MCLK Spread Spectrum – The BIOS Optimization Guide

MCLK Spread Spectrum - The BIOS Optimization Guide

MCLK Spread Spectrum

Common Options : 0.25%, 0.5%, 0.75%, Disabled

 

Quick Review

Spread spectrum clocking works by continuously modulating the clock signal around a particular frequency. This “spreads out” the power output and “flattens” the spikes of signal waveform, keeping them below the FCC limit.

The MCLK Spread Spectrum BIOS feature controls spread spectrum clocking of the memory bus. It usually offers three levels of modulation – 0.25%, 0.5% or 0.75%. They denote the amount of modulation around the memory bus frequency. The greater the modulation, the greater the reduction of EMI. Therefore, if you need to significantly reduce EMI, a modulation of 0.75% is recommended.

Generally, frequency modulation through spread spectrum clocking should not cause any problems. However, system stability may be compromised if you are overclocking the memory bus.

Therefore, it is recommended that you disable the MCLK Spread Spectrum feature if you are overclocking the memory bus. Of course, if EMI reduction is still important to you, enable this feature by all means, but you may have to reduce the memory bus frequency a little to provide a margin of safety.

If you are not overclocking the memory bus, the decision to enable or disable this feature is really up to you. If you have electronic devices nearby that are affected by the EMI generated by your motherboard, or have sensitive data that must be safeguarded from electronic eavesdropping, enable this feature. Otherwise, disable it to remove even the slightest possibility of stability issues.

 

Details

All clock signals have extreme values (spikes) in their waveform that create EMI (Electromagnetic Interference). This EMI interferes with other electronics in the area. There are also claims that it allows electronic eavesdropping of the data being transmitted.

To prevent EMI from causing problems to other electronics, the FCC enacted Part 15 of the FCC regulations in 1975. It regulates the power output of such clock generators by limiting the amount of EMI they can generate. As a result, engineers use spread spectrum clocking to ensure that their motherboards comply with the FCC regulation on EMI levels.

Spread spectrum clocking works by continuously modulating the clock signal around a particular frequency. Instead of generating a typical waveform, the clock signal continuously varies around the target frequency within a tight range. This “spreads out” the power output and “flattens” the spikes of signal waveform, keeping them below the FCC limit.

The MCLK Spread Spectrum BIOS feature controls spread spectrum clocking of the memory bus. It usually offers three levels of modulation – 0.25%, 0.5% or 0.75%. They denote the amount of modulation around the memory bus frequency. The greater the modulation, the greater the reduction of EMI. Therefore, if you need to significantly reduce EMI, a modulation of 0.75% is recommended.

Generally, frequency modulation through spread spectrum clocking should not cause any problems. However, system stability may be compromised if you are overclocking the memory bus. Of course, this depends on the amount of modulation, the extent of overclocking and other factors like temperature, voltage levels, etc. As such, the problem may not readily manifest itself immediately.

Therefore, it is recommended that you disable the MCLK Spread Spectrum feature if you are overclocking the memory bus. You will be able to achieve better overclockability, at the expense of higher EMI. Of course, if EMI reduction is still important to you, enable this feature by all means, but you may have to reduce the memory bus frequency a little to provide a margin of safety.

If you are not overclocking the memory bus, the decision to enable or disable this feature is really up to you. If you have electronic devices nearby that are affected by the EMI generated by your motherboard, or have sensitive data that must be safeguarded from electronic eavesdropping, enable this feature. Otherwise, disable it to remove even the slightest possibility of stability issues.

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