications in Bluetooth in-vehicle systems should generally be granted.

Bluetooth Headset Application Specification

Bluetooth headsets are also used as hands-free in Bluetooth in-vehicle systems. The Bluetooth headset application specification is considered to be a simplified version of the hands-free application specification and can only perform limited functions. Since the user interface of the headset is very simple, usually a small number of buttons and LED display, the headset application protocol only defines simple functions, including data link, voice link establishment and removal, and optional between the voice gateway and the in-vehicle system. Volume synchronization control support, etc. Through the headset application specification, basic hands-free calling operations such as answering calls, hanging up calls, voice dialing, etc. can be performed.

The Bluetooth headset application specification is also implemented by adding a headset control entity and an application layer to the core protocol.

Summary of this article

In the design and implementation of the Bluetooth in-vehicle system, a reasonable hardware design is needed to meet the requirements of temperature range, reliability and price of the automotive application. At the same time, a reasonable software solution is needed to meet the strict Bluetooth interoperability. Sexual requirements, good functional scalability, and good real-time and high reliability. The Bluetooth in-vehicle system needs to be applied on a large scale. In terms of technology, it is also necessary to solve the standardization problem of application specifications, and it is necessary to have complete testing means to ensure good interoperability of devices between different vendors. Currently, hands-free application specifications and headset application specifications are widely supported on Bluetooth in-vehicle systems and Bluetooth phones.

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