ISSCC: IMEC reports threshold-voltage processor

LONDON – Engineers from research institutes IMEC (Leuven, Belgium) and the Holst Center (Eindhoven, The Netherlands) have produced a processor circuit that operates at 0.4 volts.

The processor, designed for use in monitoring of electroencephalograph (EEG) and electrocardiograph (ECG) information operates at close the threshold voltage of the 40nm CMOS process and at clock frequencies of up to 1MHz.

The processor includes nine 32-bit functional unit, from FU0 to FU8, providing inherent scalabilty through the array of dynamically reconfigurable processing units, In tests based on computing Fast Fourier Transforms, IMEC said it consumed 79-microwatts, a fraction of the power consumption at standard voltages.

The architecture of the IC includes a general-purpose processor core to enable ultra-low voltage operation and a system of in-situ monitoring for computational need and automatic scaling of performance.

The ability to maintain reliable operation over a wide range of operating voltages and temperatures was achieved by forward biasing the transistors within the processor, allowing it to operate at voltages just above the threshold for the CMOS process used. The operating voltage can be adjusted between the processor’s nominal voltage of 1.1V and a minimum voltage of 0.4V depending on the performance requirements.

The natural variability of manufacturing processes can lead to voltage fluctuations causing such a processor to stop working. To prevent this IMEC/Holst included "canary" flip-flops to the most time-critical parts of the circuit. These are not part of the circuit functionality but are designed to fail before the processor circuit does. By monitoring these "canaries" the operating voltage can be scaled up before noise affects the processor. In addition unused functional units are switched off to reduce power consumption.

Harmke de Groot, ultra-low power program director at Holst, said that much of the industry research is still aimed at improving performance rather increasing energy efficiency. "At Holst Centre, we focus on low power and low voltage to enable battery-powered and energy scavenging smart devices," de Groot said in a statement issued by IMEC.





IMEC/Holst SoC for ExG applications in 40-nm CMOS operates at 1MHz and 0.4-volts


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