Article Error...well, sort of.
Posted: Wed Aug 28, 2002 9:30 pm
In "The Ultimate Underclock and Undervolt Project" article, the editor disagrees with the author's power consumption equation for a CPU with the following statement:
>" ...(Editor's note: The relationship between CPU clock speed and
> power dissipation is linear...Leo's actual...heat drop is closer to 4.5%"
This is not correct. Every CPU (and digital circuit in general) has a certain steady-state power draw due to leakage current. Decreasing the clock rate on a modern cpu certainly does _not_ result in a linear decrease in power consumption, for this very reason. I work with embedded cpus on a daily basis, underclocked to allow for battery-powered operation. For one of these, lowering the clock rate from 16mhz (yes only 16mhz) to 8, and its power consumption is 79% of nominal (not 50%). Lowering it all the way to 1 mhz reduces power only to 45% of nominal, not the 6% a linear decrease would predict. And reducing it all the way to 0.1 mhz (about as slow as it can stably function), it still uses 41% of the power the original 16mhz processor does, while running 160 times slower.
For older ICs done on .25 micron or larger processes, the leakage current term was usually rather. However, it is has grown exponentially as transistor size decreased with smaller lithography. This is one of the hottest topics in modern EE design-- at nodes below the upcoming .09 micron that Intel is now designing chips for (the P4 is, of course, on a .13 micron process), leakage current will dominate, and instead of our digital circuits being filled with on-off "switches", they're made of leaky buckets.
>" ...(Editor's note: The relationship between CPU clock speed and
> power dissipation is linear...Leo's actual...heat drop is closer to 4.5%"
This is not correct. Every CPU (and digital circuit in general) has a certain steady-state power draw due to leakage current. Decreasing the clock rate on a modern cpu certainly does _not_ result in a linear decrease in power consumption, for this very reason. I work with embedded cpus on a daily basis, underclocked to allow for battery-powered operation. For one of these, lowering the clock rate from 16mhz (yes only 16mhz) to 8, and its power consumption is 79% of nominal (not 50%). Lowering it all the way to 1 mhz reduces power only to 45% of nominal, not the 6% a linear decrease would predict. And reducing it all the way to 0.1 mhz (about as slow as it can stably function), it still uses 41% of the power the original 16mhz processor does, while running 160 times slower.
For older ICs done on .25 micron or larger processes, the leakage current term was usually rather. However, it is has grown exponentially as transistor size decreased with smaller lithography. This is one of the hottest topics in modern EE design-- at nodes below the upcoming .09 micron that Intel is now designing chips for (the P4 is, of course, on a .13 micron process), leakage current will dominate, and instead of our digital circuits being filled with on-off "switches", they're made of leaky buckets.