Power Management: Tests and Theory
Power consumption is central to this Intel platform, and they’ve made a lot of efforts in that department. Aside from the chipset, which consumes a lot of power in comparison to the processor, the Atom itself has many attractive functions.
Bus and cache
As we’ve already said, Intel has put a lot of effort into the bus and the cache: A different mode for the bus was developed (CMOS mode) and the cache can be disabled in part depending on how it’s being used. These functions reduce power consumption, as do the use of an in-order architecture and 8T SRAM for the L1 cache.
C6 power state
In addition to the low voltage (1.05 V) CPU, the Atom also introduces a new standby mode, C6. As a reminder, the C modes (0 to 6) are low-power states, and the higher the number, the less the CPU consumes. In C6 mode, the entire processor is almost totally disabled. Only a cache memory of a few kB (10.5) is kept enabled to store the state of the registers. In this mode, the L2 cache is emptied and disabled, the supply voltage falls to only 0.3 V, and only a small part of the processor remains active, for wake-up purposes. The processor can go into C6 mode in approximately 100 microseconds, which is quick. In practice, Intel claims, C6 mode is used 90% of the time, which limits overall power consumption (obviously, if you launch a program that requires a lot of CPU power or even watch a Flash video you won’t be in that mode).
We should point out, though, that the two chipsets to be used with the Atom N200s are power users: the Atom 230s use a i945GC that consumes 22 W (4 W for the CPU) and the Atom N270s ship with a i945GSE that burns 5.5 W (2.4 W for the CPU).
So is the Atom really low-power in practice? The processor is, yes. For the platform aimed at NetTop (low-cost desktop computers), the answer is yes, but... Why the “but”? Because the chipset used uses a lot of power and the processor is listed at a TDP of 4 W, compared to 2.4 W for the mobile versions. Our test motherboard consumes 59 W in standby, and we reached 62 W under maximum load (with a 3.5" hard disk and a 1 GB DDR2 DIMM). Obviously, these values are what we measured for the complete platform, not only the motherboard, and they don’t take power-supply losses into account (our test model has a yield of approximately 80%). That’s both a little and a lot – it’s not much for a desktop computer, of course, but it’s a lot in absolute terms. We should add that we recently tested a motherboard based on a 1.5 GHz Via C7, and the configuration drew less power with the same components: 49 W at idle and 59 W under load (always measured at the AC outlet).