This paper designs a Bluetooth audio gateway (AG) system based on S3C4480X, which can realize a point-to-point connection with a Bluetooth headset to establish an audio connection
And realize the pairing and voice data transmission between the two.
1 Bluetooth architecture
The Bluetooth wireless communication protocol standard includes two parts: core protocol and application framework. The purpose of the Bluetooth protocol standard is to be able to operate with each other in compliance with the normative application. In order to achieve the operation with each other, the corresponding application program on the remote device must run on the same protocol stack. The architecture of the Bluetooth protocol can be divided into a physical layer, a data link layer, an intermediate protocol layer, and an application layer. Figure 1 shows a block diagram of the system composition of the Bluetooth system protocol stack. The core protocol of Bluetooth includes 4 parts, namely baseband, link management, logical link control and adaptation protocol (L2CAP), service search protocol (SDP), etc. Among them, L2CAP is responsible for establishing logical link connections, providing protocol multiplexing, segmentation and reassembly of data packets between the upper and lower layers. SDP can provide service search and service browsing functions, and is responsible for querying and accessing services between Bluetooth devices.
The Bluetooth host control interface (HCl) can provide a command interface for the baseband controller, connection controller, hardware status, and control registers, and can provide a general interface between the host and baseband of the Bluetooth device. The protocol above the HCI layer runs on the host, while the following protocol is completed by the Bluetooth host controller hardware. This not only ensures the stability of the underlying protocol transmission, but also supports the scalability of the upper-layer application protocol.
2 System hardware structure and design
This article selects Samsung's 32-bit core ARM7TDMIS3C44BOX as the processor, and builds a system hardware platform through Bluetooth module, audio codec (CODEC), Ethernet interface, etc. This system is based on the Bluetooth BlueStack protocol stack and can realize all the functions of Bluetooth. The Bluetooth audio gateway system is based on BlueStack. S3C44BOX and RTL8019 are used to develop 10Mbps Ethernet interface, which can make the system connect to Ethernet and operate through Ethernet. Figure 2 is a block diagram of its system configuration.
2.1 Structural features of the Bluetooth module ROKl01008
This system selects ROKl0l008 module of ERICSSON company. The Bluetooth module has high integration and low power consumption. It can provide functions up to the HCI layer of the control interface. It has three host control interfaces of UART, I2C and PCMCIA, which can be easily communicated with the host. Bluetooth voice and data transmission. ROKl008 supports the Bluetooth specification v1.OB. The maximum input level of the receiver is +13 dBm, and the dynamic range is wide. It can ensure reliable communication of the transmitter. Its output power can reach Class2, and the maximum data flow of the UART is 460 kbps. At the same time ROKl01008 can support voice and data transmission functions, can achieve point-to-point connection.
The composition of ROKl01008 module is shown in Figure 3. The module has five modules: wireless transceiver, baseband controller, FLASH Memory, power management and clock. The wireless transceiver module consists of a Bluetooth transceiver and an antenna. Its function is to provide wireless channels for gateways and terminal devices. The baseband controller can complete the frequency synthesis, at the same time complete the conversion and filtering of the data bit signal of the baseband module to the radio signal, and send the radio signal with a certain power and frequency hopping frequency. The working frequency band of ROKOl008 is between 2.4 GHz ～ 2.5 GHz ISM, there are 79 channels, each channel is 1 MHz.
2.2 Hardware structure of Bluetooth audio gateway
The hardware of the Bluetooth audio gateway includes a Bluetooth chip and an audio codec (CODEC). The embedded Bluetooth audio gateway circuit is mainly used to provide audio input / output interfaces and establish audio connections. Figure 4 shows the hardware circuit of the Bluetooth audio gateway. The Bluetooth module voltage is +3.3 V. Use the UART of the Bluetooth module to connect with the UART of the S3C44BOX of the one-chip computer. Can form a communication channel. The S3C44BOX can also communicate and interoperate with the PC through the UART interface. CODEC is responsible for audio input and output.
3 Software design of Bluetooth audio gateway
The realization of the Bluetooth audio gateway is mainly based on the Bluestack protocol stack. The audio gateway software is divided into three layers, namely the BlueStack protocol stack, the AG client program and the connection manager.
BlueStack is the Bluetooth protocol software stack of Cambridge Consulting Co., Ltd., which can be implemented in C language. It follows the layered model proposed by the Bluetooth Special Interest Group and can support logical link control and adapt the protocol to use the serial port emulation protocol or binary telephone control protocol and use it as the standard layer of the transmission protocol. Based on the BlueStack protocol stack, all functions of Bluetooth can be realized.
The implementation of the entire BlueStack protocol stack is message-driven, and message queues are used to pass between layers. In the embedded Bluetooth audio gateway application, there are generally two types of messages involved: CM messages and BlueStack messages. CM messages can be passed between the client and the connection manager, while BlueStack messages are passed between the BlueStack protocol stack. The primitive types of BlueStack are DM PRIM, RFC_PRIM, SDP_PRIM and L2CAP_PRIM. In AG applications, the first three are generally used. The BlueStack message format is DM (RFC, SDC or SDS) _primitive_extension.
Two tasks are usually defined in the AG application, one is the CM task, the task number and queue number are O; the other is the AG task, the task number and queue number are 1. The AG client submits the message to the CM to the queue 1, and the CM submits the message to the queue 0 to the AG.
The application program should provide a communication interface between ARM and Bluetooth, mainly to establish two L2CAP channels (control channel and data channel) between the hosts. During initialization, the Bluetooth module should be woken up and write data (in the form of a queue), and then establish a connection by request. When the AG client sends a request signal, if the CM responds, the connection can be established. In the process of data sending and receiving, the data should be encapsulated / unsealed and classified. Figure 5 shows the software flow chart of the Bluetooth data receiving and sending process.
4 Conclusion
The S3C44BOX kernel is powerful and can embed the operating system, so an embedded operating system platform can be built through the microcontroller system. Based on the ARM platform, this paper designs a Bluetooth audio gateway based on S3C44BOX. After testing, the gateway can achieve the access of Bluetooth headsets, and successfully achieve pairing and voice data transmission. The Bluetooth headset and the audio gateway need to establish a pairing connection before interoperating, that is to say, this connection is achieved through Bluetooth authentication and binding. Once this relationship is established, it is not necessary to perform this operation again.
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