Introducing a magnetic tunneling junction (MTJ) into a flip-flop enables nonvolatile power gating (PG) but large store energy to MTJ is a critical concern. We propose an optimized two-step store (TSS) control to first perform a short store with an optimal time for all nonvolatile flip-flops (NVFFs) and then perform a long store only at the failed ones for reducing the store energy. As the key technologies to realize this, we present a verify-and-retryable NVFF (VR-NVFF) circuit enabling the TSS control and an analytical expression for the optimal short-store time (${T}_{\text {short}{\_}\text{opt}}$) minimizing the store energy. To examine the effectiveness of the optimized TSS control and the validity of analytically derived${T}_{\text {short}{\_}\text{opt}}$, we implemented the TSS control on a coarse-grained reconfigurable array (CGRA)-based accelerator chip and fabricated it in a 40-nm CMOS/MTJ hybrid process technology. Results demonstrated that analytical${T}_{\text {short}{\_}\text{opt}}$showed a good agreement with the measured value (within 8% difference) under process and temperature variations. The TSS control with${T}_{\text {short}{\_}\text{opt}}$reduced the store energy to$0.32\times $of that of the conventional long-store-only technique. The break-even time (BET), which is the minimum power-gating time to get the gain in energy savings, was shortened to 0.51–$0.7\times $by the TSS control, achieving the BET of 50–$923 \mu \text{s}$in the range of 0 °C–80 °C.

While the spin-transfer torque (STT) magnetic tunnel junction (MTJ) is a promising technique for enabling nonvolatile flip-flops (NVFFs) to perform power gating to reduce leakage power without any data losses, the large store energy (the energy to make a store operation) of MTJs needs to be addressed. The nonvolatile cool mega array series is an edge-oriented coarse-grained reconfigurable accelerator that implements an improved MTJ-based NVFF with a verify-and-retryable store method that should ideally reduce the store energy under the presence of the switching time variation originating from the stochastic nature of the MTJs. However, the energy reduction effect of the method has not been formulated or evaluated thoroughly enough to make the best use of the method in actual applications. In this study, we propose an analytical model to estimate the store energy in typical operational conditions under the assumption of switching time variations following the normal distributions based on the measurements of a real chip fabricated with a 40-nm perpendicular MTJ/CMOS hybrid process. In contrast to the tedious measurement on each different condition, the proposed model allows for an instantaneous determination of the best storing method for minimizing the store energy, with an energy reduction of up to 69% compared with a conventional one-time attempt storing method. This model is expected to be used for system-level energy simulations and, ultimately, for design explorations in pursuit of energy-optimized memory.

Level shifters to convert signal swings from low-voltage (VDDL) to high-voltage (VDDH) are required at the boundary of voltage domains in SoC employing multiple supply voltages. However, they cost delay, power and area in addition to increasing the complexity of physical design. This paper proposes a level-shifter-less (LSL) approach to use a reverse body bias (RBB) in the VDDH domain and superior threshold-voltage modulation capability of FD-SOI devices. Simulation results and measurements of a fabricated chip demonstrated that the chip applying the LSL approach correctly operates at VDDL=0.6V and VDDH=1.2V under RBB of 2V for pMOS transistors while suppressing the static dc current in the VDDH domain.

In this brief, a practical power optimization method that calculates the optimal power supply and body bias voltages, for a given target operational frequency and a temperature, is proposed and evaluated. The proposed optimization method is based upon a simple power model in which several coefficients for leakage power, switching power, temperature, and operational frequency are obtained from accurate real chip measurements. The calculated optimal-voltage settings by the proposed model can achieve minimum accuracies of 93.8%, 91.6%, and 79.5% for room-temperature, 50 °C, and 65 °C, respectively. Since the proposed methodology is based on well-known power formulas, it can be applied to the latest FD-SOI technologies.

