spread spectrum clocking_low mmio align_xilinx pre-productio

什么是Spread Spectrum：

Spread-spectrum clock generation (SSCG) is used in some synchronous digital systems, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal and because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency and its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the FCC in the United States, JEITA in Japan and the IEC in Europe).

Spread-spectrum clocking avoids this problem by using one of the methods described in the Spread-spectrum telecommunications section in order to reduce the peak radiated energy. The technique therefore reshapes the system's electromagnetic emissions to comply with the electromagnetic compatibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a modification to the equipment.

Spread-spectrum clocking has become more popular in portable electronics devices because of faster clock speeds and the increasing integration of high-resolution LCD displays in smaller and smaller devices. Because these devices are designed to be lightweight and inexpensive, passive EMI reduction measures such as capacitors or metal shielding are not a viable option. Active EMI reduction techniques such as spread-spectrum clocking are necessary in these cases, but can also create challenges for designers. Principal among these is the risk that modifying the system clock runs the risk of the clock/data misalignment.

It is important to note that this method does not reduce the total energy radiated by the system, and therefore does not necessarily make the system less likely to cause interference。 Spreading the energy over a large frequency band effectively reduces the electrical and magnetic field strengths that are measured within a narrow window of frequencies。 Spread-spectrum clocking works because the measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide。spread spectrum clockingspread spectrum clocking

[7] If the system under test were to radiate all of its energy at one frequency, it would register a large peak at the monitored frequency band。 Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits。 The usefulness of spread-spectrum clocking as a method of actually reducing interference is often debated, but it is probable that some electronic equipment with sensitivity to a narrow band of frequencies will experience less interference, while other equipment with broadband sensitivity will experience more interference。

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, some BIOS writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This may be considered a loophole, but is generally overlooked as long as the default BIOS setting provided by the manufacturer has the spread-spectrum feature enabled. An ability to disable spread-spectrum clocking for computer systems is considered useful as the spread-spectrum techniques used can affect the maximum clockspeed achievable by the components involved due to clock skew, affecting overclocking efforts.

本文来自电脑杂谈，转载请注明本文网址：
http://www.pc-fly.com/a/shouji/article-53105-1.html