IP phone design based on VoIP processor and USB interface

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Abstract: This article introduces the Tiger560B processor and the w681511 audio encoder. The design and implementation method of IP phone hardware for USB interface is given.
Keywords: USB; endpoint; codec

1 Introduction With the popularity of VoIP, the quality of IP telephony has improved significantly, and there is basically no difference compared with traditional telephony. In addition, it has the unparalleled price advantage of traditional phones, attracting more and more users to use IP phones for communication. At present, many manufacturers have developed soft IP phone terminals, but it is necessary to use a headset to make calls, which is not in line with the habit of people calling, and long-distance calls are far less than traditional telephones in terms of voice quality and user comfort. It is more difficult for users who are not very skilled in operating computers. So most people are more willing to call on the phone instead of the computer. According to this need, there are some IP telephones connected to the communication interface of the host. Among them, IP phones with USB interface are the most common.

Universal Serial Bus (USB) is a new type of interface technology. It is a serial communication standard developed and planned by companies such as Intel. USB has the following features: (1) has a higher transmission rate. USBl.1 supports both full-speed and low-speed modes. The full-speed rate is 12Mbps and the low-speed rate is 1.5Mbps. In addition to the two speed modes of USBl.1, USB2.0 also increases the high-speed mode up to 480Mbps. (2) USB supports plug and play and hot swap. (3) Two-way data support for the phone is provided. USB supports asynchronous and isochronous data transfers, allowing phones to be integrated with PCs. (4) Has high fidelity audio. Since USB audio information is generated outside the computer, the opportunity for electronic noise to interfere with the sound quality is reduced, thereby making the audio system more fidelity. In addition, USB has the advantages of high reliability, low cost, low power consumption, and easy expansion.

2 device selection
2.1 VoIP Processor Selection
From the cost reduction, simplified design and the need to have a USB interface, we chose the Tiger560B VOIP microprocessor. It offers a low-cost, high-quality IP telephony solution. It has the following features: 1) Realize all required VoIP functions and USB interfaces for VoIP; 2) Built-in support for keypad, LCD, telephone interface and ring; 3) Support for H.323 and SIP standards; No driver required, all drivers are embedded in Windows; 5) Audio features include: PCM interface, support for a variety of commonly used audio decoders: use Microsoft audio USB driver, support 8-bit μ rate codec interface , μ rate to PCM16 conversion, volume control, automatic mute, etc.; 6) Support HID audio device class, can directly use Microsoft audio USB driver, which is convenient for system development.

2.2 Codec selection
W681511 is a single-channel voice CODEC chip launched by Winbond. It is a low-cost and high-quality chipset solution for the VoIP market launched with Tiger560B. Compared with other solutions on the market, it can provide comparison. Good performance and competitive prices. W681511 is mainly used in Internet, integrated service digital networks and telephone related products. The W681511 includes all of the features of the Winbond CODEC voice chip family, offering low power consumption (only 10 microamps and 100 milliamps when the system is in a wait state and a stop state) and a single 4.5 to 5.5 volt supply. In addition, the chip is compatible with the Motorola MC14LC5480 and, in most cases, directly replaces the existing Motorola chip.

3 system implementation
3.1 The overall circuit
IP phone schematic diagram shown in Figure 1. It consists of the Tiger560B and W681511 and their peripherals and interfaces. The main circuits are: voice circuit, LCD display circuit, keyboard circuit.


3.2 peripheral circuit design
LCD module: LCD is used to display information such as phone status, time, master/called number. It is connected via the universal I/O interface of the Tiger560B.
Key circuit: Tiger560B can be directly connected to 4×4 or 4×8 keyboard through PIB port.
Audio circuit: As shown in Figure 2, the W681511 is directly connected to the serial port of the Tiger560B. The serial port contains a data clock (DCLK). One frame synchronous clock (FSC) and two data lines (DIN and DOUT). The FSC is an 8KHZ clock that transmits and receives 8-bit u-law rate audio data in one FSC cycle.


3.3 USB working mode design
The Tiger560B implements a fully compatible USB 1.1 interface that includes a USB controller and a standard 3.3 volt supply that can be directly connected to the USB bus.

USB has four modes of transmission: control transfer, bulk transfer, isochronous transfer, and interrupt transfer. The control transmission is mainly used for the transmission of configuration, status and control information between the host and the device. Any USB device must support control transfer. Interrupt transfers are used for devices that have a small amount of data, are not scheduled to transmit, allow a certain delay, and have a limited service period. Isochronous transfer of devices for large data volumes, high real-time, and allowing for certain errors.

Therefore, the IP phone sets up four endpoints: Control Endpoint 0, Interrupt Transfer Endpoint 5, and Isochronous Transfer Endpoints 6, 7.

USB Endpoint 0 is the only control endpoint on Tiger560B

USB endpoint 5 is used for interrupt transmission with a polling interval of 1 ms, which is used to transfer keyboard key values. Each time a poll occurs, two bytes are polled to the host. The first byte of data is the current state value of the current AUX pin. The second byte is the value of the specified source, and Register 0x18 defines the address of this byte. Setting the 6 and 7 bits of Register 0x18 to 1 enables the PIB to abort status polling. The polling operation does not conflict with any manufacturer commands because polling is done at each USB SOB packet. Byte 1 indicates the state of any interrupt line connected to the AUX pin. Byte 2 is the interrupt status value of the external device.

USB endpoints 6 and 7 are used to transmit 16-bit PCM audio formats. Each USB isochronous transfer may carry 8 samples and 16 bytes of data. The Tiger560B converts 8-bit u-law rates and 16-bit PCM audio encoding. The Tiger560B can use the USB SET_CUR volume command to complete the hardware volume adjustment. Both the microphone and the microphone support silent control.

3.4 Memory Configuration The
Tiger560B supports 64×16 EEPROM. After power-on, while the RESET signal is low, if the EECS pin is high, the Tiger560B will access the EEPROM and read its contents into the register.

The Tiger560B requires an address space of 256. Among them, the internal register address is 0x00 to 0xbf, accessed through the USB endpoint to control the Tiger560B.

4 Conclusion
This article introduces a new VoIP processor Tiger560B and audio codec W681511. Based on this, a prototype of an IP phone was developed. The machine is connected to the computer through a USB interface, and the SIP protocol is implemented on the computer. Combined with the RTP-based quality of service dynamic control strategy, a satisfactory call quality can be achieved.

references:
[1] Wang Ruigang, Li Yan compiled IP telephone terminal equipment - principle, circuit and application. Xi'an University of Electronic Science and Technology Press
[2] Xu Yonghe compiled the design and application of EZ-USB FX series MCU USB peripheral devices. Beijing University of Aeronautics and Astronautics Press
[3] Tigerjet network Inc. Tiger560B Advanced Information
[4]Winbond. W681511 single-channel voiceband codec Advance Data Sheet

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