TheNewbie said:
... but why do I have the raise the vcore to get a stable oc
by just dropping the multi. and raising fsb? ...
Captain Newbie said:
...
A frequency increase will usually need a voltage increase. I don't have the technical background (yet) to say why, but this is the general rule among overclocking. The person to ask is hitechjb; in the meantime, this is the general rule for Athlons:
Over 2 volts is not reccomended unless you have phase-change cooling.
Higher Vcore is necessary to get to higher frequency but not sufficient for stability. To get to stable high frequency under load, it requires
both high Vcore and low enough temperature for a given CPU and for a given cooling setup.
This is why:
CPU (chip) is made up of many transistors (towards 100 millions for Tbred/Barton, more for A64) forming logic switches to perform logic operations. Physically, each transistor is connected to some capacitors which are inherent in the transistor gate dielectric and coupling between connections and the underlying silicon. In order for the transistors to switch and perform the required logic function physically in a given CPU cycle,
electric current is needed to charge and discharge these capacitors (100 millions+) via the corresponding transistor switches.
Such switching current (usually known as Idsat) through a transistor depends on Vcore, the higher the Vcore, the larger the current (a property of transistor, without going into details here).
That is,
the higher the Vcore is,
the higher the current,
the shorter the time to switch a transistor to do a logic operation,
the shorter the cycle time of a pipeline in a CPU,
the higher the CPU frequency.
These are the mathematics and physics of the above statement.
Idsat = k1 (Vcore - Vt)^n
where k1 and n are constants, n is between 1 and 2, Vt is transistor threshold voltage. In more detail, typically, n = 2, k1 = W u e / (2 L d), where W is width of transistor gate, L is transistor channel length, u is mobility, e is gate oxide dielectric constant, d is gate oxide thickness.
Since more current can charge or discharge a capacitor faster, the (delay) time (tD) to switch a transistor (in a logic gate) varies inversely with the current, so the higher the current, the shorter the time to perform a logic operation.
tD = C Vcore / Idsat
where C is capacitance (described above).
In a CPU pipeline, within a clock cycle, typically there are 5-20 stages of such logic switching. So the shorter the deley time (tD), the shorter the CPU cycle time T or the higher the CPU frequency f (since f = 1 / T).
f = 1 / T = k2 / tD
Combining this with the above equations, taking n = 2, we can get
f = k2 Idsat / C Vcore = k2 k1 (Vcore - Vt)^2 / C Vcore
or
f ~ k3 Vcore + k4
where k2, k3 = k1 k2, and k4 are some constants.
In other word, the higher the Vcore is, the higher is the frequency, and answers the original question.
For more details:
Vcore vs processor frequency and cycle time (page 19)
Why frequency and voltage are important for overclocking performance (page 19)
This is only part of the story, without other constraints. But the bad news is, we cannot keep increasing Vcore, as there are constraints of
- heat and temperature
- gate voltage breakdown of silicon oxide under the transistor gate
What is gate break-down voltage (page 16)
This keeps the CPU frequency from going forever by Vcore increase.
The temperature constraint is due to the active power P_active, plus leakage power P_leakage, dissipated when running a CPU at frequency f with voltage Vcore.
P_active = C Vcore^2 f
where C is the equivalent capacitance of the CPU to model power.
The temperature t of a CPU is related to the power P_active + P_leakage by
t = kR (P_active + P_leakage) + tA
where kR is known as thermal resistance of a given cooling, and tA is certain temperature offset.
The higher the voltage and frequency, the higher the power and the higher the temperature. Such active power will increase the CPU to certain temperature under certain load for a given cooling.
Since carrier mobility decreases as temperature increase beyond certain temperature due to lattice scattering, transistor switching slow down as temperature increases. So the
frequency f of a CPU varies inversely with the temperature, or df / f = - k dt, mathematically.
The balancing of these two opposing actions, or the intersection of the voltage-frequency curve and the temperature-frequency curve of a CPU characteristic
naturally determines the final stable voltage/frequency/temperature operating point. If overclocking is due properly, the maximal overclocking should settle naturally at certain frequency, voltage and temperature, as desribed above, below the maximum absolute rating of voltage and temperature (as seen from Tbred/Barton, ...). A perceived stable voltage and temperature setting may not be necessary after all, if the voltage, temperature, frequency variations are monitored properly.
CPU voltage: from stock to max absolute, from efficient overclocking to diminishing return (page 19)
How does leakage current slow down future generations of chips (page 19)
