What Sets A CPU's Lowest Speed?

[DAnDan]

Hopefully this isn't a stupid question, just something that came to mind the other day... What determines a processor's lowest clock speed?

For example my laptop can have a clock speed of between 1.1 and 2.6 GHz, but that determines the lower limit? Is it simply the point where power savings below that point aren't worth the extra performance trade off, or is there more to it?

 

[Lysander]

Sausages.

 

[StriderX]

Eggs.

In all seriousness however, It is likely to do with heat, as is all electronics when it comes to power consumption, but i am no hardware guru.

 

[Pug]

Trying to be a little constructive I'd say there is a threshold relating to the % usage of the CPU, say it goes above 40% it clocks up to compensate.

 

[DAnDan]

Trying to be a little constructive I'd say there is a threshold relating to the % usage of the CPU, say it goes above 40% it clocks up to compensate.

Thanks but that's not quite what I mean. Most CPUs seem to have a lower limit as to where they'll drop the clock speed to when the CPU is under low load, so I was wondering what determines the limit.

A modern processor with a lower clock limit of 1.1 GHz, would this be able to run in the MHz range, with the obviously crippled performance, or would it not perform properly?

Eggs.

In all seriousness however, It is likely to do with heat, as is all electronics when it comes to power consumption, but i am no hardware guru.

For the upper limit of the clock speed definitely, but at lower clocks/voltages I can't imagine the heat will be the limiting factor.

 

[tres_kun]

the limiting factor is architecture
The clip has a certain size
so it becomes unstable at a low voltage
It is like a engine in a car
700RPM being the low limit
You can lower it but it will stop

 

[eddyr]

Thanks but that's not quite what I mean. Most CPUs seem to have a lower limit as to where they'll drop the clock speed to when the CPU is under low load, so I was wondering what determines the limit.

A modern processor with a lower clock limit of 1.1 GHz, would this be able to run in the MHz range, with the obviously crippled performance, or would it not perform properly?


For the upper limit of the clock speed definitely, but at lower clocks/voltages I can't imagine the heat will be the limiting factor.

It depends on the transistor design and how they respond to a voltage. This may be of interest. http://www.realworldtech.com/near-threshold-voltage/
Page 2 shows a pentum core operating as low as 3 Mhz on milliwatts. 0.3v for the logic and 0.55v for memory caches.

The lowest used limit is likely a combination of, ease of implementation, minimum acceptable performance and the tradeoff of powervperf. From what I understand, there isnt anything stopping your chip dropping to say 500mhz at its 1.1ghz voltage for some marginal power saving. Dropping voltage would better drop power consumption but will slow down the switching on/of speed of transistors, forcing use of a lower speed to prevent errors during operation.

 

[tres_kun]

nice comeback makes me look like a noob

 

[eddyr]

I'm still very much a noob. Only recently started looking more into the technical side of processor design. I have a load of bookmarked stuff I've to get through when I can, but that link was a recent one. Some good stuff up on that site.