Research Projects

Fundamental Limits in Data-Retention for Standby SRAM--Experimental Results (pJoules)

Kannan Ramchandran, Jan M. Rabaey, Animesh Kumar and Huifang Qin

With technology scaling, increasing leakage power is an issue in SRAM cache which typically occupies a significant portion of the chip area. This work explores the fundamental limits on leakage power subject to reliable data-retention in standby SRAM. Each SRAM cell has a minimum threshold voltage called DRV, for reliable data retention. This DRV parameter exhibits intra-die (and inter-die) variations in the deep submicron region. Traditional worst-case methods suggest the largest DRV (say DRV₀) of a chip as the minimum supply voltage for successful data-retention.

In contrast, this work proposes aggressive reduction of retention supply voltage below DRV₀, with the use of error-control coding to correct ensuing errors. Reducing supply voltage increases the failure rate, but reduces the leakage-power. This phenomenon is analyzed by deriving an optimum tradeoff between the leakage power and the retention-voltage dependent failure rate of SRAM cells. The optimum power per useful bit as a function of supply-voltage is derived, using information theoretic methods. It is observed that power per bit can be optimized over the choice of supply voltage. Hence, it is shown that about 23-52% optimum leakage power reduction can be achieved (with respect to worst-case method) for different SRAM chips in the 90 nm CMOS-technology. A Hamming code implementation with block-length 31 was proposed to prove our analytical results. The implementation achieves 12-46% power reduction, while including the cost of encoding-decoding, which is remarkably close to the theoretical optimum. The analytical results are verified by experiments using twenty-four test-chips [1,2] (see Figure 1).

Figure 1: Power per bit reduction (in percent) is illustrated for the optimum and the implemented Hamming scheme in twenty-four experimental chips. The optimum power reduction varies between 23-52%, while the corresponding numbers for implementation range from 12-46%.

[1]

A. Kumar, H. Qin, P. Ishwar, J. Rabaey, and K. Ramchandran, "Fundamental Redundancy Versus Power Trade-Off in Standby SRAM," ICASSP, April 2007.

[2]

A. Kumar, H. Qin, P. Ishwar, J. Rabaey, and K. Ramchandran, "Fundamental Bounds on Power Reduction During Data-Retention in Standby SRAM," ISCAS 2007, May 2007.