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Introduction: Due to the rapid spread of mobile phones and notebooks, more mobile phone manufacturers will use the embedded AV specification Bluetooth as the standard configuration, and notebook manufacturers will also have Bluetooth with EDR specifications as the standard configuration. The application space of Bluetooth products based on AG is more and more widely used. . The three-in-one solution of Bluetooth stereo headsets with FM and MP3 will be a big growth point in the market. This article briefly describes the system design of the World Health System (ESPL) based on the CSR chip FM Bluetooth stereo headset and MP3.
As shown in Figure 1, the whole system can be designed independently or integrated. According to customer needs, it can form: 1, Bluetooth stereo headset (or bring electric display); 2, headset with FM (or bring electric display); 3, headset and SD Card MP3 player function; 4, headphone, FM and MP3, four product lines, rich product line.
system structure
Figure 1 System structure
Shijian offers three Bluetooth MP3 solutions based on the high performance of SIGMATEL 35/36xx/PHILIPS PNX0102+BC4ROM and the low-cost solution based on BC3MM EXT mainly introduced in this paper.
The system uses CSR BC3MM as the main chip to implement Bluetooth function and support Bluetooth 1.2. BC3MM-EXT is highly integrated with RF, BB, on-chip 1v8 power supply, 16-bit audio CODEC (supports stereo) and supports external CODEC to provide I2S and SPDIF interfaces, and supports 8M external FLASH. In addition, KAILMA DSP is responsible for voice communication quality such as echo cancellation and SBC decoding. SD card and OTI CHIP are optional circuits for MP3 functions. In view of the stereo STREAMING and card reading channels, the SD card and the OTI CHIP are separated by a MUX, and the logic relationship should be fully considered in the circuit design. FM and OLED are also available as an optional part of the product line and are connected to the system via the I2C bus. The PHILIPS-based TEA5761 design maximizes peripheral components. Shijian offers two options, the FM module approach and the CHIP ON BOARD design. Selecting the mainstream display technology of OLED display, OLED is different from traditional LCD display mode, which can be thinner, lighter, faster, less energy-consuming, active, high-definition, low-temperature and shock-resistant, flexible and environmentally friendly. Suitable for the design of portable products.
ESPL provides fully functional integration software, including Profile: 1, A2DP (AVRCP); 2, Headset; 3, Handfree. The ESPL Reference Design MMI Configuration reserves a variety of USER Interface define for the user's discretion.
Power management
In view of the frequency range of 20Hz~20kHz heard by the human ear, the open frequency band of 2.4GHz in the working band of Bluetooth, the strict requirements of the earphone application for controlling noise, the system has high noise suppression capability requirements for the power chip. Although the 1V8 linear power supply is integrated on the chip, portable devices usually use power consumption as a hard target. It is recommended to use an external DC-DC, such as Microchip's TC1303B, with a quiescent current of only 65 UA. Not only that, the TC1303B also provides an LDO output of 2.5V@300MA to power the CSR BC3MM EXT FLASH. If you pay attention to the characteristics in the design, it is recommended to choose 1V8 FLASH. Low power consumption, such as price, 3V FLASH available. In the actual product test, using TC1303B, 3V3FLASH, when the system enters SLEEP mode, the power consumption can reach 0.8MA, and the average standby current is 1MA. If you switch to TC1303A, the power consumption can reach 1.3MA when the system enters SLEEP mode. If 1V8 FLAH is used with XCS9216 (only 1V8 is output, no need to provide 3V power for FLASH separately), when the system enters SLEEP mode, the power consumption is only 0.1MA, and the average standby current is only 0.3-0.4MA. This shows the importance of circuit design and device selection for the product.