Body bias control is an efficient means of balancing the trade-off between leakage power and performance especially for chips with silicon on thin buried oxide (SOTB), a type of FD-SOI technology. In this work, a method for finding the optimal combination of the supply voltage and body bias voltage to the core and memory is proposed and applied to a real micro-controller chip using SOTB CMOS technology. By obtaining several coefficients of equations for leakage power, switching power and operational frequency from the real chip measurements, the optimized voltage setting can be obtained for the target operational frequency. The power consumption lost by the error of optimization is 12.6% at maximum, and it can save at most 73.1% of power from the cases where only the body bias voltage is optimized. This method can be applied to the latest FD-SOI technologies.

This paper presents a design and control scheme of a microprocessor whose internal function units are power gated at instruction-by-instruction basis. Enabling/disabling the power gating is adaptively controlled under the support of on-chip leakage monitors and the operating system to minimize energy overhead due to sleep-in and wakeup. Measured results of the fabricated chip in the 65nm CMOS technology demonstrated that our approach reduces energy to 21-35% in the range of 25-85°C as compared to the non power-gated case. Energy dissipation was reduced by up to 15% as compared to the conventional fine-grain power gating technique in the same temperature range.

Power-performance efficiency is still remaining a primary concern for microprocessor designers. One of the sources of power inefficiency for recent LSI chips is increasing leakage power consumption. Power-gating is a well known technique to reduce leakage power consumption by switching off the power supply to idle logic blocks. Recently, fine-grained power-gating is emerged as a technique to minimize leakage current during the active processor cycles by switching on and off a logic blocks in much finer temporal/spatial granularity. Though fine-grained power-gating is useful, a comprehensive evaluation and analysis has not been conducted on a real LSI chips. In this paper, we evaluate fine-grained run-time power-gating for microprocessors' functional units using a real embedded microprocessor. We also introduce an architecture and compiler co-operative power-gating scheme which mitigates negative power reduction caused by the energy overhead associated with finegrained power-gating. The experimental results with a fabricated core shows that a hardware-based scheme saves power consumption of functional units by 44% and hardware compiler co-operative scheme further improves power efficiency by 5.9% when core temperature is 25 ˚C.

Presents a conference poster that addresses a perpetuum mobile 32bit central processing unit that resides on 65nm CMOS technology using reverse-body-bias via assisted sleep mode.

Cube-1 is a heterogeneous multi-core processor which can achieve the required performance with the least energy consumption as possible. It can control the performance and energy with two levels: (1) the number of accelerators can be easily changed by increasing or decreasing the number of stacked chips after fabrication, as they are connected with inductive coupling links. (2) The supply voltage for PE array of the accelerator can be controlled by the host CPU so that the required performance can be obtained with a minimum supply voltage.

We propose a multi-voltage (multi-Vdd) variable pipeline router to reduce the power consumption of Network-on-Chips (NoCs) designed for chip multi-processors (CMPs). Our multi-Vdd variable pipeline router adjusts its pipeline depth (i.e., communication latency) and supply voltage level in response to the applied workload. Unlike dynamic voltage and frequency scaling (DVFS) routers, the operating frequency is the same for all routers throughout the CMP; thus, there is no need to synchronize neighboring routers working at different frequencies. In this paper, we implemented the multi-Vdd variable pipeline router, which selects two supply voltage levels and pipeline modes, using a 65nm CMOS process and evaluated it using a full-system CMP simulator. Evaluation results show that although the application performance degraded by 1.0% to 2.1%, the standby power of NoCs reduced by 10.4% to 44.4%.

CMA-Cube is the second prototype of building block scalable reconfigurable accelerator using inductive coupling interconnect. It uses the wireless inductive coupling interconnect as a packet switching network which connects accelerators. As an accelerator core, CMA (Cool Mega Array), which consists of a large coarse-grained PE array with combinatorial circuits and tiny micro-controller, is applied. Evaluation results of Cube-1 Quad Core which consists of a host embedded CPU and three CMA-Cubes achieved 3.15 times performance acceleration as that without accelerators when JPEG decoder is executed.

