Extreme Processing Thresholds: Challenges Designing Low Power Devices

Friday, April 23rd, 2010 by Robert Cravotta

[Editor's Note: This was originally posted on the Embedded Master

The low power thresholds for processor devices continue to drive downward, but what does it take to drive those energy thresholds ever lower? In many cases, it is possible to reduce a processor’s active current draw by migrating to a more aggressive silicon node, but this comes at the cost of standby power or leakage current. To balance between lower active and standby power, processors rely on low leakage silicon variants and optimally sized transistors within each block of the architecture.

Øyvind Janbu, CTO at Energy Micro, points out that designing circuits that are going to be enabled 100% of the time, such as power supervision circuits with brown-out and power-on-reset functions, is challenging to design to an energy budget of a few nA of current. As with all parts of the processor, chip architects trade-off between speed, energy draw, and accuracy at each resource block to best meet the needs of the specific function and the overall system requirements. He believes that because flash memory is power hungry and slow, in time, it will be replaced by other non-volatile technologies in many cases.

Janbu also feels that some of the challenges when designing for extremely low power chip designs are similar to those faced by RF designers. He believes the accuracy of simulation models of transistors are being pushed outside their intended operating region, and this means that the architects must specify sufficient margins so that the designs still work in volume production. There is a need for more extracted layout simulations because of high impedant nodes due to extremely low currents.

Internal voltage regulators are another low power challenge as microprocessors continue to move into more aggressive silicon nodes. While using internal voltage regulators helps reduce active power consumption, the challenge lies in designing voltage regulators and voltage references that have zero quiescent current, so as to not sacrifice the standby power consumption. The voltage regulator and voltage reference are basically the reason why microprocessors made in 0.18 um or smaller silicon nodes have standby current consumption in the tens of uA range.

The future direction of low power processors may center on modular architectures because driving to extremely low power requirements increasingly requires a rethinking of the fundamental architecture of each module. This rethinking increases the design time and risk of the processor, especially when the architects are exploring and implementing new and unproven approaches. However, as the market finds more uses for low power devices, the increased volumes will provide the needed offsets to incur the longer design cycles and higher risk to push the power threshold even lower.


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