The battery charge management IC uses Microchip MCP73832 or pin-compatible LTC4054. The actual test comparison performance has little effect on the performance parameters of Bluetooth products. MCP73832 is recommended because of its high cost performance. The first leg of 73832 can directly control the charging status indication, which can save the PIO resources of the Bluetooth chip. Here, it is worth noting that if the design of the MP3 function is used, the system cannot be reset and must be able to access the MENORY while using USB. Charging, when designing the circuit, a CMOS tube and a diode should be separated between the battery and the load of the charging IC to ensure that the battery is still charged when the battery is very small and in the state of being turned on. This is related to the state of charge of the battery, requiring more current versus relatively low current state of charge when the system is not entering the low power mode. If the system is started before then, the battery will also supply power to the load, which will prevent the chip from entering the fast charge mode. In fact, if it is only for the design of stereo headphones, you can save these devices to replace the RC reset circuit to save costs. The internal FIRMWARE design of the underlying CSR chip, after the reset state, the system detects the battery level to determine the state of charge. If it is below a certain threshold voltage such as 2.3V, the system will not be activated. The system is not started until the system considers that the state of charge has reached the fast mode.
Audio circuit
The MIC bias circuit uses the XCS6219A272 LDO. Due to its outstanding noise suppression capability (>75dB@1kHz, >70dB@10kHz) and small package, the software can control the power supply to turn on the power only when the call is in progress to reduce power. Consumption. This part of the power supply is not good or the MIC device selection is not good will bring noise. CSR recommends a sensitivity of -40DB to -60DB, usually -54DB, with internal noise reduction, but usually this far Can not meet the design specifications requirements. There is often noise in the call state. If the line and sensitivity are OK, there is still noise, and the factor that affects it is the RF part circuit, or the gain of the MIC is lowered to achieve the desired quality. In addition, the inductance of the MIC output that filters out GSM noise should not be omitted. Can effectively eliminate the call noise. When designing the MIC circuit, it is worth noting that some MIC-L/R channels are connected in the customer stereo design. Actually, only one MIC channel needs to be connected, and there is no problem with the MIC. This is because the FIRMWARE internal PORT Configuration can be solved by software modification or hardware modification of the connection port. Figure 2 below illustrates the process of establishing a channel.
Figure 2 call status audio path
When the call is established, the telephone source or TONES is output to the SPK through the green channel PORT0 in the above picture. Because of the stereo earplugs, we need both ears to hear the other party's voice at the same time. It is very simple to implement, the software only needs to simultaneously PORT to 0 and 1 to achieve a true false stereo sound output.
Configure PCM:
PcmRateAndRoute(0, PCM_NO_SYNC, (uint32) 8000, (uint32) 8000, VM_PCM_INTERNAL_A_AND_B)
The MIC transmits the sound to the remote user via the red line. Here we can see that the MIC is connected to PCM PORT0, physical channel A. Therefore, the design hardware and software of the system must be matched, otherwise there will be no sound output from the MIC. Here, it is worth mentioning that the BC3MM does not use the MIC to detect the noise around the environment, automatically adjust the Speaker output gain, so that the headset automatically maintains the clearest and appropriate output. The current design phase is to cure the gain of the MIC, and the gain of the SPK is manually adjusted.
As shown in Figure 3 below, STREAMING stereo audio path through Bluetooth. To appreciate music, the quality of the SCO link is too bad. Therefore, the ACL link is used to send data packets, but the ACL link is designed for bursty data, not audio, so he will delay the packet transmission, and can decode the MP3 by performing packet buffering on the receiver. The device feeds a steady stream of information. This involves the application having to ensure that even if there is an error, the compression method used must allow all information to pass through the channel. Although theoretically 733.2 KBPS, it actually requires additional bandwidth. Therefore, the encoding of MP3 must be compressed to be less than the maximum value of this theory, and the existing design adopts the SBC method. DSP decodes SBC, DAC restores digital signal to analog signal, the process sound quality has great influence, with DSP fast and large amount of data computing power and its multiple algorithm support, as well as built-in 16 for DAC conversion, in sound quality and There is a considerable improvement in noise C processing power, SBC decoding supports Max 200kbps, 48.000kHz.
