BC5-MM application circuit design in car Bluetooth equipment

　　Bluetooth is a wireless technology for voice and data communication within a short distance. Compared with other wireless protocols, the Bluetooth standard is more complete and defines the implementation specifications of various applications. It ensures interoperability between devices, and its application and development are becoming more and more extensive. Here, Bluecore5-MM (BC5-MM for short) is used to design a car Bluetooth device. First, the device characteristics of BC5-MM, the hardware structure of the Bluetooth subsystem of the car Bluetooth device, and the circuit design of the RF part are introduced. Then, the development and implementation of BC5-MM specific software are introduced in detail from the aspects of single-chip development mode, communication implementation mechanism and UART communication.

　　The in-vehicle Bluetooth device adopts a dual-processor structure design of BC5-MM + 32-bit external embedded processor to realize functions such as USB music playback, Bluetooth hands-free, Bluetooth streaming music playback, and phone book synchronization download. The key features of BC5-MM are as follows: (1) It integrates the RF and baseband control of the Bluetooth system, a 16-bit MCU with a RISC structure, and a Kalimba DSP with a performance of up to 64 MI/s in a single chip. The MCU implements the Bluetooth protocol stack and application software, and the DSP implements audio and voice processing as well as the encoding and decoding of streaming music files. (2) It has an independent audio input and output system, a built-in microphone bias circuit, and a certain audio output driving capability. (3) The wireless transmission power reaches +8 dBm and the receiving sensitivity is -90 dBm. No external amplifier is required. Only a printed circuit board antenna is required to meet the Bluetooth RF specification Class 2 requirements, and the communication distance is up to 10 m. (4) It has multiple interfaces such as USB, I2C, SPI, UART, etc., and can be connected to an external host controller to realize complex applications. BC5-MM and the main processor communicate via UART, and the SPI interface is used for BC5-MM program debugging, downloading, and key PSKEY attribute settings. (5) The built-in LD0 (low-dropout linear regulator) can generate 1.8 V and 1.5 V voltages from an external 3.3 V voltage. 1.8 V is mainly used to power the I/O port, and 1.5 V is mainly used to power the core, thereby simplifying the circuit design. Figure 2 shows the Bluetooth RF subsystem circuit design. Since BC5-MM has an internal integrated Bluetooth Modem, only an external filter and antenna are required to implement the RF subsystem circuit design. Here, the filter is DBF81F104, which is a device designed by SOSHIN for Bluetooth communication reliability. A 5 ppm high-performance RF crystal oscillator is selected, and the key PSKEY-Crystal Trim value of BC5-MM is adjusted to control the initial carrier offset within a very small range to ensure that the Bluetooth mobile phone can search for the device and pair it.

　　Single chip development method

　　There are two ways to develop BC5-MM: one is to use BC5-MM as a Bluetooth baseband transmission device, and the protocol stack and application software are all implemented by an external main processor; the other is to use BC5-MM as a Bluetooth single chip, implement the protocol stack and some application software, and complete related applications through communication with an external main processor. This design uses the latter development method. The software development of BC5-MM is carried out on the Roadtunes-SDK provided by CSR. The Roadtunes-SDK provides a complete protocol stack and profile library, a software solution for in-vehicle applications, and a virtual machine VM for single-chip embedded development. Software is developed based on this solution, and its applications include hands-free calls, signal strength, battery power and SMS reminders, streaming music playback and remote control, reading TAGID3 of Bluetooth player music, incoming call ringtone reminder, downloading the phone book in the Bluetooth mobile phone, automatic connection, etc. The following discusses the relevant mechanism principles and development from the aspects of communication implementation mechanism and UART communication.

　　UART Communication

　　Due to the limited resources of the BC5-MM processor, simple applications can be implemented, but complex applications require the help of an external processor. Here, BC5-MM + external main processor is selected. This architecture uses UART communication to realize the interaction between processors and comprehensively complete related applications. The UART communication protocol includes two parts: the underlying transmission and the application protocol. Among them, the transmission protocol defines the frame format and selects the appropriate verification mechanism and error handling mechanism. The frame format is: frame header (0x55+0Xaa) + id (indicates the meaning of the frame) + dlc (data field length) + data (data field) + checksum (16-bit arithmetic checksum). According to the frame format, the UART driver combines frames to send and parses received data frames. The combined frame is relatively simple, and the parsed frame defines the corresponding state according to the frame structure and parses the ID and data with the concept of the state machine.

　　The application protocol first defines the corresponding signals according to the specific application, and then combines and encapsulates each signal in the frame. The state quantity and control quantity are defined in the form of bit signals to control the frame length. The formulation of the application protocol is closely related to the specific application, and the corresponding interaction method needs to be defined for the specific application. The Bluetooth subsystem structure design and BC5-MM device characteristics are introduced. The BC5-MM software design characteristics and implementation methods are explained from the aspects of single-chip development mode, communication implementation mechanism and UART communication, so as to provide design reference and reference for engineers who use BC5-MM development.