Design of Bluetooth Headset Based on MBM02 Module

There are more and more Bluetooth modules with highly integrated functions for different applications. Hitachi Maxell's MBM02 is a universal Bluetooth module that integrates RF circuit, baseband circuit, crystal oscillator and unbalanced transformer (Balun). Using MBM02 to develop Bluetooth devices can make the structure more compact and the design more concise.

Hitachi Maxell Module Introduction

The MBM02 module complies with the Bluetooth V1.1 standard, with a maximum transmission power of 2.5mW (4dbm) and an operating voltage of 3±0.3V. The module is a 11.5mm×11.5mm square with a thickness of only 1.8mm, making it extremely suitable for mobile devices such as Bluetooth headsets that have high requirements for size and weight.

In the entire module structure, the BlueCore2-External Bluetooth chip is the core of the module. The chip has its own MCU, integrated RF and baseband control circuits, and provides

It provides a variety of peripheral interfaces, including UART, USB, PCM, 12 PIO, AIO, SPI, etc. It should be noted that the USB and UART ports must be used separately. The PCM interface is used for the transmission of voice signals, the PIO interface is a general I/O port that can be connected to buttons and indicator lights, and the SPI interface is generally used for debugging, which can be used to upgrade the software code in the Flash.

The module has a built-in 16MHz crystal oscillator to provide the working clock for the chip, and a 4M Flash is used to store the code. As can be seen from Figure 1, the module integrates an unbalanced transformer and a bandpass filter, so that the microstrip antenna can be directly connected when the module is used. In addition, the module also has a built-in voltage regulator circuit to provide 1.8V working voltage for Balun and other devices. Therefore, from the perspective of the entire module structure, the hardware development of some simple Bluetooth devices can be completed by expanding the power control circuit and the corresponding peripheral interface circuit.

Bluetooth headset design

Overall structure

The development of Bluetooth headsets using MBM02 mainly includes the following parts: function buttons and status indicators, PCM codec, power supply and monitoring circuits.

Function buttons and status indication circuit

The main consideration is how to use the PIO port of the module. The module opens 8 input/output ports, PIO0~PIO7. The usage in the design is as follows: PIO7~PIO4 are used to expand four function buttons to control answering and hanging up, volume increase, volume decrease, and conversion control between sleep mode and standby mode.

PIO3~PIO2 are connected to LED indicators for status indication, indicating whether the current state is matching mode or connection standby mode.

PIO1 and PIO0 are multiplexed I/O ports, which can control the internal amplifier and can also be used as general I/O ports. Since the communication distance of Bluetooth headsets is generally maintained at 3~5m, there is no need to turn on the internal amplifier, so PIO1 is used as a general I/O port to control the opening and closing of the PCM codec, and PIO0 is reserved.

PCM codec circuit

The PCM codec is directly connected to the Bluetooth module through a 13-bit bidirectional pulse code modulation interface, and the module sends and receives digital voice signals through this interface. The PCM interface is mainly composed of four signals: PCM_SYNC is an 8KHz synchronization signal, PCM_CLK is a continuous clock signal of up to 256KHz, PCM_IN receives the encoded microphone voice signal, and PCM_OUT sends the received digital voice signal from the remote Bluetooth to the codec, which drives the speaker output after decoding. Motorola's MC145483 codec chip is used as the PCM codec here. This is a 13-bit linear pulse code modulation chip that can provide high-quality voice communication, and the peripheral circuit is simple. The gain of the microphone and speaker can be changed by adjusting the corresponding resistance ratio.