Cube-2 is a prototype of building block scalable reconfigurable accelerator using an inductive coupling interconnect. It is consisting of a ultra low leakage embedded processor Geyser and coarse-grained reconfigurable accelerators CMA (Cool Mega Array). A Geyser chip and multiple CMA chips are stacked, and a powerful network is formed by using the inductive coupling interconnect. The performance can be enhanced by increasing the number of CMA chips. JPEG decoder is implemented with a cooperation of Geyser and CMAs, and low power execution by controlling the power supply voltage of CMAs is demonstrated.

Recently, run-time sleep control scheme using multiple sleep modes have been studied. In those studies, each sleep mode has its own sleep depth. Deeper sleep mode provides higher leakage saving but incurs larger overhead energy.Use of multiple modes is helpful for further leakage saving if an appropriate mode is selected, but the best mode depends on the idle period whose length cannot be told in advance. Although the implementations how to realize different sleep depths have been well studied, few attention has been paid to the method of how to select the best sleep depth dynamically during execution. This paper proposes a simple but novel sleep control scheme, called stepwise sleep depth control, which aims to select the best depth among provided multiple sleep depths.Our scheme automatically applies deeper depth in a step-by-step manner after an idle state starts. It successfully reduces leakage energy while only a small modification is required for circuit implementation. This paper also proposes a methodology for optimizing control parameters of our sleep control scheme according to program behavior and temperature. Experimental result shows that stepwise sleep depth control applied to body biasing circuit improves net leakage saving of up to 43% for FPAlu at 1.0GHz, 75°C compared to conventional reverse body biasing.

Geyser-2 is the second prototype MIPS CPU which provides a fine-grained run-time power gating (PG) controlled by instructions. Geyser-l, the first prototype only provides the fine-grained run-time PG core. Although it demonstrated the leakage power reduction on a real chip, the operational frequency is limited at 60MHz because of the limitation of the I/O speed. Geyser-2 with cache and TLB mechanism is implemented to show (1) run-time PG works at least with 200MHz which is commonly used clock for embedded systems, and (2) it is also efficient on the environment with real application programs with an operating system.

A highly energy efficient reconfigurable accelerator called CMA (Cool Mega-Array) is proposed. It consists of a large Processing Element (PE) array without memory elements for maintain result of ALU and configuration data, a small simple programmable micro controller for data management, and the data memory. Unlike traditional coarse grained reconfigurable processors, the power consumption for hardware context switching, storing intermediate data in registers, and clock distribution for them are eliminated from PE array which occupies large area of a chip. Configuration registers are collected to small area of micro controller. The data flow graph mapped on the PE array is static during execution. Various application programs can be implemented by making the best use of flexible data management instructions with the micro controller. When the delay time in the PE array is longer than the data handling time with the micro controller, the supply voltage for the PE array is scaled to reduce the power consumption without degrading the performance. In the opposite case, wave pipelining is applied to enhance PE array performance. A prototype chip CMA-1 with 8 × 8 PE array with 24-bit data width was fabricated in 2.1 × 4.2mm265-nm CMOS technology, and achieves 2.4-GOPS/11.2-mW sustained performance. This energy efficiency is comparable to that of the most energy efficient accelerators that have been reported.

This paper proposes the ultrafine-grained run-time power gating of on-chip routers, in which the power supply to each router component (e.g., virtual-channel buffer, virtual-channel multiplexer, and crossbar multiplexer and output latch) can be individually controlled based on the applied workload. Since only the router components that are transferring a packet are activated, the leakage power of the on-chip network can be reduced to a near-optimal level. However, such techniques inherently increase the communication latency and degrade the application performance, since a certain amount of wakeup latency is required to activate the sleeping components. To mitigate this wakeup latency, an early wakeup method that can preliminarily detect the next packet arrival and activate the corresponding components is essential. We designed and implemented an ultrafine-grained power-gating router using a commercial 65 nm process. We propose four early wakeup methods and combine them with the power-gating router. The proposed router with the early wakeup methods is evaluated in terms of its application performance, area overhead, and leakage power reduction taking into account the on/off energy overhead. The simulation results showed that it reduces the leakage power by 54.4-59.9% on average even when the application programs are fully running, at the expense of 4.6% of the area and 0.7-3.7% of the performance overheads when we assume a 1 GHz operation.

