A Reconfigurable 8T Ultra-Dynamic Voltage Scalable (U-DVS) SRAM in 65 nm CMOS
Sinangil, M. E.
Verma, N.
Chandrakasan, A. P.
This paper appears in: Solid-State Circuits, IEEE Journal of
Publication Date: Nov. 2009
Volume: 44,
Issue: 11
On page(s): 3163-3173
ISSN: 0018-9200
Digital Object Identifier: 10.1109/JSSC.2009.2032493
Current Version Published: 2009-11-03
Abstract In modern ICs, the trend of integrating more on-chip memories on a die has led SRAMs to account for a large fraction of total area and energy of a chip. Therefore, designing memories with dynamic voltage scaling (DVS) capability is important since significant active as well as leakage power savings can be achieved by voltage scaling. However, optimizing circuit operation over a large voltage range is not trivial due to conflicting trade-offs of low-voltage (moderate and weak inversion) and high-voltage (strong inversion) transistor characteristics. Specifically, low-voltage operation requires various assist circuits for functionality which might severely impact high-voltage performance. Reconfigurable assist circuits provide the necessary adaptability for circuits to adjust themselves to the requirements of the voltage range that they are operating in. This paper presents a 64 kb reconfigurable SRAM fabricated in 65 nm low-power CMOS process operating from 250$~$mV to 1.2$~$V. This wide supply range was enabled by a combination of circuits optimized for both subthreshold and above-threshold regimes and by employing hardware reconfigurability. Three different write-assist schemes can be selectively enabled to provide write functionality down to very low voltage levels while preventing excessive power overhead. Two different sense-amplifiers are implemented to minimize sensing delay over a large voltage range. A prototype test chip is tested to be operational at 20 kHz with 250 mV supply and 200 MHz with 1.2 V supply. Over this range leakage power scales by more than 50 X and a minimum energy point is achieved at 0.4 V with less than 0.1 pJ/bit/access.
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