Routing resources in modern FPGAs use 50% of the silicon real estate and are significant contributors to critical path delay and power consumption; the situation gets worse with each successive process generation, as transistors scale more effectively than wires. To cope with these challenges, FPGA architects have divided wires into local and global categories and introduced fast dedicated carry chains between adjacent logic cells, which reduce routing resource usage for certain arithmetic circuits (primarily adders and subtractors). Inspired by the carry chains, we generalize the idea to connect lookup tables (LUTs) in adjacent logic cells. By exploiting the fracturable structure of LUTs in current FPGA generations, we increase the utilization of the existing LUTs in the logic cell by providing new inputs along the logic chain, but without increasing the I/O bandwidth from the programmable interconnect. This allows us to increase the logic density of the configurable logic cells while reducing demand for routing resources, as long as the mapping tools are able to exploit the logic chains. Our experiments using the combinational MCNC benchmarks and comparing against an Altera Stratix-III FPGA show that the introduction of logic chains reduce the average usage of local routing wires by 37%, with a 12% reduction in total wiring (local and global); this translates to improvements in dynamic power consumption of 18% in the routing network and 10% overall, while utilizing 4% fewer logic cells, on average. Copyright 2011 ACM.

A new method to synthesize clusters of floating-point addition operations on FPGAs is presented. Similar to Altera's floating-point data path compiler, it performs normalization once, at the output of the cluster operation. All significands in the clustered operation are denormalized in parallel with respect to the largest exponent: a fixed-point compressor tree then sums the aligned significands, followed by normalization and rounding. Compared to Altera's floating-point datapath compiler, our method reduces the critical path delay by as much as 20%, and area by as much as 29% on Altera Stratix III FPGAs. © 2010 IEEE.

Fast carry chains featuring dedicated adder circuitry is a distinctive feature of modern FPGAs. The carry chains bypass the general routing network and are embedded in the logic blocks of FPGAs for fast addition. Conventional intuition is that such carry chains can be used only for implementing carry-propagate addition; state-of-the-art FPGA synthesizers can only exploit the carry chains for these specific circuits. This paper demonstrates that the carry chains can be used to build compressor trees, i.e., multi-input addition circuits used for parallel accumulation and partial product reduction for parallel multipliers implemented in FPGA logic. The key to our technique is to program the lookup tables (LUTs) in the logic blocks to stop the propagation of carry bits along the carry chain at appropriate points. This approach improves the area of compressor trees significantly compared to previous methods that synthesized compressor trees solely on LUTs, without compromising the performance gain over trees built from ternary carry-propagate adders. ©2009 IEEE.

We propose a new DSP block for use in modern high-performance FPGAs. Current DSP blocks contain fixed-bitwidth multipliers that can be combined efficiently to form larger multipliers. Our approach is similar, but includes a bypass layer following the partial product generator that exposes the compressor tree used for partial product reduction directly to the user. As a consequence, the proposed DSP block can accelerate multi-input addition operations in addition to multiplication. To increase the flexibility of the device, the partial product reduction tree used within our DSP block uses a fixed-function compression logic along with a field programmable compressor tree (FPCT), the latter of which is user-configurable to meet the needs of the application at hand. Multi-input addition operations can be mapped directly onto the FPCT without compromising any of the other functionality of the DSP block. © 2009 IEEE.

In floating-point datapaths synthesized on FPGAs, the shifters that perform mantissa alignment and normalization consume a disproportionate number of LUTs. Shifters are implemented using several rows of small multiplexers; unfortunately, multiplexer-based logic structures map poorly onto LUTs. FPGAs, meanwhile, contain a large number of multiplexers in the programmable routing network; these multiplexer are placed under static control of the FPGA's configuration bitstream. In this work, we modify some of the routing multiplexers in the intra-cluster routing network of a CLB in an FPGA to implement shifters for floating-point mantissa alignment and normalization; the number of CLBs required for these operations is reduced by 67%. If shifting is not required, the routing multiplexers that have been modified can be configured to operate as normal routing multiplexers, so no functionality is sacrificed. The area overhead incurred by these modifications is small, and there is no need to modify every routing multiplexer in the FPGA. Experiments show that there is no negative impact in terms of clock frequency or routability for benchmarks that do not use the dynamic multiplexers. © 2012 ACM.

This paper advocates the use of 3D integration technology to stack a DRAM on top of an FPGA. The DRAM will store future FPGA contexts. A configuration is read from the DRAM into a latch array on the DRAM layer while the FPGA executes; the new configuration is loaded from the latch array into the FPGA in 60ns (5 cycles). The latency between reconfigurations, 8.42μs, is dominated by the time to read data from the DRAM into the latch array. We estimate that the DRAM can cache 289 FPGA contexts. ©2009 IEEE.