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Using sub-threshold techniques for IC design

Posted: 07 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Sub-threshold? energy consumption? transistors? super-threshold? non-volatile memory?

So the overall strategy is to use sub-threshold circuits throughout by default, use super-threshold in those few cases where it's possible, and use near-threshold in those few cases where required for speed or bandwidth.

Ambiq Micro leverages all of these techniques as needed; this diversity of options is a critical characteristic of the SPOT platform. In particular, Ambiq's circuits involve the extensive use of dynamic, adaptive strategies that keep the circuits operating optimally even as conditions change. Sub-threshold design can be frustrating, with solutions to one problem creating new problems in whack-a-mole fashion. To some extent, it's simply hard work done by engineers skilled in sub-, near-, and super-threshold design that has allowed Ambiq to design circuits that overwhelmingly rely on sub-threshold circuits.

Design and logistics impact
A great deal of effort has gone into ensuring that Ambiq's sub-threshold circuits leverage existing established flows wherever possible. Custom processes might make life easier, but they're not required, and Ambiq's focus is on using what is already known to work well.

The design flow was impacted based on the number of custom cell libraries and the sheer number of corners to be verified, given the various design techniques available to manage the circuit sensitivities. These design flow challenges are being encountered in the super-threshold world at the 28-nm process nodes, so solutions exist. It's just that Ambiq has leveraged those solutions at more widely available process nodes. Importantly, Ambiq's SPOT technology can also be scaled to lower geometry processes for even more energy savings as those nodes become more mainstream.

Testing challenges such as the need to measure low currents were addressed by creating complex custom probe cards and on-chip test circuitry. Those cards include specific custom current-measuring circuits that handle the measurements that the tester itself cannot manage.

Finally, the characterisation flow had to be much more thorough than what would typically be done for a super-threshold design. It necessitated more detailed measurement under many more conditions and combinations of conditions than would typically be done. The impact of this is greater confidence in the robustness of the product.

In general, no step of the design and manufacturing flow has escaped scrutiny. Where elements of the standard flows have fallen short, Ambiq has modified them to ensure that the resulting product is indistinguishable from something built using super-threshold techniques C with the exception of energy consumption.

Reliability
Creating novel circuits means not only building something that works now, but also ensuring that the circuits will operate correctly for the life of the chip. For a system designer, reliability expectations will be the same regardless of the particular circuit techniques involved.

For that reason, sub-threshold circuits built on Ambiq's unique SPOT platform have been subjected to the usual battery of reliability tests, involving multiple lots exposed to extreme conditions over extended time periods as well as other standard tests such as electrostatic discharge (ESD).

Conclusions
The use of sub-threshold techniques can be a powerful way to create circuits that consume dramatically less energy than those built using standard design practices. It's a fact that sub-threshold design is difficult. But, given the right experience and diligence, it is a solvable problem, and one that Ambiq continues to solve via their patented SPOT technology.

The result of these efforts are circuits that provide the same functions as more traditional ones using a fraction of the energy. There is no compromise in performance, robustness, or reliability; Ambiq's chips can operate alongside their traditional counterparts with no externally-visible difference C except for the amount of energy required to drive them. They can provide important energy savings to designers building energy-efficient systems. Because of the fundamental nature of these innovations, sub-threshold design techniques can be applied to virtually any type of IC device.

About the author
Mike Salas is the vice president of marketing at Ambiq Micro.


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