A new energy-efficient chip designed by researchers at MIT may use so little power that someday human body heat will be able to charge implantable medical devices.
The new chip design, which researchers say will reduce power consumption by 10 times, was unveiled earlier this month at the International Solid State Circuits Conference in San Francisco. The chip, still in the proof-of-concept stage, is expected to be used in portable electronic devices, like cellphones, PDAs and even implantable medical systems.
"We intend to implant these new low-voltage techniques as quickly as we can," says Dennis Buss, chief scientist at Texas Instruments. TI engineers worked with MIT researchers on the two year project. "To get to where we'd need to be with this will take about five years. Doing a research demonstration is an important step, but making it robust for commercial production will require some work."
The key to the chip's improved energy efficiency lies in making it work at a reduced voltage level, says Joyce Kwong, a graduate student in MIT's Department of Electrical Engineering and Computer Science and a member of the chip design project team. Most of the mobile processors today operate at about 1 volt. The requirement for MIT's new design, however, drops to 0.3 volts.
When you use the standard equation to calculate how much power a chip will use, the voltage is squared. That means if voltage is increased, power consumption will jump. However, if voltage is decreased, power consumption will plummet.
"Voltage is critical," says Jim McGregor, an analyst at In-Stat. "All these handheld devices are being asked to do more and more. To be able to decrease voltage lets you increase silicon complexity to handle more functions, and it also increases battery life, which is a critical component in multiple applications."
However, the voltage required by a cellphone or handheld device depends on what the device is doing. TI's Buss explains that if the chip is idle, 0.3 volts will be enough to operate the chip. However, if the device is doing something that requires high speed, the chip will need to use more voltage. And the chip is designed to scale between higher and lower voltages.
"The concept we have calls for dynamic voltage scaling," he adds. "The voltage can be increased or decreased depending on how much computation you have to do. When you need to go fast, you turn up the voltage. When you have a sleep mode or standby, then you can drop the voltage and save substantial power."
Buss explains that the chip uses DC-to-DC converters, which control the voltage levels. Multiple converters, he adds, can set different levels depending on a device's function is at a specific moment. Moving to low-voltage levels whenever possible will save energy consumption.
"If I was using this chip design in a Blackberry, I could get substantially more battery life," he says. "How much more depends on the applications being used."
MIT's Kwong says researchers had to redesign the memory and logic circuits to get the chip down to a 0.3 voltage. "Basically, in the memory, instead of the classic six-transistor bit cell, we needed to use an eight-transistor bit cell," she says. "This helps with reading from the memory. With more transistors, it makes it harder to disturb the data within the cell when we do the read operation. If you slip the data in the cell, then you've caused an error because now the data is wrong, and that would cause the chip to go to a higher voltage."
She also says the MIT team re-designed the building blocks of the logic circuits to make them less susceptible to variations in the manufacturing process.
Buss says the power needed to operate the chip is becoming so small that someday human body heat or even movement could be converted to power the chips that are running in implantable medical devices, like heart monitors.
"Power consumption is getting to the level that scavenging becomes possible," he says. "That means you could obtain voltage from body heat or motion. In a watch, for example, as you move your wrist, the motion generates power. If we can make these medical functions as low power as a watch, then we can think about converting body heat or motion. It's possible but it's not in production today. It's a research topic still."
Dean McCarron, president of Mercury Research, notes that in the 1960s and early 1970s, an average computer chip used about 12 volts. Ten years ago, that number was down to 5 volts, and it only dropped down to 1 to 2 volts within the past three years, he says.
"Lowering voltage is actually the standard for lowering system power," he says. "The challenge is that when voltage gets to a certain level, generally around 0.8 to 0.9 volts, making the chip work becomes more difficult. You know, 0.9 was thought to be the floor, and these guys have broken through the floor."