0.3-1.5 V Embedded SRAM Core with Write-Replica Circuit Using Asymmetrical Memory Cell and Source-Level-Adjusted Direct-Sense-Amplifier

Toshikazu SUZUKI
Yoshinobu YAMAGAMI

IEICE TRANSACTIONS on Electronics   Vol.E88-C    No.4    pp.630-638
Publication Date: 2005/04/01
Online ISSN: 
DOI: 10.1093/ietele/e88-c.4.630
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Low-Power LSI and Low-Power IP)
Category: Memory
SRAM,  low-voltage,  wide-voltage,  SoC,  

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This paper describes the access-timing control for an embedded SRAM core which operates in low and wide supply voltage (Vdd = 0.3-1.5 V). In the conventional SRAMs, a wiring-replica with replica-memory-cell (RMC) to trace the delay of memory core is used for this purpose. Introducing a wiring-replica control provides the better tracing capability in the wide range of Vdd above 0.5 V than those of using the control with logic-gate-delay. However, as Vdd is reduced below 0.5 V and gets close to the threshold voltage (Vth) of transistors, the access-timing fluctuation resulting from the variation in memory-cell-transistor-drain current (Id) is intolerably increased. As a result, the conventional control is no longer effective in such a low-voltage operation because the RMC can not replicate the variation in transistor-drain current (Id) and Vth of the memory-cells (MCs). Thus, to trace the access-timing fluctuation caused by the variation in Id and Vth is the prerequisite for the SRAM control in the range of Vdd = 0.3-1.5 V. To solve this issue, we have proposed new access-control scheme as follows; 1) a write-replica circuit with an asymmetrical memory cell (WRAM) making it possible to replicate the variation in Id and Vth for the write-access timing control, and 2) a source-level-adjusted direct-sense-amplifier (SLAD) to accelerate the read-access-timing even in low Vdd. These circuit techniques were implemented in a 32-Kbit SRAM in four-metal 130-nm CMOS process technology, and have been verified a low-voltage operation of 0.3 V which has not ever reported with 6.8 MHz. Moreover it operated in wide-voltage up to 1.5 V with 960 MHz. The required 27 MHz operation for mobile applications has been demonstrated at 0.4 V which is 6-times faster than the previous reports. The WRAM scheme enabled the write operation even at 0.3 V. And the access time which can be restricted by the slow tail bits of MCs has been accelerated by 73% to 140 nsec at 0.3 V by using SLAD scheme.