FPGA computing systems: Background knowledge and introductory materials

Traditionally, computing was classified into General-Purpose Computing performed by a General-Purpose Processor (GPP) and Application-Specific Computing performed by an Application-Specific Integrated Circuit (ASIC). As a trade-off between the two extreme characteristics of GPP and ASIC, reconfigurable computing has combined the advantages of both. On one hand reconfigurable computing can have better performance with respect to a software implementation but paying this in terms of time to implement. On the other hand a reconfigurable device can be used to design a system without requiring the same design time and complexity compared to a full custom solution but being beaten in terms of performance. The main advantage of a reconfigurable system is its high flexibility, while its main disadvantage is the lack of a standard computing model. In this module we are presenting a first definition of reconfigurable computing, describing the rationale behind it and introducing how this field has been influenced by the introduction of the FPGAs.

FPGA design tools must provide a design environment based on digital design concepts and components (gates, flip-flops, MUXs, etc.). They must hide the complexities of placement, routing and bitstream generation from the user. This module is not going through these steps in details, an entire course will be needed just for this, but it is important at least to have an idea of what it is happening behind the scene to better understand the complexity of the processes carried out by the tools you are going to use. Within this context, this module guides you through a simple example, which is abstracting the complexity of the underlying FPGA, starting from the description of the circuit you may be willing to implement to the bitstream used to configure the FPGA.

The reconfiguration capabilities of FPGAs give the designers extended flexibility in terms of hardware maintainability. FPGAs can change the hardware functionalities mapped on them by taking the application offline, downloading a new configuration on the FPGA (and possibly new software for the processor, if any) and rebooting the system. Reconfiguration in this case is a process independent of the execution of the application. A different approach is the one that considers reconfiguration of the FPGA as part of the application itself, giving it the capability of adapting the hardware configured on the chip resources according to the needs of a particular situation during the execution time. In this case we are referring to this reconfiguration as dynamic reconfiguration and the reconfiguration process is seen as part of the application execution, and not as a stage prior to it. This module illustrates a particular technique, which is extending the previous two, that has been viable for most recent FPGA devices, Partial Dynamic Reconfiguration. To fully understand what this technique is, the concepts of reconfigurable computing, static and dynamic reconfiguration, and the taxonomy of dynamic reconfiguration itself must be analyzed. In this way partial dynamic reconfiguration can be correctly placed in the set of system development techniques that it is possible to implement on a modern FPGA chip.