USB通信系统设计方案-AET-电子技术应用

USB通信系统设计方案

来源：维库电子

摘要：　　USB（Universal Serial Bus）是一种目前通用的串行总线技术，它是通过PCI总线和PC的内部系统数据线连接，实现数据的传送，具有即插即用的功能，支持热拔插，设备间线缆的长度可达5 m左右。

关键词： PCI USB PC

Abstract：

Key words :

　　1USB通信技术

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　　USB接口信号线有4条线，两条是电源线与地线（Vbus与GND），两条是以差分方式产生的信号线（D+与D-），从而实现了主机系统与USB设备之间的数据通信，见图1。

USB通信数据流

　　图1 USB通信数据流

　　USB总线121技术是基于分组交换方式的总线通信。首先把数据分成若干块，然后在每块数据前添加上同步信号、包标识，后面添加上CRC校验，形成USB封包。USB总线通信使用一种差分的输出驱动器来控制数据信号在USB电缆上的发送，即通过控制D+和D一线从空闲态到相反的逻辑电平，实现源端口的包发送。包发送结束后，D+和D-上的输出驱动器均处于高阻抗状态。

　　2 USB通信系统设计

　　2.1 硬件设计

　　USB通信接口是连接Pc机和汽车尾气检测仪内嵌单片机通信的桥梁，目前采用USB接口芯片来实现PC机与单片机AT89C5 1之间的连接是一种比较好的方法。

　　由于汽车尾气检测系统内嵌单片机是采用串口（TXD、RXD）以RS232方式进行通信的，所以USB芯片与单片机AT89C51之间的连接方式也必须是RS232串口通信方式。因此，本文采用PROLIFIC公司推出的可实现USB接口转RS232串口的转换芯片PL一2303HX。根据汽车尾气检测仪USB通信系统设计要求，USB通信接口电路设计如图2所示。

USB通信接口

　　图2 USB通信接口

　　USB插头与USB芯片间的连接通过USB—D+、USB—D一、GND和VO一3.3这4个端点实现，由于USB芯片PL2303HX支持类RS232接口，含有全双工发送和接收（RXD、TXD）串口，所以，PL2303HX芯片与单片机AT89C51的RS232串口间直接可以用3根线RXD、TXD、GND连接，即USB芯片PL2303HX的RXD、TXD和GND分别与单片机AT89C51的TXD、RXD和GND连接，双方都可发送和接收。

　　2.2 软件设计

　　2.2.1 通信方式

　　本文USB通信系统采用异步半双工通信方式，即通信的某一方发送和接收交替进行。通信的数据格式为每帧10位，包括1位起始位、8位数据位和1位停止位，见图3。

字节传输序列

　　图3 字节传输序列

　　这里设置单片机串行口采用T作模式l，为10位异步通信，片内定时器Tl作为波特率发生器，波特率为2400bps；由串行口控制寄存器SCON的工作模式选择位SM0、SMI，分别设为0和l，从而设置其串行口为模式l；当定时器／计数器Tl控制寄存器TMOD中C／T=0和M1M0=10时，AT89C51定时器／计数器Tl为定时器模式，设置为方式2。作为波特率发生器，其波特率表示为：