
Rockbox mail archiveSubject: Re: Power Efficiency TradeoffsRe: Power Efficiency Tradeoffs
From: Bertrik Sikken <bertrik_at_sikken.nl>
Date: Mon, 21 Jan 2008 23:43:53 +0100 Burelli Luca wrote: > > On Sat, 12 Jan 2008, Catalin Patulea wrote: > >>  I believe (correct me if I'm wrong) that, in general, power >> consumption is proportional to the core frequency. Let the power >> consumptions of the cores be P_1 = k*f_1 and P_2 = k*f_2. > > An accurate estimation of power consumption in digital electronics is > not so easy to figure out. However, as a rule of thumb, you may assume > there's a constant power dissipation which is due to leakage (nonideal > switches allowing current to flow even where and when it should not), > and a "dynamic" power dissipation that is proportional to Vdd (the > switching voltage) and to _the square_ of the switching frequency. So > the above would be better written as P_1 = P_{1,leak} + k * f_1^2, > meaning that if you run the CPU twice as fast, you need four times the > energy (ignoring leakage). That's why clock throttling helps _a lot_ in > reducing battery drain! > > Hope I remembered things correctly from my University classes :) I think you have the relations mixed up. In one clock tick, a bunch of internal nodes acting as tiny capacitors need to be charged or discharged, dissipating an amount of energy equal to the energy contained in those capacitors. A charged capacitor C has energy E = 1/2 * C * V * V, which demonstrates the quadratic relation between voltage and consumed dynamic power. Increasing the clock simply means that this happens more often each second, which points to a linear relation between frequency and dynamic power. Kind regards, Bertrik Received on 20080121 Page template was last modified "Tue Sep 7 00:00:02 2021" The Rockbox Crew  Privacy Policy 