The path is configured as follows:
PcmRateAndRoute(0, PCM_NO_SYNC, SampleRate, SampleRate, VM_PCM_INTERNAL_A_)
PcmRateAndRoute(1, PCM_NO_SYNC, SampleRate, SampleRate, VM_PCM_INTERNAL_B_)
Figure 3 Bluetooth stereo audio path
The headphone driver amplifier uses TI's TPA6112A2, which provides 150 MW of stereo output. It has three main advantages for Bluetooth applications, first, support for fully differential inputs, second, built-in POP REDUCE circuitry, and third, internal MID-RAIL circuitry. Pay attention to the processing of power supply filtering in the design. Excessive capacitance will affect the delay of the startup, the value of the feedback power group and the value of the parallel capacitor. If the noise is too large and directly affects the noise, it is usually recommended to 1PF or not.
It is worth noting that the bias circuit of the MIC usually considers a PIO to control the power supply to save power. The underlying software of the CSR has already made the PIO output high during the voice call and call (both SCO/ACL link establishment).
It is used to control the bias voltage, but at the same time, some circuit designs expect this voltage to supply power to the audio amplifier circuit at the same time, and save power by controlling the power supply. However, there is a problem here. After the SCO/ACL link is established, the user can answer the key or respond to the song by pressing the button. There will be noise. This is because of the control of the underlying FIRMWARE of the CSR, the link will have a process of disconnecting the channel before the application is established, and the applied software cannot control the timing. This process will cause a sudden change in the power supply. This mutation is often easy to bring transient noise, and the user can experience the instantaneous noise of answering the phone or playing the song button. You can also use the oscilloscope to capture the process. There are several possible methods. First, use the software to mute or delay the 1-2S response after the current state button action, to remove the transient noise during this period, but it is difficult to do so. To ensure the accuracy of the software setting time, it often causes some wrong actions. For example, when the STREAM MP3 is pressed to play the down button, the user may hear the mute--current song (short)--the process of the song. Second, the hardware button delay circuit, the experiment proved feasible, but there are also accuracy problems. Third, use the SHUTDOWN function of the TPA6112A2. The audio amplifier provides an uncontrolled power supply, using the PIO SHUTDOWN pin, which avoids the noise generated by the switching of the power supply. Some users may raise doubts about power consumption. In the SHUTDOWN state, the DATASHEET of the TPA6112A2 usually consumes only 10 UA.
Headphones are used as phones. The function of BUZZER is very necessary. It is precisely because of this feature that many consumers choose Bluetooth headsets, because you can also hear ringtones when you are not in your ear. Although the CSR chip PIO port can simulate high and low pulse changes, but because FIRMWAE sets PIO output priority level is the lowest, the output pulse is often inaccurate, and PIO can not directly simulate the frequency pulse for driving BUZZER, so This part of the design is not like the usual MCU design directly with the GPIO analog driver (such as the phone's BUZZER can do this), but by means of the PIO port interrupt to generate a very low frequency pulse, this pulse is used to control the BUZZER rhythm The frequency required to drive BUZZER is selected as hardware implementation, such as the 555 circuit.
RF circuit
The quality of the antenna design and the degree of matching play a great role in the quality and distance of the call. For customers with higher voice quality requirements, it is recommended to select the MTCL antenna. Both walsin and CSG have good price/performance ratio, while gigaant's antenna provides the best performance. The matching design process is very simple. It can be done by using network analyzer for 10 minutes. The key is to choose the antenna manufacturer and get technical support. .
Using the BC3MM EXT chip CHIP ON BOARD, plus DBF81F106 BALUN, the actual test RF maximum power can be as large as 4DB, fully meet the requirements. Need to understand is not every manufacturer can do it
Such good RF characteristics, which have a lot to do with layout, routing, device selection, power supply design, and FIRMWARE settings, especially for RF parameters. This is why some optimizations can only achieve an upgrade of 0.5DB, and the same optimization can achieve 3 DB improvement in different PCBA and different FIRMWARE environments.
RF testing of Bluetooth products is a must, not only RF testing, Bluetooth address code writing, and crystal adjustment is necessary for product and crystal consistency. Shijian system provides complete hardware and software. The test plan, this part of the content is more, limited to the length, will be introduced in detail.
Conclusion
ESPL not only provides complete Bluetooth stereo headset + MP3 + FM software and hardware solutions, but also provides the establishment of production testing. From the selection of devices to the full support of production, the speed of product launch is greatly accelerated.
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