CPU mhz question (Bonus Question)

[joker_927]

All semiconductor devices used in computers are capacitive by nature. Thsi capacitance is unwanted because its creates a delay. Use this phenomenon and explain why computers have a maximum clock frequency speed. (of about 4Ghz today)

I thought they had max clock speeds for other reasons. Oh well. It's due in 25 minutes. Let's see what google can do...

[Hungrycookpot]

oh its got nothing to do with capacitance. You see, the little men that carry the information back and forth between your different systems can only run so fast, but as technolodgy gets better, we can hire tiny little men that are in better shape, so they can run faster.


I have no idea

[RobRich]

You will find plenty of spelling mistakes (before I was EiC at TIMO), but here ya' go:

http://www.techimo.com/articles/index.pl?photo=24

[RobRich]

Also check here:

http://www.sysopt.com/articles/soi/

SOI is very important in regards to capacitance.

The diagram above represents a MOS circuit (left) as compared to an SOI circuit (right). The MOS circuit is slower because a capacitance develops between the impurity layers and the silicon substrate. This boundary area is known as the junction capacitance layer. As noted earlier, a MOS circuit must spend a large amount of its operational time discharging and recharging this capacitance. In comparison, the SOI circuit employes silicon oxide layered over the pure silicon substrate. The SOI circuit's capacitance will be negligible since the silicon oxide provides an efficient insulation barrier. The junction capacitance area is eliminated by SOI, thus the transistor will be able to operate faster since the charging process is eliminated.

[joker_927]

The time the CPU has to detect the "high" or "low" states, is limited by the CPU clock. If the clock is set to higher speed, the time for detection, the "decision cycle", becomes shorter.

The signal itself is not at all digital. Its analogue. Which means, it doesn't jump from "low" to "high" and back in no time, it gradually rises from the lower level to the higher and back. This is caused by parasitic capacitance's and resistors. They are called "parasitic", because you rather would like them not to be there, but given the current semiconductor manufacturing process, you can't avoid them.

In the design of the CPU, it is attempted to keep the parasitics as low as possible. Sometimes you run into a quagmire. If you make the resistance lower (for example wider metal lines have less resistance), you might increase the capacitance (wider metal lines have more capacitance). Which means, you will always end up with having the parasitics in one way or another.



thx!!