Abstract
To minimize energy consumption of a digital circuit, logic can be operated at sub- or near-threshold voltage. Operation at this region is challenging due to device and environment variations, and resulting performance may not be adequate to all applications. This article presents two variants of a 32-bit RISC CPU targeted for near-threshold voltage. Both CPUs are placed on the same die and manufactured in 28 nm CMOS process. They employ timing-error prevention with clock stretching to enable operation with minimal safety margins while maximizing performance and energy efficiency at a given operating point. Measurements show minimum energy of 3.15 pJ/cyc at 400 mV, which corresponds to 39% energy saving compared to operation based on static signoff timing.
Highlights
With constantly tightening power budgets and incentives to add new features, energy consumption has become one of the most important aspects of portable electronics
The minimum energy consumption of digital logic is traditionally achieved by utilizing a supply voltage which is below the transistor threshold voltage [1]
With the exception of time borrow detector (TBD) and level shifters, all gates were from vendor standard cell library, which was re-characterized at the target operation point
Summary
With constantly tightening power budgets and incentives to add new features, energy consumption has become one of the most important aspects of portable electronics. The minimum energy consumption of digital logic is traditionally achieved by utilizing a supply voltage which is below the transistor threshold voltage [1]. Operation in this sub-threshold -region has several practical limitations such as radically reduced performance and exponentially increased variability. Recent manufacturing processes have pushed the minimum energy point (MEP) towards the threshold voltage, which mitigates performance and variance concerns and makes near-threshold operation much more attractive and practical. Near-threshold computing has recently gained increasing amount of attention due to its suitability to IoT and other extremely energy-limited applications which yet have moderate performance requirements
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