Towards generic low-power area-efficient standard cell based memory architectures

Digital IC designers often use SRAM macrocells to implement on-chip memory functionality. In this paper we argue that in several situations, standard cell based memories (SCMs) can have advantages over SRAM macrocells. Various ways to implement SCMs are presented and compared to each other for different CMOS technologies and standard cell libraries and to corresponding macrocells, aiming for finding the most adequate memory option for each application. The benefits and drawbacks of SCMs compared to macrocells are illustrated with the example of a low-power low-density parity check (LDPC) decoder

Two-Port Low-Power Gain-Cell Storage Array: Voltage Scaling and Retention Time

The impact of supply voltage scaling on the retention time of a 2-transistor (2T) gain-cell (GC) storage array is investigated, in order to enable low-power/low-voltage data storage. The retention time can be increased when scaling down the supply voltage for a given access statistics and a given write bit-line (WBL) control scheme. Moreover, for a given supply voltage, the retention time can be further increased by controlling the WBL to a voltage level between the supply rails during idle and read states. These two concepts are proved by means of Spectre simulation of a GC-storage array implemented in 180-nm CMOS technology. The proposed 2-kb storage macro is operated at only 40% of the nominal supply voltage and leverages the GCs to enable two-port operation with a negligible area-increase compared to a single-port implementation

Standard-Cell Based Memories (SCMs): from Sub-VT to Error-Resilient Systems

Embedded memories consume an increasingly dominant part of the overall area and power of a large variety of systems-on-chip [ITRS’09]: 1) biomedical implants and wireless sensor networks require robust memories operating in the sub-VT domain; 2) many handheld devices and microprocessors are operated near to threshold-voltage; and 3) fault-tolerant systems/error-resilient computing has attracted interest due to increaing process variations. Standard-cell based memories (SCMs) entail minimum design effort and are immediately functional in any system from reliable sub-VT to error-resilient high-performance. In particular, sub-VT SCMs ensure robustness and improve access bandwidth and energy-efficiency compared to sub-VT SRAM macros. Adding only one custom cell (low-leakage latch) to a commercial standard-cell library further improves energy-efficiency of sub-VT SCMs. In fault-tolerant systems requiring small data retention times, a small amount of errors in the memory content does not severely impede system functionality, and dynamic latches yield SCMs smaller than commercial 6T SRAM macros for storage capacities up to at least 2kb. Various silicon-prooven SCM architectures are presented, and the best-practice SCM implementations for both sub-VT and above-VT applications are derived. To reduce leakage power in sub-VT SCMs, a latch with few highly resistive VDD-ground path is designed using transistor stacking and stretching. For the benefit of smaller silicon area, but at the cost of reduced robustness, various dynamic latches are integrated in the SCM compilation flow

Replica Bit-Line Technique for Embedded Multilevel Gain-Cell DRAM

Multilevel gain-cell DRAMs are interesting to improve the area-efficiency of modern fault-tolerant systems-on-chip implemented in deep-submicron CMOS technologies. This paper addresses the problem of long access times in such multilevel gain-cell DRAMs, which are further aggravated by process parameter variations. A replica bit-line (BL) technique, previously proposed for SRAM, is adapted to speed up the multilevel read operation at a negligible area-increase. Moreover, the same replica column is used to improve the write access time. An 8-kb DRAM macro implemented in 90-nm CMOS technology shows that the replica column is able to successfully track die-to-die process, voltage, and temperature variations to generate control signals with optimum delay. Finally, Monte-Carlo simulations show that a small timing margin of 100 ps is sufficient to also cope with within-die process variations

A 15.8 pJ/bit/iter quasi-cyclic LDPC decoder for IEEE 802.11n in 90 nm CMOS

We present a low-power quasi-cyclic (QC) low density parity check (LDPC) decoder that meets the throughput requirements of the highest-rate (600 Mbps) modes of the IEEE 802.11n WLAN standard. The design is based on the layered offset-min-sum algorithm and is runtime-programmable to process different code matrices (including all rates and block lengths specified by IEEE 802.11n). The register-transfer-level implementation has been optimized for best energy efficiency. The corresponding 90nm CMOS ASIC has a core area of 1.77mm2 and achieves a maximum throughput of 680 Mbps at 346MHz clock frequency and 10 decoding iterations. The measured energy efficiency is 15.8 pJ/bit/iteration at a nominal operating voltage of 1.0V

