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USB interface the application of the serial communication circuit

Published on Oct 28 2010 // Micrcontroller circuits

USB interface the application of the serial  communication circuit

Abstract: USB interface, serial communication applications, discuss the USB interface, RS-232 interface with the conversion, and simplified design of modem USB interface, and gives the actual product in the drawing.

Keywords: USB RS-232 communications UniModem

1 USB Interface Overview

USB or Universal Serial Bus, can hot swap, daisy chain up to 127 devices can be connected by bus to provide power, and error detection, error correction to ensure correct data transmission.

USB in the PC, application, PC’s operating system needs to support USB protocols, this time on a PC, the software consists of two parts: USB initialization software and operating parts. The former structure of the peripherals and define a unique ID, which is called the enumeration process to obtain a description of each peripheral table.

USB host-side software other hierarchical classes for a variety of peripherals, such as large memory, peripherals, communications, peripherals, audio peripherals, human interface peripherals.

USB differential signal transmission cable, all communication from the root hub initialization, reproduction by an external hub, the connection is point to point, with half-duplex. USB data lines no clock signal, so as asynchronous communication between nodes on the bus all the peripherals rated communication rate 12MHz, peripherals by way of over-sampling the received signal from the bus. The receiver depends on the signal changes regularly to maintain synchronization with the transmitter, USB specification does not return to zero by flipping the book to achieve its data lines to send code.

USB to RS-232 converter

RS-232 is a popular interface. In the MS-DOS, four serial interface known as COM1, COM2, COM3 and COM4, and about most of the windows application can have up to four peripherals, but if the user wants to expand more peripheral, it must be with plug-in serial card or an external switch box implementation.

RS-232 point to point connection, a serial port can only connect a peripheral. The USB is a multi-point, high-speed connection, the use of the connection hub to achieve more. The basic part of the USB interface, serial interface engine SIE, SIE from the USB transceiver to receive data bits into significant byte sent to SIE interface; the other hand, SIE interface, can also receive bytes into serial bits to the USB bus .

Figure 1 is a schematic converter plus serial peripherals. Converter and peripherals in the receiver side have a microcontroller, in fact, the two can be combined microcontroller, MCS-51 series can be I2C bus controller or micro-controllers.

Figure 2 to use an external USB connection diagram of the micro-controller, general-purpose micro-controller selects MCS-51, USB peripheral chip is Philips’s PDIUSBD11, the chip with an I2C interface, the SIE interface includes an interrupt and a large register, with in on the I2C bus access. When a packet arrives, PDIUSBD11 will break them down, turn the microcontroller and the interrupt handler, the first interrupt status register read PDIUSBD11, and then read the other registers in order to obtain the data packet type and packet in the data. After each send a packet will generate an interrupt request, interrupt handler control sequence to monitor and detect any errors.

3 with a USB connection 56kbps modem design

USB protocol includes a communications class, defines a structure to cover all telecommunications services for medium-speed network services and equipment. Thus, peripherals can identify itself as a communications class peripherals.

3.1 The structure of a traditional modem

Conventional modem generally include:

* Micro-controller: the decoding of control (AT command set)

• Digital signal processor (DSP): the signal modulation and demodulation

• Analog modulation and demodulation circuit

• Data access management circuit

* Isolation: the isolation of high voltage or high current telephone line

Click to enlarge

3.2 simplify the design

PC serial port as the maximum rate of 115.2kbps, so that the structure in Figure 3 to form a speed bottleneck. RS-232 system includes two serial signal path, and its opposite, respectively, for the transmission of commands and data, and command and status must be intertwined with the data; and USB support for the separation of command and data channels and to allow independent status report .

In the Windows operating system, the communication layer UniModem driver (Figure 4) can be achieved modem command set. Microsoft VCOMM port driver software to re-locate the original output for the serial port will be directly passed to Windows Peripheral Driver Module (WDM-Windows Device Driver Model) class driver, the WDM class driver outside the output to USB set. The reverse order using USB peripherals can be sent to the Uni-modem input drive and eventually returned to the application.

In addition, because the host PC has enough processing power, in dealing with user application at the same time, can achieve real-time signal processing, and Intel retained in a processor MMX instruction set. The instruction set has the advantages of DSP algorithms, is also through “soft” treatment to implement, and relative to the physical DSP chip, the instruction set of signal processing algorithms can be easily upgraded.

Through the “soft treatment” only to retain the traditional modem modulation level, signal isolation, and a data access manager. Figure 5 shows a simplified design with a transformer for isolation.

This article discusses the technology in USB interface, serial communication applications, by adding a microcontroller peripherals, and to use several Windows support communications, the traditional USB interface to simplify the modem design. The paper gives a simplified design of an actual modem, using LabView software and the VB language can be simulated on the modem