Geyser-1 is a MIPS CPU which provides a fine-grained run-time power gating (PG) controlled by instructions. Unlike traditional PGs, it uses special standard cells in which the virtual ground (VGND) is separated from the real ground, and a certain number of the sleep transistors are inserted for quick power shut-down and wake-up. In Geyser-1, the fine-grained run-time PG is applied to computational modules in the execution stage. The power shut-down and wakeup are controlled with architectural and software level. This implementation is the first available CPU with this type of run-time PG technique. Geyser-1 has both time and spatial fine-grained PG and works well with a real chip.

This paper proposes an ultra fine-grained run-time power gating of on-chip router, in which power supply to each router component (e.g., VC queue, crossbar MUX, and output latch) can be individually controlled in response to the applied workload. As only the router components which are just transferring a packet are activated, the leakage power of the on-chip network can be reduced to the near-optimal level. However, a certain amount of wakeup latency is required to activate the sleeping components, and the application performance will be degraded. In this paper, we estimate the wakeup latency for each component based on circuit simulations using a 65 nm process. Then we propose four early wakeup methods to overcome the wakeup latency. The proposed router with the early wakeup methods is evaluated in terms of the application performance, area, and leakage power. As a result, it reduces the leakage power by 78.9%, at the expense of the 4.3% area and 4.0% performance when we assume a 1 GHz operation.

Provides an abstract of the tutorial presentation and a brief professional biography of the presenter. The complete presentation was not made available for publication as part of the conference proceedings.

One of the benefits of coarse grained dynamically reconfigurable processor array(DRPA) is its low dynamic power consumption by operating a number of processing elements(PE) in parallel with low clock frequency. However, in the future advanced processes, leakage power will occupy a considerable part of the total power consumption, and it may degrade the advantage of DRPAs. In order to reduce the leakage power, a fine grained Power Gating(PG) is applied to a DRPA, MuCCRA-2.32b, and leakage power and area overhead are measured. We evaluated the effect of two control modes; Pair and Unit Individual based on layout design and real applications. It appears that by applying PG for ALUs and SMUs in PEs individually, 48% of leakage power can be reduced with 9.0% of area overhead.

A fine-grain dynamic power gating is proposed for saving the leakage power in MIPS R3000 by sleep control and applied to a processor pipeline. An execution unit is divided into four small units: multiplier, divider, shifter and other (CLU). The power of each unit is cut off dynamically, based on the operation. We tape-outed the prototype chip Geyser-0, which provides an R3000 Core with the power reduction technique, 16 KB caches and translation lookaside buffer (TLB) using 90 nm CMOS technology. The evaluation results of four benchmark programs for embedded applications show that 47% of the leakage power is reduced on average with 41% area overhead.

So far, low power design for SoC has mainly focused on techniques to reduce dynamic power and standby leakage power. In further scaled devices, design technology to reduce active leakage power at the operation mode becomes indispensable. This is because the share of leakage power in the total operation power continues to increase as the device gets scaled. This paper gives a brief overview on the conventional leakage reduction techniques and describes novel approaches to use run-time power gating for active leakage reduction.

One of the critical issues in MTCMOS design is how to estimate a circuit delay quickly. In this paper, we propose a delay modeling and static timing analysis (STA) methodology targeting at MTCMOS circuits. In the proposed method, we prepare a delay look-up table (LUT) consisting of the input slew, the output load capacitance, the virtual ground length, and a power-switch size. Using this LUT, we compute a circuit delay for each logic cell by applying the linear interpolation. Experimental results show that the proposed methodology enables to estimate the critical path delay in a good accuracy

Leakage power dissipation becomes a dominant component in operation power in nanometer devices. This paper describes a design methodology to implement runtime power gating in a fine-grained manner. We propose an approach to use sleep signals that are not off-chip but are extracted locally within the design. By utilizing enable signals in a gated clock design, we automatically partition the design into domains. We then choose the domains that will achieve the gain in energy savings by considering dynamic energy overhead due to turning on/off power switches. To help this decision we derive analytical formulas that estimate the break-even point. For the domains chosen, we create power gating structure by adding power switches and generating control logic to the switches. We experimentally built a design flow and evaluated with a synthesizable RTL code for a microprocessor and a 90nm CMOS device model both used in industry. Results from applying to a datapath showed that the break-even point that achieves the gain exists in the number of enables controlling the power switch. By applying the domains controlled by up to 3 enables achieved the active leakage savings by 83% at the area penalty by 20%.

