Reduction of Power Consumption in FPGAs - An Overview <s> Power Reduction at System Level Design: <s> This paper investigates experimentally the quantitative impact of pipelining on energy per operation for two representative FPGA devices: a 0.13μm CMOS high density/high speed FPGA (Altera Stratix EP1S40), and a 0.18μm CMOS low-cost FPGA (Xilinx XC2S200). The results are obtained by both measurements and execution of vendor-supplied tools for power estimation. It is found that pipelining can reduce the amount of energy per operation by between 40% and 90%. Further reduction in energy consumption can be achieved by power-aware clustering, although the effect becomes less pronounced for circuits with a large number of pipeline stages. <s> BIB001 </s> Reduction of Power Consumption in FPGAs - An Overview <s> Power Reduction at System Level Design: <s> This paper presents experimental measurements of the differences between a 90nm CMOS FPGA and 90nm CMOS Standard Cell ASICs in terms of logic density, circuit speed and power consumption. We are motivated to make these measurements to enable system designers to make better informed hoices between these two media and to give insight to FPGA makers on the deficiencies to attack and thereby improve FPGAs. In the paper, we describe the methodology by which the measurements were obtained and we show that, for circuits containing only combinational logic and flip-flops, the ratio of silicon area required to implement them in FPGAs and ASICs is on average 40. Modern FPGAs also contain "hard" blocks such as multiplier/accumulators and block memories and we find that these blocks reduce this average area gap significantly to as little as 21. The ratio of critical path delay, from FPGA to ASIC, is roughly 3 to 4, with less influence from block memory and hard multipliers. The dynamic power onsumption ratio is approximately 12 times and, with hard blocks, this gap generally becomes smaller. <s> BIB002 </s> Reduction of Power Consumption in FPGAs - An Overview <s> Power Reduction at System Level Design: <s> This article introduces an automatic design procedure for determining the sensitivity of outputs in a digital signal processing design to small errors introduced by rounding or truncation of internal variables. The proposed approach can be applied to both linear and nonlinear designs. By analyzing the resulting sensitivity values, the proposed procedure is able to determine an appropriate distinct word-length for each internal variable in a fixed-point hardware implementation. In addition, the power-optimizing capabilities of word-length optimization are studied. Application of the proposed procedure to adaptive filters and polynomial evaluation circuits realized in a Xilinx Virtex FPGA has resulted in area reductions of up to 80p (mean 66p) combined with power reductions of up to 98p (mean 87p) and speed-up of up to 36p(mean 20p) over common alternative design strategies. <s> BIB003 </s> Reduction of Power Consumption in FPGAs - An Overview <s> Power Reduction at System Level Design: <s> Performance of applications can be boosted by executing application-specific instruction set extensions (ISEs) on a specialized hardware coupled with a processor core. Many commercially available customizable processors have communication overheads in their interface with the specialized hardware. However, existing ISE generation approaches have not considered customizable processors that have communication overheads at their interface. Furthermore, they have not characterized the energy benefits of such ISEs. We present a soft-processor customization framework that takes an input 'C' application and realizes a customized processor capturing the microarchitectural details of its interface with the specialized unit. We are able to accurately measure the speedup, energy, power and code size benefits of our ISE approach on a real system implementation by applying the design flow to a popular Xilinx Microblaze soft-processor core synthesized for four real-life applications. We show that only one large ISE per application is sufficient to get an average 1.41/spl times/ speedup over pure software execution in spite of incurring communication overheads in the ISE implementation. We also observe a simultaneous savings in energy (up to 40%) and power (up to 12% peak power reduction) with this increased performance. <s> BIB004 </s> Reduction of Power Consumption in FPGAs - An Overview <s> Power Reduction at System Level Design: <s> Contemporary field-programmable gate array (FPGA) design requires a spectrum of available physical resources. As FPGA logic capacity has grown, locally accessed FPGA embedded memory blocks have increased in importance. When targeting FPGAs, application designers often specify high-level memory functions, which exhibit a range