Using the Plasma RISC CPU from OPENCORES.org in an communication system

A success story

In our research group, a project funded by the German Research Foundation (DFG) aims at developing novel network protocols for mobile ad hoc networks. A special aspect of this project is the evaluation of our protocols in a real implementation. The implementation consists of a test bed that contains a number of mobile robots. Each robot is equipped with a communication module. The communication module contains a transceiver module and an FPGA module. The processing of the network protocols is done within the FPGA module. For this purpose, we have implemented a System-On-Programmable-Chip that contains a general purpose processor and hardware assists to accelerate specific protocol functions. Since the capacity of the used Xilinx Virtex E 300 FPGA is limited, we had to search for a processor core small enough to be fitted into the used FPGA.

Block diagram of the communication module for the mini robot Khepera.

After some research on the web, we found out that OPENCORES.org offered suitable processor cores whose license allows the use without additional fees and with full source code available. We did some tests with the available cores and decided that the Plasma CPU (also called MLITE) is the most suited core for our purposes. We integrated it in our system together with the necessary transceiver hardware to control the communication module. During the integration, we found some minor errors in the instruction set decoder of the processor and fixed them. We also replaced multiplexers within arithmetic units by tri-state buffers to further reduce the area requirements of the processor. We ended up with a system that is able to execute our network protocols and performs the necessary signal processing for the transceiver module. With the help of Steve Rhoads, the developer of the Plasma CPU core, the changes were also contributed back to OPENCORES.org. The current version of the Plasma CPU core, which can be downloaded at OPENCORES.org, contains all of our bug fixes and enhancements.

We started to work with the Plasma CPU core in April 2003. In September 2003, we successfully demonstrated a working prototype with four mini robots during an inspection by the German Research Foundation. Our project has been extended until the end of 2006. We are currently working on larger scale experiments with up to 20 mini robots.

Further Information

• Mobile ad hoc networks: Prototypical implementation with efficient system architectures
• DFG Sonderforschungsbereich 376 -- Teilprojekt C6: Mobile ad hoc networks