Building distributed embedded systems with RTLinux-GPL

Linux has revealed in the last few years as an appealing option for developing embedded systems but there are some extra requirements of embedded systems that Linux does not fulfill, such as real-time capabilities, file system size and specific hardware support. Real-time features can be achieved with Real-Time Linux GPL (hereafter RTLinux), which is a small, deterministic, real-time kernel that handles time-critical tasks and runs Linux as its lowest priority execution thread. However RTLinux has also important drawbacks. One of them is that real-time tasks cannot make use of Linux services and, in particular, TCP/IP networking. This paper describes RTL-lwIP, which is a TCP/IP stack for embedded systems based on the lwIP (lightweight) TCP/IP stack that runs on RTLinux and can be used by real-time tasks. RTL-lwIP allows real-time tasks to communicate directly with remote real-time tasks or even with Linux user processes. The importance of introducing TCP/IP on RTLinux is that it enables the possibility of developing real-time distributed embedded systems based on CORBA, thus allowing interoperability with other platforms and Web-integration. This paper describes the porting of the lwIP TCP/IP stack to RTLinux and gives some guidelines in order to implement RTLinux drivers for Ethernet cards using as example the implementation of a RTLinux driver for the 3Com905C-X NIC (Network Interface Card).