Energy Consumption Modeling and Optimization for SRAMs

This thesis is the second in a series that will be made available on this server.

The recent trends in portable computing technologies have established the need for energy efficient design strategies to maximize the operation time available between rechargings of the battery systems. There has been a significant amount of work directed at reducing the energy consumption through system level design changes. Opportunities exist for improving the energy consumption at the circuit level by altering the individual architectures and circuit designs of the CMOS chips.

To achieve these minimum energy design goals, system designers need a technique to accurately model the energy consumption of their design alternatives without performing a full physical design and full-circuit simulation. This technique must provide the designer with the appropriate amount of modeling detail to achieve their required accuracy goals while minimizing the development effort.

This dissertation presents and compares five approaches for modeling the energy consumption of CMOS circuits. These five modeling approaches have been chosen to represent the various levels of model complexity and accuracy found in the current literature. These modeling approaches are applied to the energy consumption of SRAMs to provide examples of their use and to allow for the comparison of their modeling qualities.

A mixed characterization model, built from a simple CV^2 energy model on digital- behavior circuit subsections, and circuit simulations on analog-behavior subsections, is best to use for estimations of the absolute energy consumption of a circuit. The mixed characterization model is also the best for predictions of the optimum organizations for minimizing the energy consumption in a given circuit.

The optimum organizations for minimized energy consumption in our SRAM designs are found to be non-square, containing more rows than columns. As the size and density of the memories increase, the number of rows and subarrays increases while the number of columns remains small. The scaling of the technology to smaller feature sizes does not affect the optimum energy organization. The optimum organizations for minimizing the memory access time are much more square (more equal number of rows and columns) than those for minimizing the energy consumption. The choice of the optimum energy organization will result in a significant tradeoff in the access time of the device.

An analysis of several common architectures and circuit designs for SRAMs shows that global, rather than local improvements, produce the largest energy savings. The best architecture determined for our process is a Multiplexed Internal Address Lines design, which distributes the internal address lines to the subarrays through a multiplexor, as opposed to the usual bussing architecture. The best architecture for minimizing the access time of the memory is also the Multiplexed design. Multi-bit word organizations produce a savings in the energy per bit read, suggesting that the energy consumption can be further optimized by placing as many bits per word accessed in the same physical memory device. Sequential-address access memories do not provide a significant energy savings over singly-addressed devices, although they can save energy at the system level. Sharing sense amps amongst multiple columns and subarrays will lead to a small energy savings, but this should ideally be minimized as the speed penalty is substantial.

The best precharge design for minimizing energy consumption contains two strong pullup transistors to limit the voltage swing of the column lines during the read cycles. The through-current associated with the pullup transistors during the read/write cycles is not significant in the selection of an optimum design. For timing considerations, the best precharge designs contain no pullup transistors, which produces the fastest bitline voltage response.

The analysis of the standard SRAM decoder circuits shows that the best design is an intermixed repeater/driver design, with the rows/columns physically arranged in Gray Coding order. These decoder design options offer no improvements in the time delay through the decoder stages of the device.

Robert J. Evans, 1994 (Under the supervision of Dr. Paul D. Franzon)