Table of Contents

Abstract: A low complexity digital VLSI architecture for the computation of an algebraic integer (AI) based 8-point Arai DCT algorithm is proposed. AI encoding schemes for exact representation of the Arai DCT transform based on a particularly sparse 2-D AI representation is reviewed, leading to the proposed novel architecture based on a new final reconstruction step (FRS) having lower complexity and higher accuracy compared to the state-of-the-art. This FRS is based on an optimization derived from expansion factors that leads to small integer constant-coefficient multiplications, which are realized with common sub-expression elimination (CSE) and Booth encoding. The reference circuit [1] as well as the proposed architectures for two expansion factors α† = 4.5958 and α′ = 167.2309 are implemented. The proposed circuits show 150% and 300% improvements in the number of DCT coefficients having error ≤ 0:1% compared to [1]. The three designs were realized using both 40 nm CMOS Xilinx Virtex-6 FPGAs and synthesized using 65 nm CMOS general purpose standard cells from TSMC. Post synthesis timing analysis of 65 nm CMOS realizations at 900 mV for all three designs of the 8-point DCT core for 8-bit inputs show potential real-time operation at 2.083 GHz clock frequency leading to a combined throughput of 2.083 billion 8-point Arai DCTs per second. The expansion-factor designs show a 43% reduction in area (A) and 29% reduction in dynamic power (P_D) for FPGA realizations. An 11% reduction in area is observed for the ASIC design for α† = 4.5958 for an 8% reduction in total power (P_T ). Our second ASIC design having α′ = 167.2309 shows marginal improvements in area and power compared to our reference design but at significantly better accuracy.

Abstract: The need for ultra low power circuits has forced circuit designers to scale voltage supplies into the sub-threshold region where energy per operation is minimized [1]. The problem with this is that the traditional 6T SRAM bitcell, used for data storage, becomes unreliable at voltages below about 700 mV due to process variations and decreased device drive strength [2]. In order to achieve reliable operation, new bitcell topologies and assist methods have been proposed. This paper provides a comparison of four different bitcell topologies using read and write V_MIN as the metrics for evaluation. In addition, read and write assist methods were tested using the periphery voltage scaling techniques discussed in [4–13]. Measurements taken from a 180 nm test chip show read functionality (without assist methods) down to 500 mV and write functionality down to 600 mV. Using assist methods can reduce both read and write V_MIN by 100 mV over the unassisted test case.

Abstract: We present a low voltage, low power operational transconductance amplifier (OTA) designed using a Fully Depleted Silicon-on-Insulator (FDSOI) process. For very low voltage application down to 0.5 V, two-stage miller-compensated OTAs with both p-channel MOSFET (PMOS) and n-channel MOSFET (NMOS) differential input have been investigated in a FDSOI complementary metal oxide semiconductor (CMOS) 150 nm process, using 0.5 V threshold transistors. Both differential input OTAs have been designed to operate from the standard 1.5 V down to 0.5 V with appropriate trade-offs in gain and bandwidth. The NMOS input OTA has a simulated gain/3 dB-bandwidth/power metric of 9.6 dB/39.6 KHz/0.48 µW at 0.6 V and 46.6 dB/45.01 KHz/10.8 µW at 1.5 V. The PMOS input OTA has a simulated metric of 19.7 dB/18.3 KHz/0.42 µW at 0.4 V and 53 dB/1.4 KHz/1.6 µW at 1.5 V with a bias current of 125 nA. The fabricated OTAs have been tested and verified with unity-gain configuration down to a 0.5 V supply voltage. Comparison with bulk process, namely the IBM 180 nm node is provided and with relevant discussion on the use of FDSOI process for low voltage analog design.

Abstract: A novel Single Event Upset (SEU) tolerant flip-flop design is proposed, which is well suited for very-low power electronics that operate in subthreshold ( < Vt ≈ 500 mV). The proposed flip-flop along with a traditional (unprotected) flip-flop, a Sense-Amplifier-based Rad-hard Flip-Flop (RSAFF) and a Dual Interlocked storage Cell (DICE) flip-flop were all fabricated in MIT Lincoln Lab’s XLP 0.15 μm fully-depleted SOI CMOS technology—a process optimized for subthreshold operation. At the Cyclotron Institute at Texas A&M University, all four cells were subjected to heavy ion characterization in which the circuits were dynamically updated with alternating data and then checked for SEUs at both subthreshold (450 mV) and superthreshold (1.5 V) levels. The proposed flip-flop never failed, while the traditional and DICE designs did demonstrate faulty behavior. Simulations were conducted with the XLP process and the proposed flip-flop provided an improved energy delay product relative to the other non-faulty rad-hard flip-flop at subthreshold voltage operation. According to the XLP models operating in subthreshold at 250 mV, performance was improved by 31% and energy consumption was reduced by 27%.

Abstract: This paper presents the first known timing-error detection (TED) microprocessor able to operate in subthreshold. Since the minimum energy point (MEP) of static CMOS logic is in subthreshold, there is a strong motivation to design ultra-low-power systems that can operate in this region. However, exponential dependencies in subthreshold, require systems with either excessively large safety margins or that utilize adaptive techniques. Typically, these techniques include replica paths, sensors, or TED. Each of these methods adds system complexity, area, and energy overhead. As a run-time technique, TED is the only method that accounts for both local and global variations. The microprocessor presented in this paper utilizes adaptable error-detection sequential (EDS) circuits that can adjust to process and environmental variations. The results demonstrate the feasibility of the microprocessor, as well as energy savings up to 28%, when using the TED method in subthreshold. The microprocessor is an 8-bit core, which is compatible with a commercial microcontroller. The microprocessor is fabricated in 65 nm CMOS, uses as low as 4.35 pJ/instruction, occupies an area of 50,000 μm², and operates down to 300 mV.