I finally got around to trying to undervolt my notebook's CPU to get a little better battery life out of it. I know the rule for the standard-voltage C2Ds: the minimum Vcore the processor will accept is the lowest figure in the idle voltage range, as per the Intel C2D specs- 0.950 V. So I grab a copy of the linux-PHC patches and downgrade my OS to the 32-bit version because the PHC patches won't work on x86_64. I get it patched and away I go.

My particular chip isn't a standard-voltage C2D T-series but a U7500 ULV unit. It has two speeds for EIST- the 800 MHz idle as it's on a 533 MHz bus and the 1.067 GHz top speed. The stock voltages are 0.850 volts for idle and 0.875 volts for top speed. The C2D documentation says that the idle voltage range for the ULV series ranges between 0.750 and 0.900 volts and the top-speed range is 0.800-0.975 volts. So I try to drop the idle voltage down one VID notch to 0.8375 volts and nothing happens. Odd. I drop the 1.067 GHz VID down a notch to 0.8625 V and it works. It will also run at 1.067 GHz at the same 0.850 V as the idle speed, but I cannot drop the voltage below the stock idle voltage. I booted the machine with Windows and RMclock ran into the same problems.

So do the ULV chips simply have the minimum voltage lock at the stock idle speed voltage rather that at the minimum idle voltage as reported in the documentation like the standard-voltage chips? If so, that's pretty crummy as I suppose I didn't really get the luck of the draw with the chips then, and there's not much I can do about it (it's a BGA chip.)

I am running Kubuntu. I normally downgrade the speed from the GUI. There is a tiny battery icon, and right click I can reduce the speed. I wonder if it helps the battery?