of sizes and control structures. These logical memories must be mapped to FPGA embedded memory resources such that physical design objectives are met. In this paper, a set of power-efficient logical-to-physical RAM mapping algorithms is described, which converts user-defined memory specifications to on-chip FPGA memory block resources. These algorithms minimize RAM dynamic power by evaluating a range of possible embedded memory block mappings and selecting the most power-efficient choice. Our automated approach has been validated with both simulation of power dissipation and measurements of power dissipation on FPGA hardware. A comparison of measured power reductions to values determined via simulation confirms the accuracy of our simulation approach. Our power-aware RAM mapping algorithms have been integrated into a commercial FPGA compiler and tested with 34 large FPGA benchmarks. Through experimentation, we show that, on average, embedded memory dynamic power can be reduced by 26% and overall core dynamic power can be reduced by 6% with a minimal loss (1%) in design performance. In addition, it is shown that the availability of multiple embedded memory block sizes in an FPGA reduces embedded memory dynamic power by an additional 9.6% by giving more choices to the computer-aided design algorithms <s> BIB005
Looking at research works carried out to minimize power reduction techniques at system level design can be categorized in the following three parts:  Basic techniques: Following basic techniques have been explored so far at system level design: -Preferably use coarse-grained embedded blocks rather than the fine-grained configurable logic blocks in an FPGA, since the former are more power efficient than the latter for the same function BIB002 . While using course-grained, it is to be ensured that power consumption for routing would not increase significantly. -To obtain the best trade-off in speed, area, power consumption, flexibility, and accuracy, word-length optimization can be BIB003 . -Clock gating is a simple and effective method for reducing dynamic power consumption. It decreases dynamic power by eliminating unnecessary toggling on the outputs of flip-flops of a circuit, gates in the fan-out of the flip-flops, and clock signals. Fig. 14 depicts a classic example of clock gating. In part (a) of the figure, we see two register files on the left, each feeding a combinational logic circuit. The outputs of the two combinational logic circuits feed the inputs of a multiplexer. The multiplexer's select signal, sel, selects which combinational circuit's output is passed to the input of a destination register file. Part (b) of the figure shows the circuit after clock gating has been applied. The signal sel is used to derive a clock enable signal on clocks feeding the input register files. Power is reduced through several mechanisms. First, the capacitive loading on the clock network itself is reduced. Second, unnecessary toggling within the combinational circuits is eliminated. In particular, in the optimized circuit, toggling only occurs in the combinational circuit whose output is selected by the multiplexer to be passed to the destination register file. Clock gating has been used extensively in ASICs for power optimization. However, clock gating has not been explored in depth for FPGAs as well and is different from ASICs because of the fixed pre-fabricated clock interconnection network. It can be used to reduce dynamic power consumption to prevent signal transitions by disabling the clock for the inactive regions. The circuitry in an operator is gated when not in use if it can be combined with word-length optimization [62] . -It is found that, at a given clock speed, pipelining which is a simple and effective way of reducing glitching can reduce the amount of energy per operation by between 40% and 90% for applications such as integer multiplication, CORDIC, triple DES, and FIR filters BIB001 . -By using dynamic voltage scaling to adapt the dynamic supply voltage to the FPGA as the temperature changes, to minimize power consumption. Power reduction between 4% and 54% can be achieved for various arithmetic circuits  .  Techniques with run-time reconfigurability -Runtime reconfiguration with word-length optimization can be combined to adopt the smallest design at a given time, as long as the energy reduction in execution is greater than the energy overhead for reconfiguration [62] . -In order to adapt to run-time conditions, runtime reconfiguration can be applied to change a design. A more powerful but less energy efficient turbo coder can be used to maintain a fixed bit error rate, and vice versa when a communication channel becomes more noisy  .  