27.3 A 210mV 5MHz variation-resilient near-threshold JPEG encoder in 40nm CMOS

Abstract

Operating circuits in the near-threshold region enables large energy savings. However, such circuits also pose many challenges, such as increased delay, unwanted leakage paths and high sensitivity to variations. Working in advanced nanometer CMOS technologies compromises the robustness of circuits even more due to the increased variability. Nonetheless, these technologies offer higher operating frequencies for ultra-low-voltage circuits. Transitioning to smaller technologies is attractive for digital near-threshold circuits, provided that the impact of the increased variability can be mitigated. Few prior works have considered the design of variation-resilient ultra-low-voltage circuits in CMOS technologies smaller than 65nm. The aim of this work is to design a large system that advances the state-of-the-art by not only reaching very low energy consumption, but also clock frequencies of tens of MHz, while providing high variation resilience. We present a full JPEG encoder fabricated in a 40nm CMOS technology, fully functional down to a supply voltage of 210mV. The JPEG encoder is able to operate at clock frequencies in a range from 5 to 275MHz for supplies from 210 to 550mV, and thus achieves very high ultra-low-voltage speed. At the minimum-energy point (MEP), the JPEG encoder consumes only 29.01pJ/pixel at an operating frequency of 41MHz (V DD =330mV). The variation (σ/μ) of 26 dies at this point is only 9.4% for the frequency and 6.0% for the energy consumption.

A 16.07pJ/cycle 31MHz fully differential transmission gate logic ARM Cortex M0 core in 40nm CMOS

“…The structure of a TG allows to create any logic function with the same generic scheme and layout, making it easy to create a library consisting of lots of cells with different functionalities. The benefits of stacking and differential TG logic (see [4]) more than outweigh their increased design complexity. Signal degradation due to a lack The cell library is characterized in a differential way.…”

“…Operating the microcontroller at ultra-low voltage can help to meet these strict energy requirements, yet this strategy is plagued by 3 conflicting design challenges: 1) ensuring reliable low voltage operation under the increased variability of advanced nanometer CMOS technologies; 2) maintaining an operating speed >10MHz, necessary to cover a wide range of applications, under ultra-low voltage operation; 3) integrating low voltage variability robust design into a standard cell design flow, avoiding the need for full custom design. Current state-of-theart low-voltage 32-bit microcontrollers fail to combine these 3 aspects, either suffering from sub-MHz operating speed [1]- [3] or needing full custom circuit design to ensure variation resiliency [4].…”

New devices for internet of things: A circuit level perspective

“…Currently most energy efficient NT processors have a V DD ranging from 200 to 400 mV. [4]. While lowering the supply voltage a small off current should be maintained.…”

“…This enables the design of the high performance processing systems of today, operating at a clock frequency up to a few GHz. Energy efficient IoT nodes run at a clock frequency which is at least a factor of 10 lower: 10 to 100 MHz [3] [4]. Since the delay performance is actually governed by the equation t d =CV dd /I, devices with a current capability of 50 to 100 A/ m are acceptable for the design of IoT nodes.…”

On the effect of technology scaling on variation-resilient sub-threshold circuits

“…The MAC was chosen since it is a block that is very frequently used in DSP designs and because it is a complex block that includes feedback. Since the MAC is the critical component of DSP designs, it is also possible to design a processor that achieves similar ultra-lowvoltage characteristics as this MAC with the same design principles [10]. Important to note is that the aim was to design variation-resilient circuits which are able to operate at both very low energy consumptions and n × 10 MHz-speed to increase the industrial relevance of ultra-low-voltage circuits.…”