Design and failure analysis of logic-compatible multilevel gain-cell-based DRAM for fault-tolerant VLSI systems

This paper considers the problem of increasing the storage density in fault-tolerant VLSI systems which require only limited data retention times. To this end, the concept of storing many bits per memory cell is applied to area-efficient and fully logic-compatible gain-cell-based dynamic memories. A memory macro in 90-nm CMOS technology including multilevel write and read circuits is proposed and analyzed with respect to its read failure probability due to within-die process variations by means of Monte Carlo simulations

Review and Classification of Gain Cell eDRAM Implementations

With the increasing requirement of a high-density, high-performance, low power alternative to traditional SRAM, Gain Cell (GC) embedded DRAMs have gained a renewed interest in recent years. Several industrial and academic publications have presented GC memory implementations for various target applications, including high-performance processor caches, wireless communication memories, and biomedical system storage. In this paper, we review and compare the recent publications, examining the design requirements and the implementation techniques that lead to achievement of the required design metrics of these applications

Exploration of Sub-VT and Near-VT 2T Gain-Cell Memories for Ultra-Low Power Applications under Technology Scaling

Ultra-low power applications often require several kb of embedded memory and are typically operated at the lowest possible operating voltage (VDD) to minimize both dynamic and static power consumption. Embedded memories can easily dominate the overall silicon area of these systems, and their leakage currents often dominate the total power consumption. Gain-cell based embedded DRAM arrays provide a high-density, low-leakage alternative to SRAM for such systems; however, they are typically designed for operation at nominal or only slightly scaled supply voltages. This paper presents a gain-cell array which, for the first time, targets aggressively scaled supply voltages, down into the subthreshold (sub-VT) domain. Minimum VDD design of gain-cell arrays is evaluated in light of technology scaling, considering both a mature 0.18 μm CMOS node, as well as a scaled 40 nm node. We first analyze the trade-offs that characterize the bitcell design in both nodes, arriving at a best-practice design methodology for both mature and scaled technologies. Following this analysis, we propose full gain-cell arrays for each of the nodes, operated at a minimum VDD. We find that an 0.18 μm gain-cell array can be robustly operated at a sub-VT supply voltage of 400mV, providing read/write availability over 99% of the time, despite refresh cycles. This is demonstrated on a 2 kb array, operated at 1 MHz, exhibiting full functionality under parametric variations. As opposed to sub-VT operation at the mature node, we find that the scaled 40 nm node requires a near-threshold 600mV supply to achieve at least 97% read/write availability due to higher leakage currents that limit the bitcell’s retention time. Monte Carlo simulations show that a 600mV 2 kb 40 nm gain-cell array is fully functional at frequencies higher than 50 MHz

4T Gain-Cell with internal-feedback for ultra-low retention power at scaled CMOS nodes

Gain-Cell embedded DRAM (GC-eDRAM) has recently been recognized as a possible alternative to traditional SRAM. While GC-eDRAM inherently provides high-density, low-leakage, low-voltage, and 2-ported operation, its limited retention time requires periodic, power-hungry refresh cycles. This drawback is further enhanced at scaled technologies, where increased subthreshold leakage currents and decreased in-cell storage capacitances result in faster data deterioration. In this paper, we present a novel 4T GC-eDRAM bitcell that utilizes an internal feedback mechanism to significantly increase the data retention time in scaled CMOS technologies. A 2 kb memory macro was implemented in a low-power 65nm CMOS technology, displaying an over 3× improvement in retention time over the best previous publication at this node. The resulting array displays a nearly 5× reduction in retention power (despite the refresh power component) with a 40% reduction in bitcell area, as compared to a standard 6T SRAM