The Gigabyte GA-X48T-DQ6 - Redefining the High End?by Rajinder Gill on January 2, 2008 3:15 AM EST
- Posted in
More BIOS Details
Command Rate (CMD)
The options are Auto, 1N and 2N. A setting of 1N can be used almost exclusively while overclocking, bringing a performance advantage of around 2ns to memory access latency. If memory speeds over DDR3-1800 are desired then a setting of 2N may be required for stability.
CPU/PCI-E Clock Driving Control
The default setting is 800mv, with a range of voltage control offered between 700mv-1000mv. As this is a differential amplifier circuit, increasing voltage may actually decrease the clock signal accuracy due to increased power supply noise. Differential circuits are used in preference to single ended circuits because of their noise rejection and low voltage operating capabilities. Increasing voltage to these circuits in turn increases "nasties" such as overshoot and output clock signal jitter. This in turn counteracts the benefits of using a differential amplifier in the first place. We did experiment with various levels of overvoltage and found no gains in stability whatsoever, further cementing our beliefs that more is not always better.
CPU Clock Skew Control and (G)MCH Clock Skew Control
These two clock skew settings are directly related to the voltage control circuit above. They control the PLL output to both the CPU and Northbridge. Again, as a differential amplifier is used, the level of offset required should never exceed 150ps (Pico seconds) of skew to either the CPU or Northbridge reference clocks. At most, PLL circuits such as these should be "good enough" to retain a jitter level of around 150-200ps (lower is better). For those wishing to experiment, adjustments in the range of 0-200ps are of interest for both of the clock skew functions. As a rule, start with the lowest voltage possible, tune either skew setting, and then monitor for effects if any.
DDR3 Overvoltage Control
Base VDimm is 1.5V. The scale offers between 1.5V-3.05V, more than enough to fry any DDR3 modules. 3.05V is certainly more than enough voltage for extreme benchmarking.
PCI-E Overvoltage Control
a level of overvoltage to the PCI-E bus. The available voltage range is between
1.5V-2.25V. We recommend staying close to stock voltage; higher voltages cause the
board to shut down during the boot cycle. Most modern graphics cards do not benefit
from higher PCI-E voltages. Increasing this voltage will increase noise and magnetic interference into nearby sensitive circuits. Remember, more voltage always has side effects, it is best to let graphics cards draw their power from the PSU PCI/E connectors rather than through the motherboard.
FSB Overvoltage Control
This voltage setting is more commonly known as VTT. Default is 1.10V, with a maximum of 1.41V available. This voltage is critical for quad-core overclocking. 400FSB will require 1.41V right off the bat if stability is desired. (Ed: When isn't that the goal?) Unfortunately, we have no direct control over GTL (Gunning Transceiver Logic) reference values, which are locked at 67% of VTT for CPU die 1 and 63% of VTT for CPU die 2. On a top-end board like this, we had expected to find GTL adjustments, which can be crucial for finding quad-core CPU stability at high FSB speeds.
(G)MCH Overvoltage Control
Default voltage is 1.45V, and the available voltage scale provides an overvoltage of 0.025-0.775V. There an actual undervoltage of 0.2V from the BIOS set overvoltage. For example, as the stock voltage is set to 1.45V a BIOS setting of +0.50V would imply a voltage of 1.95V. However setting +0.50V gives an actual voltage of 1.75V, suggesting an undervoltage of 0.2V.
Setting this to Enabled reduces Intel's specification voltage droop to the CPU when significant power is drawn. Setting enabled places more strain on the PWM circuit and increases voltage overshoot (within the corrective feedback loop) and settling time from load oscillation, which in turn can affect CPU overclock stability. The trade off is that disabling this setting will require a higher Vcore increasing idle CPU temperatures. Vdroop under load is in the region of 0.06V. Therefore, if a full load voltage of 1.35V is required for stability, a setting of 1.41V will be required for Vcore with Loadline Calibration set to Disabled. The Enabled setting gives a Vdroop of around ~0.02V under load. Users are advised to pick a method that suits their cooling, and use common sense to work out how much voltage is safe at idle if Loadline Calibration is set to Disabled.
A range from 0.50V to 2.35V is available, making this board suitable for every type of perceivable over and underclocking. Please note we have not experimented with voltages higher than 1.90V with our cascades cooling, so we cannot guarantee voltages over 1.90V actually work.