Design of a non-chip mini speaker based on AU7845 single chip

With the popularization of computers, networks and digital products, a large number of storage cards and USB flash drives make it easy for people to obtain songs in MP3/WMA format through the Internet; a product that specifically plays this type of music, the inorganic MP3 mini player , came into being, which maximizes people's demand for enjoying music anytime and anywhere. For this type of inorganic market application, Mountain View Integrated Circuit Company has developed the AU7845 series chips. The functions of this chip include: support for USB flash drives and SD/MMC cards, support for MP3/WMA dual decoding; built-in RTC module, support for clock and alarm functions, support for independent power supply of RTC module; built-in 4-channel 10-bit ADC; the chip has a built-in 64KB OTP ROM and supports external flash for online firmware upgrade; built-in high-performance audio DAC to achieve high-quality audio output; built-in 11 sound effects functions to facilitate different styles of hobbies; built-in enhanced 8051 core MCU, which is convenient for customers to carry out secondary development through the application layer of the software; built-in IR decoding, support NEC, TOSHIBA and SANYO format codes.

The following figure is the internal block diagram of the mini speaker (BOOMBOX) product designed with AU7845. It is a product designed by Mountain View for a domestic brand BOOMBOX. The entire solution adopts the AU7845 single-chip design to control the work of all peripheral modules. Since the chip uses the 51 core, the software is easy to use, the hardware design is simple, and the cost is low. The whole machine realizes the following functions: support MP3, WMA decoding; through IPOD DOCKING and IPOD decoding chips, all IPOD controls can be realized, including IPOD video decoding; support FM radio reception; support two completely independent alarm clocks with snooze function, support all sound sources and "BB sound" as alarm sound; support external music input; support all sound source electronic volume and EQ control; segment code LCD display. All operations and status are clear at a glance, and support remote control. The whole machine is cost-effective and has passed relevant EMC and other certifications. The design of each functional module is as follows:

> The decoding part adopts the Mountain View main chip AU7845 design, which is a solution with the highest cost performance among similar chips on the market. This solution has low power consumption and no heat generation; the MP3 playback sound quality can reach more than 65dB in terms of signal-to-noise ratio; the frequency response is flat; it supports both MP3 and WMA file formats; and has built-in USB and SD (MMC) interfaces.

> The alarm clock uses a built-in RTC and is powered by a rechargeable button battery. It can keep the clock running normally when the power is off. It uses a completely independent dual alarm design. Each alarm can independently set the ringing time, ringing sound source, and snooze time. This part is the focus of the design. The customer is most concerned about the function of this part. The function seems to be relatively simple at first, which makes it easy to ignore at the beginning of the design, resulting in very troublesome modifications later. It is mainly reflected in the fact that the system has five playback modes and standby mode: "USB, SD, FM, IPOD, AUX". The sound source of the ringing sound also has "BB sound (this solution uses software to simulate BB sound)" in addition to the five playback modes. In the selection of sound sources in the playback mode and standby mode, there are many different combinations that need to be processed. In addition, when the ringing sound, the current state is processed, the ringing sound is controlled from small to large, the maximum volume is limited, the response to the key during the ringing process, the conflict of dual alarm time settings, including the snooze time (snooze) conflict processing, and finally the display content processing.

> IPOD part, currently the common practice is to use an independent chip to be responsible for the communication with IPOD and decoding chip, which will increase the interconnection between chips and complicate the design. However, our solution only needs an IPOD base and an IPOD video decoding IC, and no additional chips are required. AU7845 is fully used as a bridge between IPOD and decoding IC, which reduces chip costs and makes the system more reliable. The main difficulty lies in the data reception of IPOD. Different versions of IPOD send data in inconsistent formats, especially the new version of IPOD sends data continuously and uninterruptedly. In this process, it is necessary to pay attention to giving priority to receiving IPOD data. If the chip handles other events, data loss is likely to occur, which makes the verification fail in the design.

> The radio part uses the SI4702 chip, which is well-known in the industry. When used with the AU7845 chip master, the AU7845 needs to be configured to work in down-frequency mode through software to reduce interference to FM, improve FM reception sensitivity, and also reduce the EMI radiation of the product. This solution uses 4-frequency division, which can take into account both system response speed and EMI radiation reduction.

> For the power supply part, due to the need to meet green environmental protection requirements, the power consumption of the whole machine is less than 1 watt. A linear transformer cannot be used for power supply. A switching power supply with a low quiescent current LDO (linear regulator) is required. The power supply of the corresponding module is disconnected in standby mode, and only the power supply of the chip, IPOD socket and display part is retained. The power supply of the IPOD socket is retained mainly to charge the IPOD in standby mode. At the same time, certain processing is also required in the software. Since the user may turn on the IPOD when inserting it, it is necessary to send a shutdown command to the IPOD socket at regular intervals in standby mode to put the IPOD inserted in this mode in shutdown charging mode.

> The button part uses the built-in ADC for button scanning to reduce button wiring. The built-in ADC module has a resolution of 10Bit, high precision, large distinction between buttons, and high reliability of button reading. At the same time, the chip has built-in IR hardware decoding to realize remote control operation.

> The electronic sound effect IC uses the commonly used PT2314 on the market, supports 4-way audio source input, and realizes volume and sound effect control of all audio sources through AU7845 I2C control.

> The display is a segment code blue-bottom silver LCD, with a universal HT1621 LCD driver IC. The display data is detailed, all states and operations are displayed accordingly, and the human-machine interface is friendly.

> Using AME's dedicated reset chip AME8500 makes the system more stable and can be reset reliably even with rapid power on and off.

Figure: Block diagram of the mini speaker (BOOMBOX) system designed with AU7845.

Conclusion

From the above design, we can see that the coreless MP3 U disk player is simple in design and has no active mechanical structure, so the system is much more stable than cassette players or CD players. The product is easy to produce, low in cost, and very easy to obtain sound sources, which greatly reduces the cost of people enjoying music; the coreless MP3 U disk player has replaced the cassette player in terms of function and performance, causing the cassette player to gradually withdraw from the market. As for CD players, they have also faded out of the market due to difficulties in obtaining and storing sound sources. In addition, the sound quality of the coreless MP3 mini player designed with Mountain View chip can meet the needs of popular users. Due to the coreless design, it can be small, smart, beautiful and exquisite, which can better meet the pursuit of modern people's pursuit of high-quality lifestyle. The coreless MP3 U disk player can become the mainstream trend of mini audio development.