Overclocking: what _exactly_ happens in the chip?

[Elledan]

I'm a bit confused on this topic. When overclocking, are you actually 'pushing' more electrons through the chip, so that everything just goes faster?

Please help me

 

[Demon-Xanth]

Overclocking is simply running the chip at a higher than RATED speed. Nothing is really moving faster, just changing states faster.

 

[Sohcan]

The clock on a CPU controls when the logic changes state. There are a number of steps involved in executing an instruction...the basic ones are instruction fetch, instruction decode / register fetch, execute, memory access, write back. Modern CPUs employ a number of other performance techniques like register renaming and out-of-order execution, so that's why the basic pipelines of the Athlon and P4 are 12 and 20 stages, respectively. Therefore, it takes 12 or more clock cycles for the Athlon to execute an instruction. By forcing the clock signal to go faster, you are forcing the CPU to change states more often...thus, it executes instructions faster. The reason that overclocking can introduce instability (besides heat issues) is that each stage of the pipeline takes a certain amount of time to complete its task. Say, for example, you have a CPU with a 10ns clock period (100 MHz), and it's arithmetic logic unit (used during the execute stage) has an 8ns delay for an add instruction. Therefore, if you force the clock period to be shorter than 8ns (125 MHz), the CPU might change state before the add is completed. The result is that the CPU will latch an incorrect result into the register file, and errors will ensue.

 

[Elledan]

Just by making the clock in the chip run faster? If so, how?

I must admit that I'm an overclock-newbie

 

[Elledan]

Thanks, Sohcan

I'm still not certain what exactly this clock is. I know what it does, but what is it?

And how does increasing the FSB make the clock go faster? Also, can you explain what the FSB is and does?

Sorry for asking so many questions, but please allow me to learn something

 

[Sohcan]

A clock (there are many of them in a computer) is a line that changes from high voltage to low voltage in the shape of a square wave on a regular interval. Sequential circuits use the clock signal to change state on a regular interval...the clock is used to time all the components. Essentially, a clock is used to prevent everything from happening at once (just as in physics, time exists to prevent everything from happening at once ).

On the motherboard is a clock generator that produces the clock signal (at 8 or 16 MHz I believe?). A PLL is then used to multiply the clock signal by and integer or fraction to increase or decrease the period of the clock signal. A PLL in the CPU uses the FSB frequency as its input; therefore, by changing the multiplier on the PLL, or increasing the frequency of the FSB, you can overclock the CPU.

The Front Side Bus is the means of communication between the northbridge of the motherboard chipset and the CPU.

 

[alpha88]

The internal CPU clock is based off a multiplier of the FSB speed.

 

[Elledan]

I understand it now. Thanks guys

 

[obsidience]

<< A clock (there are many of them in a computer) is a line that changes from high voltage to low voltage in the shape of a square wave on a regular interval. Sequential circuits use the clock signal to change state on a regular interval...the clock is used to time all the components. Essentially, a clock is used to prevent everything from happening at once (just as in physics, time exists to prevent everything from happening at once ).

On the motherboard is a clock generator that produces the clock signal (at 8 or 16 MHz I believe?). A PLL is then used to multiply the clock signal by and integer or fraction to increase or decrease the period of the clock signal. A PLL in the CPU uses the FSB frequency as its input; therefore, by changing the multiplier on the PLL, or increasing the frequency of the FSB, you can overclock the CPU.

The Front Side Bus is the means of communication between the northbridge of the motherboard chipset and the CPU. >>



Sohcan, you are a stud. I've read many books on the workings of computers dating back 12 years. I knew about clock and bus frequencies but never really knew what they were until now.

So my question is how does the motherboard get its clock signal? From quartz (like a watch)? And how does it read this signal? Thanks!

Obsidience

 

[Train]

capacitors my friends, capacitors!

 

[obsidience]

I thought capacitors regulate the power output and also function as a sort of short term battery.

 

[pm]

The motherboard generates it's primary clock using a quartz crystal (a little silver shaped capsule on the board with the number like &quot;14.5323&quot; on it, and two wires coming out of it) and then creates the motherboard's synchronized clock using a PLL chip..

 

[CP]

How do we create a clock multiplier circuit to bring up the default clock generator on mobo? Any reference URL or picture would be really great.

Thanks

 

[pm]

The clock generator is a piezo-electric crystal - the circuit for getting this to resonate is really simple. Is this what you want a sch for?

The phase-locked loop that synchronizes all of the clocks and generates the faster clocks based on the slower signal is a bit more interesting (IMO) and is a little more complex from a circuit perspective. Still, they are available in integrated modules these days: like the TI N54LS297, and the NatSemi LM560-564's.

They can be tricky to design by hand using discrete components. One of my college projects was to do this and it took us longer than I expected it would to get it working correctly. There's a block diagram of the function and some description here.