Partial re-configuration

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Partial re-configuration is the process of configuring a portion of a field programmable gate array while the other part is still running/operating.

Hardware, like software, can be designed modularly, by creating subcomponents and then higher-level components to instantiate them. In many cases it is useful to be able to swap out one or several of these subcomponents while the FPGA is still operating.

Normally, reconfiguring an FPGA requires it to be held in reset while an external controller reloads a design onto it. Partial reconfiguration allows for critical parts of the design to continue operating while a controller either on the FPGA or off of it loads a partial design into a reconfigurable module. Partial reconfiguration also can be used to save space for multiple designs by only storing the partial designs that change between designs.

A common example for when partial reconfiguration would be useful is the case of a communication device. If the device is controlling multiple connections, some of which require encryption, it would be useful to be able to load different encryption cores without bringing the whole controller down.

Partial reconfiguration is not supported on all FPGAs. In current versions of software, Xilinx supports partial reconfiguration on Virtex II, Virtex II Pro, and Virtex 4 FPGA lines. A special software flow with emphasis on modular design is required. Typically the design modules are built along well defined boundaries inside the FPGA that require the design to be specially mapped to the internal hardware.

From the functionality of the design, partial reconfiguration can be divided into two groups:

  • dynamic partial reconfiguration, also known as an active partial reconfiguration - permits to change the part of the device while the rest of an FPGA is still running;
  • static partial reconfiguration - the device is not active during the reconfiguration process. While the partial data is sent into the FPGA, the rest of the device is stopped (in the shutdown mode) and brought up after the configuration is completed.

There are two styles of partial reconfiguration of FPGA devices from Xilinx: module-based and difference-based.

Module-based partial reconfiguration permits to reconfigure distinct modular parts of the design. To ensure the communication across the reconfigurable module boundaries, special bus macros ought to be prepared. It works as a fixed routing bridge that connects the reconfigurable module with the rest part of the design. Module-based partial reconfiguration requires to perform a set of specific guidelines during at the stage of design specification. Finally for each reconfigurable module of the design, separate bit-stream is created. Such a bit-stream is used to perform the partial reconfiguration of an FPGA.

Difference-based partial reconfiguration can be used when a small change is made to the design. It is especially useful in case of changing LUT equations or dedicated memory blocks content. The partial bit-stream contains only information about differences between the current design structure (that resides in the FPGA) and the new content of an FPGA. There are two ways of difference-based reconfiguration known as a front-end and back-end. The first one is based on the modification of the design in the hardware description languages (HDLs). It is clear that such a solution requires full repeating of the synthesis and implementation processes. The back-end difference-based partial reconfiguration permits to make changes at the implementation stage of the prototyping flow. Therefore there is no need for re-synthesis of the design. The usage of both methods (either front-end and back-end) leads to creation of a partial bit-stream that can be used for a partial reconfiguration of the FPGA.

Note: Partial Reconfiguration is only the update of a portion of the FPGA, it does not require that any portion of the FPGA continue running during the update. Xilinx refers to reconfiguring a portion of the FGPA while the remainder continues to run by the term "partial dynamic reconfiguration".

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