PCIE Spread Spectrum from The Tech ARP BIOS Guide!

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PCIE Spread Spectrum

Common Options : Down Spread, Disabled

 

PCIE Spread Spectrum : A 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 PCIE Spread Spectrum BIOS feature controls spread spectrum clocking of the PCI Express interconnect.

When set to Down Spread, the motherboard modulates the PCI Express interconnect’s clock signal downwards by a small amount. Because the clock signal is modulated downwards, there is a slight reduction in performance.

The amount of modulation is not revealed and depends on what the manufacturer has qualified for the motherboard. However, the greater the modulation, the greater the reduction of EMI and performance.

When set to Disabled, the motherboard disables any modulation of the PCI Express interconnect’s clock signal.

Generally, frequency modulation via this feature should not cause any problems. Since the motherboard only modulates the signal downwards, system stability is not compromised.

However, spread spectrum clocking can interfere with the operation of timing-critical devices like clock-sensitive SCSI devices. If you are using such devices on the PCI Express interconnect, you must disable PCIE Spread Spectrum.

System stability may also be compromised if you are overclocking the PCI Express interconnect. Therefore, it is recommended that you disable this feature if you are overclocking the PCI Express interconnect.

Of course, if EMI reduction is still important to you, enable this feature by all means, but you may have to reduce the PCI Express interconnect frequency a little to provide a margin of safety.

If you are not overclocking the PCI Express interconnect, 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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PCIE Spread Spectrum : The Full 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.

Clock signal (courtesy of National Instruments)

The same clock signal, with spread spectrum clocking

The PCIE Spread Spectrum BIOS feature controls spread spectrum clocking of the PCI Express interconnect.

When set to Down Spread, the motherboard modulates the PCI Express interconnect’s clock signal downwards by a small amount. Because the clock signal is modulated downwards, there is a slight reduction in performance.

The amount of modulation is not revealed and depends on what the manufacturer has qualified for the motherboard. However, the greater the modulation, the greater the reduction of EMI and performance.

When set to Disabled, the motherboard disables any modulation of the PCI Express interconnect’s clock signal.

Generally, frequency modulation via this feature should not cause any problems. Since the motherboard only modulates the signal downwards, system stability is not compromised.

However, spread spectrum clocking can interfere with the operation of timing-critical devices like clock-sensitive SCSI devices. If you are using such devices on the PCI Express interconnect, you must disable PCIE Spread Spectrum.

System stability may also be compromised if you are overclocking the PCI Express interconnect. 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 this feature if you are overclocking the PCI Express interconnect. 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 PCI Express interconnect frequency a little to provide a margin of safety.

If you are not overclocking the PCI Express interconnect, 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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