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    Tech Marketing Manager HQ Array Raja@ASUS's Avatar
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    Apr 2011

    Overclocking Tips - Part Two

    Overclocking Tips Part Two

    Part one of this guide was posted over in the Rampage IV section of the forum a year ago. The central focus of that guide was to increase awareness of what goes on at the electrical level as we overclock a system and run out of headroom (using DRAM as an example). I had planned on adding more to it eventually – it makes sense to do so now. As the subject matter is related and reliant on part one, we’ll copy over some of the key sections, making edits to accommodate new info and then add a new section below. The new “module” adds info on impedance together with how things can affect POST and general system stability. Enjoy!

    A few Overclocking Technicalities Analogized

    Why does a system become unstable when it is overclocked? There are numerous reasons, actually. More than one could cover in a single article. Many require electrical engineering backgrounds to both write and to understand. Electrical engineers we are not… Well, most of us (including me) are not anyway, so we’re going to try and keep things simple.

    Fundamentally, the role of a processor is to calculate, write and read data. At the core level, this data is represented and moved around the system as 1’s and 0’s (binary patterns). Let’s look at a crude visual representation of how data is represented at the electrical level:

    Click image for larger version. 

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    The “wavy” line is the signal alternating between a high and low voltage to represent 1 and 0. In this brief example the data pattern being transmitted from the memory bus to the processor is 101010.

    VOH (voltage output high) is the high voltage output level of the transmitter that represents a logic 1, while VOL is the low output voltage (voltage output low) representing a logic 0.

    VREF is the reference voltage. The reference is typically set at the midpoint between VOL and VOH.

    VDDQ (not shown, and known as DRAM Voltage on motherboards) is the voltage supply for VOH, VOL and the voltage from which VREF is derived.

    Most signal stages have three states: VOH, VOL and “off” - known as a tri-state transceiver/s. Typically, “off” state will be a certain level lower than VOL but above ground potential (or centered). The off state is required to prevent inadvertent data transmission. This standing voltage (bias) in “off” state is attached to a compensation network to hold the voltage in steady state when the signal line is not transmitting. Compensation is usually in the form of a resistor to ground, but can be something more elaborate if required. The reason a certain level of bias is present and the line it is not at ground potential in “off” position is due to a number of factors which fall outside the scope of this article.

    For this example, let us assume VOH is around 80% of VDDQ, VOL is 20% of VDDQ, while VREF is 50% of VDDQ. The signal swings between VOL and VOH to represent data as a 1 or 0 while a data strobe compares the signal against VREF. If the voltage is higher than VREF it is interpreted as a logic 1, or if the signal voltage is below VREF it is interpreted as a logic 0. Needless to say, the process of transferring the data and interpreting it accurately requires that the transmitter and the receiver device be in close timing sync. If there is a lack of synchronization between the transmitted signal and the strobe, the data could be read erroneously.

    In the ideal world, the signal waveform would be perfectly symmetrical as it transitions between high and low states. Never crossing above VOH or below VOL. The keen eyed among you will notice in the diagram above that the signal varies slightly from one transition to the next. That’s mostly because I’m crap at drawing, but in this case, fortunately, the rendition fits! The waveforms are non-symmetrical and have different levels of excursion past VOH or VOL (overshoot and undershoot). There are various reasons why these issues occur; power supply fluctuation, jitter, impedance issues and noise just to name a few. We won’t go into all of the factors leading to these problems as many fall outside the scope of this article. However, we can break down what happens as a result of them by using a real-world analogy in a context we can relate to.

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