Extreme Voltage Scaling
Apr 15, 2009
Leyla Nazhandali won an NSF CAREER award to pursue work in subthreshold voltage investigations.
ECE’s Leyla Nazhandali hopes to cut the energy consumption of some embedded systems by more than 90 percent with an extreme version of a well-known power-saving mode that is currently used in many digital devices including laptops. She is applying subthreshold voltage technique, which she expects will also improve the security of embedded microprocessors by protecting them from power side channel attacks.
She received a National Science Foundation (NSF) Faculty Early Career Development Program (CAREER) Award to support her efforts. CAREER grants are NSF’s most prestigious awards for creative junior faculty members who are considered likely to become academic leaders.
Operating in the twilight zone
In subthreshold operation, the voltage is reduced below the level that turns transistors on and off. “Subthreshold voltage technique can be considered the extreme case of voltage scaling, which lowers the voltage source of the hardware,” she says. “As you reduce the voltage, energy consumption is reduced quadratically. That is a very good gain for what you are losing.”
Traditional voltage scaling typically is limited to about half the circuit’s nominal operating voltage. In subthreshold-voltage design, however, the voltage is reduced below the level needed to turn the system on.
“Designers typically avoid this twilight zone region,” she explains. When you operate in this region, the switch is not completely off and you are left with leakage current.”
The transistors are not switching as usual, but are able to complete the computation by modulating the leakage current that passes through them.
“Your computation is fine, it’s just much slower,” she comments. “In fact, the performance degradation becomes exponential in subthreshold region as you lower the voltage.”
Subthreshold-voltage operation with its unique advantage of lowering the power consumption by several orders of magnitude has opened new avenues of low-power design, according to Nazhandali. Most of the work to date has been simple FFT chips and microprocessors with low performance. “They have been targeted for applications that can tolerate a low speed, such as environmental or structural health monitoring sensors that wake up once a day, then go back to sleep,” she explains.
Her goal is to achieve the energy savings through subthreshold operation, but boost the performance by using parallel coprocessors making it possible for applications such as image and signal processing. “The subthreshold cores combined will deliver the performance expected from a single superthreshold core while collectively consuming less than 10 percent of its power consumption,” she says.
Nazhandali’s team simulated a prototype design for a popular DSP application. Using the same settings as a superthreshold embedded system, the multi-processor system at subthreshold will last 30 days running on a AA battery. The comparable conventional superthreshold system will last 10-12 days. “That’s a big difference. The longer battery use is particularly important in applications where battery access is difficult, such as heart defibrillators or remote operations,” she says.
While the energy savings are significant, there are several challenges to overcome with her technique. Her 16-core system is six times larger than a conventional system. “We are working to improve this, but the larger chips will still be the same size or smaller than the battery. When you add the sensors, the chip size becomes even less important,” she says. Potential applications include hand-held land mine detectors and security cameras.
Thwarting side channel attacks
Seeking the Encryption Key in Sub-Threshold Voltage Circuits:
Nazhandali meets with Steven Griffin, an undergraduate working in her laboratory. Griffin spent last semester using attack techniques to attempt to discover the encryption key in sub-threshold voltage circuits. This semester, he is developing a hands-on project for high school students to illustrate how important encryption is to everyday life. Griffin says that doing research as an undergraduate helps the students learn the skills they've used “while developing new and cutting edge ideas that may even have real world applications.”
Saving energy isn’t Nazhandali’s only goal. She also wants to improve the security of embedded systems. She expects her technique will help protect against power side channel attacks in which the secret key and data can be inferred by analyzing the power consumption pattern.
“In subthreshold operation, there are fewer peaks, because most of the power is leakage power. The power profile is not as revealing as with superthreshold technology,” she explains. “We can more easily hide the information that is given away by power consumption in regular designs.”
Her designs and architectures are partitioned into security-critical and non-critical regions, with security-critical running at subthreshold voltage. “You need very sensitive tools in order to measure power consumption of a device running at subthreshold. Plus, we can drown out the subthreshold operation with higher power elsewhere and still remain in low power.”
E-passports and credit cards
While subthreshold technology can improve security and energy use, it is slower than conventional operation. “We are initially targeting applications where security is important, but speed is not,” she says.
She suggests e-passports and credit cards as ideal candidates for the technology. “Being low-power is an important factor for e-passports as they are either running on a small battery that should last as long as the passport is not expired or they are getting power from tiny amounts of remotely induced power. However, speed is not critical because the interview and questions take longer than processing the passport,” she explains. “When security is very high, you are able to compromise on speed.”
Before the technology can be widely used in any application, however, “we need to develop architectures that can exploit the concept, along with design and simulation tools,” she says.
Nazhandali is collaborating with fellow ECE CAREER award recipient Patrick Schaumont on the security issues. “This is one of the wonderful aspects of Virginia Tech,” she says. “I wasn’t involved in security issues, but after talking with him about his work on side-channel attacks, I realized my technique had strong security potential.”
Encouraging women in computer engineering
Every CAREER grant has an educational component, and Nazhandali’s goal is to encourage women and underrepresented groups to pursue studies in computer engineering. “The under-participation of women and minorities in computer engineering significantly impacts society,” she says. “It threatens the competitive vitality of the workforce and the profession, limits the creativity of future technologies, and restricts employment opportunities of more than half the U.S. population.”
From a number of studies on the issue, she has concluded that illustrating how computer engineers benefit society is important to recruiting and retaining women students. “We suspect this is important for all other students as well,” she says. She also believes that a lack of role models and mentorship is an important reason why women leave the field.
Nazhandali prepares a circuit board for a hands-on unit to introduce younger students to embedded systems technology and design.
To combat this problem, she is developing an outreach program she calls “embedded for life.” Her team is developing three discovery-based teaching modules for pre-college and early college students. Each module presents a problem that can be solved with embedded systems and offers a hands-on project that engages students in problem solving with embedded systems.
The first module involves highway safety, discussing embedded systems such as anti-lock brakes, electronic stability control, and cruise control. Students then are asked to program a “smart” car to steer along a black line. Another module describes using biometrics for security authentication and involves the students in work with photo sensors and simplified fingerprints.
Nazhandali is arranging for the modules to be used with ongoing Virginia Tech campus outreach programs sponsored by the National Society of Black Engineers, the Society of Women Engineers, Association of Women in Computing, and Center for Enhancement of Engineering Diversity. The modules will also be offered at the national level in collaboration with the IEEE Teacher in Service Program.The smart vehicle project won an IEEE Real World Engineering Award in 2007 and is posted at www.realworldengineering.org.
Students are asked to program a “smart” car that was built by Nazhandali’s team to achieve several tasks including steering over a curvy black line. The car sports two independently controlled motors. By turning one motor slower than another, the car can steer left or right. The car has two infrared sensors that can detect if they are over the black line. The students do not need programming skills and only need to solve the problem in general terms, such as deciding which motor should run faster.
Students eventually improve on their solution as they try to increase the speed of the car and steer through sharper turns. In addition to learning about how embedded microprocessors can better our lives, the students discover actual physical constraints when designing such systems, including the tradeoff between speed of the car and accuracy of steering.