Power is the heart of the matter

Low-power design often involves apparently counter-intuitive decisions. In an experimental wearable heart monitor developed at the European research institute IMEC, it made sense to do more local signal processing on the battery-powered sensor module itself than to offload that to a host computer.

The reason for the change lay in the wireless interface between the sensor module and the computer that would record and store the heart-beat signal from a patient. Speaking at IMEC’s International Technology Forum last month, Harmke de Groot, project director of of ultralow power digital signal processing (DSP) and wireless, said: “The radio represents 80 per cent of the power consumption. We could do two things. We could make a better radio. But the other thing we could do is more local processing so that we have less data to be sent over the air.”

In the first iteration of the design, the researchers hunch about the radio was correct. They slashed power consumption there. “But the processing power needed from a commercial microprocessor was so much that the gain from compressing the data for radio transmission was lost. We decided we needed a specialised, biological processor that could do this.”

The group put together what it calls the BioDSP, which adds hardware support for wavelet transforms – which have proven to be very useful in compressing biological data such as ECG measurements compared with the cosine- and Fourier-based techniques used in audio and video – to a conventional DSP core. IMEC reckons that, in terms of power efficiency, the BioDSP is ten times more power efficient than Texas Instruments’ MSP430 16bit microcontroller, which is very popular in systems such as power meters because of its low energy usage.

The second-generation BioFlux processor uses the CoolFlux DSP core from NXP Semiconductors. The team kept the architecture and used it as the basis for trying out a number of low-level power-reduction techniques, such as controlling the forward and reverse bias of transistors and specialised low-voltage logic cells.

“We focused on making sure that the standby and leakage power was reduced significantly. The processor is only active 0.13 per cent of time,” said de Groot, so low current leakage in the standby state is crucial as that is where most energy will be wasted. “Our innovation is not the processor core but everything around it. The BioFlux is being used as the driver for all sorts of low-voltage techniques.”

De Groot explained that the processor runs at close to the threshold voltage to try to reduce power but does not delve into the subthreshold region where it is possible to make further savings, at the expense of circuit performance.

The move to subthreshold is likely to come with a follow-on architecture that sees another level of partitioning to try to save power. The team noticed with the BioDSP that using a separate analogue front-end can consume a lot of power. So, the analogue circuitry will move onto what de Groot called the ECG SoC. This SoC will see the reintroduction of specialised execution units for operations such as wavelet transforms and noise reduction.

“We are looking to see if we can do certain parts using subthreshold logic. There is a problem in that library characterisation for the target process is not as good as it could be in the subthreshold region,” she explained. “We are now comparing the models to test chips we have made. One of the questions we need to answer is that the active power goes down but the leakage power goes. We are looking to see whether a lower voltage of 0.2V or going to 0.3V or higher will give us the optimum power consumption.

The work on this project at IMEC is a microcosm of the trade-offs that engineers need to make to eke the most power out of low-energy designs. I’ll be returning to those themes on this new blog that will focus on low-power design – from the system level down to the physical.

Posted by Chris Edwards

The Low-Power Design Blog is sponsored by Mentor Graphics. The company has focused years of R&D on low-power design techniques and is glad to support a resource that highlights creative methods for reducing the power consumption of electronic systems.