Low-power techniques for FPGA-based soft processors -Instruction set extensions based on an iterative improvement method to the Micro Blaze soft processor have been proposed in BIB004 wherein up to 40% reduction in energy and 12% reduction in peak power has been reported -Combined application of instruction recoding and power-aware scheduling techniques can be used to optimize a soft processor at multiple levels of abstraction. Dynamic power reduction of up to 74% has been reported in  . a) Before Gating b) After Gating The new medium thickness oxide transistors provide slightly less performance than thin oxide transistors, but leak significantly less power. These are used in the configuration memory and the switches that are controlled by this memory in the latest FPGAs. The oxide thickness does not affect the performance of the corresponding switches because the configuration memory remains static during the operation of the device, FPGA vendors in addition to smaller device geometries that reduce the average node capacitance, use a low-k dielectric between metal layers which reduce the parasitic capacitance and hence reduces the correspondingly dynamic power. Since the dynamic power has a quadratic relationship (CV2f ) with the supply voltage, it can be reduced further by lowering the supply voltage. Xilinx reduces the core supply voltage from 1.2V being used in Virtex 4 to 1.0V in its Virtex 5 FPGAs that cuts core power significantly. Altera and Xilinx have also made a number of architecture-level changes to their latest devices to reduce static and dynamic power like both have recently increased the size of the LUTs (lookup tables) within the logic blocks. Since LUTs are implemented using smaller transistors (compared to transistors in the routing resources), which leak less and dissipate less dynamic power therefore, both static and dynamic power are reduced by increasing the size of the basic logic elements, from 4-input LUTs to 6 and 7-input LUTs, since more logic is implemented within each LUT and less routing is needed between the LUTs. Other features at architecture-level that reduce overall power are use of the embedded memories, adders, and multipliers. Although each of these functions can be implemented using the programmable logic fabric, its implementation as a fixedfunction embedded block is more power-efficient since circuitry to make it flexible is not needed, and it can be turned off when not used. Moreover, vendors have also modified their routing architectures that reduce the average capacitance of the routes, which improves both power and performance. A number of low-power techniques have also been incorporated into the commercial FPGA CAD tools. Detailed power models have been integrated within the Altera Quartus II [68] and Xilinx ISE CAD tools  that provide a spreadsheet utility to make early power predictions before the design is complete and a detailed power model that can be used when the design is complete. The detailed power models provide estimates after the application has been placed, routed, and simulated whereas early power estimates are based on estimated resource usage, I/O types, clock requirements, clock frequencies, and environmental conditions. The estimations from the detailed power models are more accurate than those from the early power models, since detailed capacitance, leakage, and switching activity information is known for each node in the application circuit. Power-aware CAD techniques have also been incorporated into the commercial CAD flows. Power is minimized during technology mapping, placement and routing by minimizing the capacitance of high-activity signals in Quartus II. As described in BIB005 , power is also minimized by optimizing the mapping to the embedded memories and to the embedded DSP blocks. In ISE, power is minimized during placement and routing by minimizing the capacitance of high-activity signals. Dynamic power dissipation is further minimized by strategically setting the configuration bits within partially used (some inputs are not used) LUTs to minimize switching activity. Altera reports that Stratix III FPGAs are over 50% more power efficient than Stratix II FPGAs [68] by combining the above techniques and similarly, Xilinx reports that Virtex-5 FPGAs consume over 35% less dynamic power than Virtex-4 FPGAs, with even greater savings when embedded components are used  . Xilinx also points out that low leakage techniques are already incorporated in their Virtex-4 FPGAs, resulting in 70% lower static power consumption when compared with competing FPGAs. A low-power alternative to SRAM-based FPGAs is flash-based FPGA technology as flash-based memory dissipates significantly less leakage power compared to SRAM memory. Actel's devices which are Flash-based FPGAs, are inherently more efficient and it is reported that their low-power FPGAs dissipate 4 times less leakage power than their nearest competitors [71] .