FD-SOI, Body-Biasing Shine in 10x Faster DSP With Ultra-Wide Voltage Range
Posted date : Feb 20, 2014

Body-biasing design techniques, uniquely available in FD-SOI, have allowed STMicroelectronics and CEA-Leti to demonstrate a DSP that runs 10x faster than anything the industry’s seen before at ultra-low voltages (read press release here).

In the mobile world (not to mention the IoT), the role of DSPs is becoming ever more important. All those things you do with your phone – snapping pictures, listening to music, watching video, oh yeah, even talking and listening –  involve the DSP’s number-crunching prowess. It takes real-world analog information that describes sound, pressure, light, and temperature and mathematically optimizes and processes them in real-time, so the data can be displayed, analyzed, compressed, enhanced, or converted. The DSP’s raison d’etre is to maximize work-per-clock-cycle.

When product designers talk about “user engagement” with their portable device, chances are there’s a DSP involved. And the better the DSP does its job, the cleaner your sound, the clearer your picture, the faster your download, and the more easily you can converse. But all this processing comes at a price – an energy price.  Chip designers are always looking for better ways to improve the power-performance trade-off, so that we as users get all this great performance without running out of batteries.

Power is directly proportional to clock frequency (MHz, GHz) and the square of the voltage that’s supplying the device. So, if you have a device that operates at higher frequencies with lower supply voltages, you’ve got a big edge on saving power – and of course, the less power you pull from your battery, the longer your battery will keep you snapping, listening, watching, and conversing.

That’s what’s at play here with this Leti/ST news. They’ve demonstrated a DSP that can hit 500 MHz while pulling just 460mV – that’s ten times better than anything the industry’s seen so far.

In fact, Fabien Clermidy, head of Digital Design and Architecture at Leti told ASN that this could mean extending your battery life by about another 30% for typical usages.

Leti and ST showed the FD-SOI DSP at ISSCC – the IEEE’s International Solid-State Circuits Conference (February 2014), which is widely considered the premier forum for presenting advances in solid-state circuits and SOCs.


Ultra-Wide Voltage Range

Specifically, at ISSCC Leti and ST presented the successful demonstration of an ultra-wide-voltage range (UWVR) DSP, based on 28nm ultra-thin body buried-oxide (UTBB = ST and Leti’s flavor of) FD-SOI technology.

This may be the first you’ve heard of Leti/ST’s UWVR, but it’s been making the conference rounds over the last year. Leti/ST presented it at the DATE Conference in 2013. (You can get the paper on the IEEEXplore site – click here.) In that paper, Leti and ST engineers demonstrated the technology on an ARM A9, where they showed performance boosted by 40% to 200% without added energy cost. Conversely, when saving power is more important than boosting performance (which turns out to be about 90% of the time!), FD-SOI reduced leakage power by a factor of 10 using Reverse Body Biasing.

Leti and ST also presented FD-SOI in the memories section at ESSCIRC ’13, where they applied it to a 28nm FD-SOI SRAM bitcell array, noting “…over 10x energy gain is achieved from 1.2V down to 0.35V range while measuring high speed at nominal voltage.” (This paper is also available from the IEEEXplore site – click here.)

In the case of the demonstrator DSP presented at ISSCC, the demonstrator was  produced by ST in their 28nm UTBB FD-SOI process technology. The UTBB FD-SOI allows:

  • body-bias-voltage scaling from 0V to +2V,
  • decreased minimum circuit operating voltage,
  • and clock-frequency operation of 460MHz at 400mV.

To the UWVR innovation, ST and Leti have added optimized standard-cell libraries they had developed to cover the 0.275V to 1.3V range. They then were able to leverage the voltage scaling they get with FD-SOI with system clocking techniques in the optimized cells, including:

  • non-overlapping pulses;
  • fast pulse-triggered flip-flop devices designed for variability tolerance at low voltage;
  • monitoring on-chip timing-margins to dynamically adjust the clock frequency to a few percent of the maximum operating frequency, independent of supply-voltage value, body-bias-voltage value, temperature, and process technology.

As a result, even at 0.4V, the DSP exhibits 10x better operating frequency than the previous state-of-the-art.


(Courtesy: ISSCC, STMicroelectronics, CEA-Leti)

Clermidy also adds that their innovative design techniques reduced design margins, thus avoiding over-design. Again, doing this in FD-SOI rather than bulk was key, since FD-SOI really reduced variability issues.

The Leti/ST ISSCC paper, which is entitled, “A 460MHz at 397mV, 2.6GHz at 1.3V, 32b VLIW DSP, Embedding Fmax Tracking,” was presented on Feb. 12, during Session 27, “Energy-Efficient Digital Circuits..  A demonstration kit was shown to attendees. As of this writing, it’s not yet been posted on the IEEEXplore site, but it should be in the weeks to come.

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