COTS Dynamic Voltage-Frequency Scaling

The dynamic nature of BASNs can be leveraged to achieve power efficient, on-node signal processing through dynamic voltage-frequency scaling (DVFS). For example, in tremor analysis, there will be periods of little activity where resource requirements will be lower since data can either be ignored or fewer processing can occur.  One scenario might process a tremor signal with the Teager Energy Transform on a body-worn inertial node when tremor occurs, but only monitor gross movement when no tremor is detected.  However, most COTS components that are used on BASN nodes are not specifically designed for DVFS operation. Our work [1] has presented techniques for scaling frequency and voltage operation on the TI MSP430, a typical BASN microcontroller found in TEMPO 3.1.

The work adjusts voltage operating point by setting the output of a voltage regulator using a digital potentiometer. The figure below shows the possible steady-state operating points of the MSP430 as measured on actual hardware. There are other combinations of frequency and voltage, but this assumes that we operate at the lowest voltage possible for a given frequency to maximize energy savings.

An equation for power consumption of the MSP430 can be extended from that of a general CMOS switching circuit to involve a constant from external circuitry overheads and frequency operation requirements of the MSP430. When a fit of this form is performed to the power consumption data shown in the figure above, the following equation is derived for the power consumption at any voltage (assuming the maximum operating frequency at that voltage):

 

This fit-line has a Pearson correlation coefficient of 0.999 and is shown as the thick trace in the figure above. Using the fit equation, a management scheme could calculate the desired voltage given a power requirement or the power consumption given an operating voltage in real-time.
Transient analysis was also performed to measure the overheads associated with switching between any two voltages. This determines the “break-even” cycles/times. In other words, switching between any two voltages for less than the break-even cycles would result in energy loss. The figure below shows the break-even cycles and times for the experiment setup in [1]. One should note that the times are on the order of a single sampling period for motion applications. This suggests that the data is changing very slowly in comparison with how fast the voltage and frequency can be adapted with energy savings.  Using DVFS with COTS components allows for low-cost, low-power wireless nodes to adjust operating parameters to application and environment requirements.  

 

Relevant Projects:

TEMPO 3.1          Tremor Assessment          Energy-Fidelity Scalability


References

  1. Powell, Jr HC, Barth AT, Lach J.  2009.  Dynamic voltage-frequency scaling in body area sensor networks using COTS components. Proceedings of the Fourth International Conference on Body Area Networks. :1–8.