Speedstep lower limit

[Martijn van Duijn]

Hi,

I got a little bit puzzled by the frequency range of Speedstep CPU's
(and their powerNOW counterparts). My laptop switches back and forth
between 1000 and 1600 MHz, which is nice. However, when idle I get only
a few % CPU utilization ever at the lower speed. There seems to be lots
of room at the bottom to save some power and heat. What prevents a CPU
from throttling down a lot more, say to a modest 100 or even 10 MHz,
which should suffice for most idle machines?

Thanks,

Martijn

 

[Yousuf Khan]

Hi,

I got a little bit puzzled by the frequency range of Speedstep CPU's
(and their powerNOW counterparts). My laptop switches back and forth
between 1000 and 1600 MHz, which is nice. However, when idle I get only
a few % CPU utilization ever at the lower speed. There seems to be lots
of room at the bottom to save some power and heat. What prevents a CPU
from throttling down a lot more, say to a modest 100 or even 10 MHz,
which should suffice for most idle machines?

It's probably set by the individual laptop manufacturer. My Thinkpad
Pentium-M has a range between 600Mhz and 1.6Ghz. My Gateway Turion has a
range between 800 and 1.8Ghz.

Yousuf khan

 

[Nate Edel]

It's probably set by the individual laptop manufacturer. My Thinkpad
Pentium-M has a range between 600Mhz and 1.6Ghz. My Gateway Turion has a
range between 800 and 1.8Ghz.

There are two kinds of throttling; the multiplier-based throttling is a
preset capability in the processor - original speedstep (P3M/P4M) was two
settings (high/low multiplier/voltage), while Enhanced Speedstep (P-M/Core
Duo) and Cool'n'Quiet support multiple intermediate steps.

Clock-rate throttling is also usually done, and that can be set by the
individual laptop manufacturer although they tend to stick pretty closely to
the CPU/chipset makers' recommendations.

 

[Martijn van Duijn]

There are two kinds of throttling; the multiplier-based throttling is a
preset capability in the processor - original speedstep (P3M/P4M) was two
settings (high/low multiplier/voltage), while Enhanced Speedstep (P-M/Core
Duo) and Cool'n'Quiet support multiple intermediate steps.

Clock-rate throttling is also usually done, and that can be set by the
individual laptop manufacturer although they tend to stick pretty closely to
the CPU/chipset makers' recommendations.

Yeah, that's clear. It just seemed that it is not physically possible to
stably run a 1600 MHz cpu at e.g. 10 MHz (otherwise, why would the
manufacturer not enable us to do so?), so I wondered why this was the case.
Thanks for your replies

Martijn

 

[krw]

Yeah, that's clear. It just seemed that it is not physically possible to
stably run a 1600 MHz cpu at e.g. 10 MHz (otherwise, why would the
manufacturer not enable us to do so?), so I wondered why this was the case.

An extremely low clock rate may not give you the power reduction
you think. Leakage can be a large portion of the power and is
dependant on the voltage only. While a slower clock does allow a
lower voltage, this only works to a certain point (Vmin) where the
circuits stop operating. The higher percentage of static power
(leakage) makes any operations less power efficient. I.e. it may
be more power efficient to put the processor to sleep
intermittently and run it at a higher power inbetween.

There are also circuits that have a minimum frequency they'll
operate. Any dynamic (domino) logic will have a minimum frequency,
as will the phase locked loop.