bjw105 said:

Hi all...

Can someone help me? Physicsy question I'm not too sure about:

If you've got a circuit, for example a microprocessor, running at one

clock frequency and all of a sudden you double the clock frequency -

you'd expect the power consumption of the device to go up.

Yes. CMOS dynamic power is proportional to the clock frequency.

Question one (reality check): Is this indeed the case?

Yes.

Question two: How much would you expect the power consumption to go up?

Double? Is there a simple relationship? Is it linear?

First, there are two components of power. Dynamic power (switching

power) is dissipated by changes in state. Dynamic power is goes pretty

much along the classical P~FV^2 relationship. The more changes in state

per unit time (Frequency) the higher the power. Static power is the

power dissipated with no state changes and is proportional to V^2

(sorta, see below).

Older true CMOS processes have very little static power dissipation so

the power dissipated in an older processor goes pretty much along the

FV^2 dynamic power formula. However, newer processes have a lot of

leakage, so static power becomes significant. Thus the linear

relationship doesn't hold (unless you subtract off the static power).

Question three: It has been suggested to me that this only applies for

CMOS devices and bipolar devices have no frequency-power dependance. Any

thoughts?

Certainly there is a frequency/power relationship in bipolar devices as

well. The difference is that a bipolar device has a far higher static

power so it'll tend to mask the change in the dynamic power. Advanced

CMOS processes have high static power consumption (leakage is getting

to be a big problem) so the relationship is getting more complicated.

Can anyone offer a quick explanation to set me straight on this?

I hope that was quick enough. ;-)