Power management, 2011

Jack Ganssle

January 30, 2011

Jack Ganssle

TI takes DVFS even further with their SmartReflex. This is a gestalt of the hardware resources noted above, extra undocumented hardware in the silicon, plus software that dynamically and automatically manages power to squeeze the most out of every electron drawn from the battery. It's very proprietary and secret but a paper gives some details.⁶ TI claims SmartReflex can reduce leakage current by three orders of magnitude using esoteric techniques that apply odd biases to “modulate the body voltage of transistor cells or blocks.” I have no idea what that means or how it works, but it's clear an awful lot is going on at the transistor level in the silicon.

Then there's battery management, which, in the case of the OMAP takes place either in the external TWL5030 power controller or yet another IC. Two general approaches are used—or, sometimes, both together. An A/D can monitor the battery's voltage, but that's a poor indicator of reserve capacity. Most batteries have a discharge curve with a rather sharp knee; pass the knee and the battery will rather suddenly run out of juice. Better is to measure amp-hours charged into the cell and withdrawn, and then apply corrections for battery temperature and aging. In fact, the TWL5030 companion chip also measures the battery's temperature, looks for over-voltage conditions, manages charging via a USB interface, and looks for over-current conditions. There's a lot going on!

This is but a simplified discussion of the OMAP's PRCM since the thing is bewilderingly complex. And there's a lot going on that TI won't divulge. I doubt that it's possible to actually use one of these parts without a lot of interaction with the vendor, but in the mobile world the volumes are so high that I'm sure vendors and customers form close engineering partnerships.

The world of power management is far bigger than the simple sleep modes we use on our smaller controllers. It's great for consumers who get devices that will run for days on a charge. I wonder if these techniques will find their way into other consumer appliances, like TVs, in the days ahead, which promise perhaps significantly higher energy costs.

Jack Ganssle (jack@ganssle.com) is a lecturer and consultant specializing in embedded systems' development issues. He has been a columnist with Embedded Systems Design and Embedded.com for over 20 years. For more information on Jack, click here.

Endnotes:

1. Altera. “40-nm FPGA Power Management and Advantages,” white paper, December 2008, v.1.2, available at www.altera.com/literature/wp/wp-01059-stratix-iv-40nm-power-management.pdf.
2. Fallah, Farzan and Massoud Pedram. “Standby and Active Leakage Current Control and Minimization in CMOS VLSI Circuits,” IEICE Trans. on Electronics, Special Section on Low-Power LSI and Low-Power IP, Vol. E88-C, No. 4 Apr. 2005, pp. 509-519, available at SPORT Lab: http://atrak.usc.edu/~massoud/Papers/IEICE-leakage-review-journal.pdf.
3. Texas Instruments. MSP430x5xx/ MSP430x6xx Family User's Guide, June 2008/revised December 2010, available at http://focus.ti.com/lit/ug/slau208h/slau208h.pdf.
4. Microchip. “PIC16F/LF1826/27 Data Sheet 18/20/28-Pin Flash Microcontrollers with nanoWatt XLP Technology,” 2010, available at http://ww1.microchip.com/downloads/en/DeviceDoc/41391C.pdf.
5. Texas Instruments. OMAP35x Applications Processor Technical Reference Manual, available at http://focus.ti.com/lit/ug/spruf98m/spruf98m.pdf.
6. Carlson, Brian and Bill Giolma. “SmartReflex Power and Performance Management Technologies: reduced power consumption, optimized performance,” white paper, Texas Instruments, February 2008, available at http://focus.ti.com/lit/wp/swpy015a/swpy015a.pdf.