imec And Holst Centre Present ULP Processor Operating At Near-Threshold Voltage
At this week’s International Solid State Circuits Conference, imec and Holst Centre presented an ultra-low power processor that operates reliably at near-threshold voltages. The processor delivers clock speeds up to 1MHz at voltages down to 0.4 V. In tests based on a Fast Fourier Transform use case, it consumed only 79 µW – a fraction of the power consumption at standard voltages.“E
The new energy-efficient processor platform is customized for biomedical applications such as ECG and EEG monitoring. This was realized by creating an interface architecture around a general-purpose processor core to enable ultra-low voltage operation and automatic scaling of performance to improve energy efficiency, plus in-situ monitoring to guarantee reliability and high yield.
One of the key developments was the ability to reduce the operating voltage while delivering enough performance to meet application needs, and maintaining that performance over a range of operating voltages and temperatures. That 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 1.1 V and a minimum voltage of 0.4 V depending on the current performance requirements.
Natural variations in manufacturing processes can lead to voltage fluctuations when a processor is being used. At near-threshold voltages, these fluctuations can be enough to stop the processer working. To avoid this and ensure reliability, the team connected “canary flip-flops” to the most timing-critical parts of the processor. These are designed to fail before the processor’s circuits do and can be monitored – allowing the operating voltage to be scaled up before noise affects the processor. In addition, automatic bias control eliminates the usual voltage drop across the power switches that control the processor, further enhancing energy efficiency and reliability under near-threshold conditions.
To reduce energy consumption even further, the interface can control the state of individual components on the chip separately, for example turning off the processor core or reducing the voltage in the memory when these components are not required. The software interface can also dynamically switch the processor between various performance modes, optimizing the number of active functional units in the core to suit the algorithm being performed. Unused functional units are switched off to reduce power consumption.