This paper describes an automated design technique to selectively use multi-threshold CMOS (MTCMOS) in a cell-by-cell fashion. MT cells consisting of low-Vth transistors and high-Vth sleep transistors are assigned to critical paths, while high-Vth cells are assigned to non-critical paths. Compared to the conventional MTCMOS, the gate delay is not affected by the discharge patterns of other gates because there is no virtual ground to be shared. We applied this technique to a test chip of a DSP core. The worst path-delay was improved by 14% over the single high-Vth design without increasing standby leakage at 10% area overhead.

This paper describes a gate-level power minimization methodology using dual supply voltages.Gates and flip-flops off the critical paths are made to operate at the reduced supply voltage to save power.Core technologies are dual-V DD circuit synthesis and P&R.We give a brief overview on existing low-power EDA technologies as background and discuss advantages and challenges of the dual-VDD approach.Through real design examples, we will show that the approach reduces power effectively while keeping the performance at ne gligible area overhead.

We have developed a function-level power estimation methodology for predicting the power dissipation of embedded software. For a given microprocessor core, we empirically build the “power data bank”, which stores the power information of the built-in library functions and basic instructions. To estimate the average power of an embedded software on this core, we first get the execution information of the target software from program profiling/tracing tools. Then we evaluate the total energy consumption and execution time based on the “power data bank”, and take their ratio as the average power. High efficiency is achieved because no power simulator is used once the “power data bank” is built. We apply this method to a commercial microprocessor core and get power estimates with an average error of 3%. With this method, microprocessor vendors can provide users the “power data bank” without releasing details of the core to help users get early power estimates and eventually guide power optimization.

This paper describes an automated layout design technique for the gated-clock design. Two issues must be considered for gated-clock circuits to work correctly. One is to minimize the skew for gated-clock nets. The other is to keep timing constraints for enable-logic parts. We propose the layout design technique to taking these things into consideration. We developed gated-clock tree synthesizer for the first issue, and timing constraints generator and clock delay estimator for the second. We applied it to a practical gated-clock circuit. By our technique, the clock-skew could be less than 0.2 ns keeping timing constraints for enable-logic parts.

This paper describes a fully automated low-power design methodology in which three different voltage-scaling techniques are combined together. Supply voltage is scaled globally, selectively, and adaptively while keeping the performance. This methodology enabled us to design an MPEG4 codec core with 58% less power than the original in three week turn-around-time.

We present a low-power design method that utilizes the multiple supply voltages. The proposed method reduces the power consumption of random logic circuits by 47% on the average, with up to 15% area overhead, by the combination of Clustered Voltage Scaling (CVS) and by Row optimized Power Supply (RRPS) scheme. By the CVS scheme, the optimal netlist, that uses the minimized number of the level converters and the maximized number of the low Vdd gates under the timing constraints, is generated. To avoid the wiring resource consumption and the increase of the interconnect delay caused by the layout constraints of the multiple-supply-voltage design, a new power bus wiring scheme called RRPS is proposed. The proposed method is applied to a media processor chip Mpact/sup TM/ and achieved the above mentioned results. In this paper, the emphasis is put on the interrelation between the generation of the two-supply-voltage netlist with the CVS scheme and the layout technology, such as the power supply scheme and the placement of the multiple-supply-voltage gates. The clocking scheme for the multiple supply voltages is also discussed.

This paper describes a new datapath generator that generates high-density mask layouts equivalent to hand-crafted ones.