XAPP1026 v2.0：嵌入式系统用lwIP实现网络功能详解

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"XAPP1026（v2.0）是一份详细的Xilinx应用笔记，针对嵌入式系统开发者，着重介绍了如何在搭载PowerPC或MicroBlaze处理器的Xilinx嵌入式系统上集成轻量级IP（lwIP）网络库。该应用笔记发布于2009年6月15日，lwIP是一个开源的TCP/IP协议栈，专为嵌入式设备设计，旨在提供高效、低功耗的网络功能。这份文档详细指导了如何利用lwIP进行网络功能的开发，包括实现以下关键应用：Echo服务器、Web服务器、TFTP服务器以及接收和发送数据的性能测试。这些功能对于需要互联网连接的嵌入式项目来说，是必不可少的基础组件。提供的资源包括针对ML505、ML507和Spartan-3AN FPGA Starter Kit板卡的参考系统示例，链接位于：<https://secure.xilinx.com/webreg/clickthrough.do?cid=107743.zip>。硬件需求方面，读者需要拥有其中任意一款Xilinx的ML505、ML507或Spartan-3AN Starter Development Board，这将确保所使用的开发环境能够与lwIP库无缝集成。软件需求方面，读者需要确保他们的系统安装了与所选硬件平台兼容的Xilinx Embedded Development Kit (EDK)，该工具集针对Xilinx嵌入式系统进行了lwIP软件的定制和优化，以便更好地支持MicroBlaze处理器。此外，对基本的编程技能，特别是C语言编程和网络编程原理，也是进行此类项目开发的必要基础。 XAPP1026（v2.0）不仅提供了技术指导，还为嵌入式开发者提供了一个实际操作的平台，帮助他们理解和利用lwIP在网络嵌入式应用中的优势，提升系统的网络通信能力。"

Software Applications

XAPP1026 (v2.0) June 15, 2009 www.xilinx.com 3

The RAW API provides a callback style interface to the application. Applications using the RAW

API register callback functions to be called on significant events like accept, read or write. A

RAW API based echo server is single threaded, and all the work is done in the callback

functions. The main application loop is structured as follows.

while (1) {

xemacif_input(netif);

transfer_data();

}

The function of the application loop is to receive packets constantly (xemacif_input), then

pass them on to lwIP. Before entering this loop, the echo server sets up certain callbacks:

/* create new TCP PCB structure */

pcb = tcp_new();

/* bind to specified @port */

err = tcp_bind(pcb, IP_ADDR_ANY, port);

/* we do not need any arguments to callback functions */

tcp_arg(pcb, NULL);

/* listen for connections */

pcb = tcp_listen(pcb);

/* specify callback to use for incoming connections */

tcp_accept(pcb, accept_callback);

This sequence of calls creates a TCP connection and sets up a callback on a connection being

accepted. When a connection request is accepted, the function accept_callback is called

asynchronously. Because an echo server needs to respond only when data is received, the

accept callback function sets up the receive callback by performing:

/* set the receive callback for this connection */

tcp_recv(newpcb, recv_callback);

When a packet is received, the function recv_callback is called. The function then echoes

the data it receives back to the sender:

/* indicate that the packet has been received */

tcp_recved(tpcb, p->len);

/* echo back the payload */

err = tcp_write(tpcb, p->payload, p->len, 1);

Although the RAW API is more complex than the SOCKET API, it provides much higher

throughput because it does not have a high overhead.

Web Server

A simple web server implementation is provided as a reference for a TCP based application.

The web server implements only a subset of the HTTP 1.1 protocol. Such a web server can be

used to control or monitor an embedded platform via a browser. The web server demonstrates

the following three features:

• Accessing files residing on a Memory File System via HTTP GET commands

• Controlling the LED lights on the development board using the HTTP POST command

• Obtaining status of DIP switches (push buttons on the ML403) on the development board

using the HTTP POST command

The Xilinx Memory File System (xilmfs) is used to store a set of files in the memory of the

development board. These files can then be accessed via a HTTP GET command by pointing

a web browser to the IP address of the development board and requesting specific files.

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