SSC spread spectrum

What is meant by SSC：

Spread frequency spread spectrum(SSC), All electronic components that generate frequency signals are potential electromagnetic interference （ EMI ） Source , These signals will affect, for example, the radio 、 The normal operation of electronic products such as TV or mobile phone .SSC Let the clock slowly change a little bit to disperse the energy of the output clock . Its principle is similar to that in Figure 1 The example shown is , Let the water pressure （ Spectrum energy ） A certain amount of water from a small hole （ Fixed frequency ） Out of the , This and getting it out of the multiple holes in the shower head （ Dispersion frequency ） The effect of the spray is different . The water pressure of the latter is dispersed , The strength of the water （ Radiated noise ） It will be much smaller .

 

SSC Not only for clock oscillation frequency （ Fundamental wave ） It has an inhibitory effect , It also suppresses the peak value of higher harmonic . chart 2 As shown in the for SSCG The frequency spectrum waveform of the output fundamental wave and higher harmonic wave

 

Spread spectrum design ：

1、 add to SSCG device

add to SSCG IC The device realizes the ideal control of the clock frequency , Can achieve very good EMI Inhibition effect

 

2、 Software implementation

There are some CPU Of clock Registers provide SSC Spread frequency control function , Through the operation of register by software SSC function , It can also achieve the spread frequency effect . This paper mainly uses software to realize SSC function .

Let's look at it first A7 datasheet About China Clock Introduction to ：

 

SYS3_SSCPLL Support SSC function , Register found , By default SSC Function is off , open SSC Need to set up SSCPLL register ：

 

poky/sources/u-boot/board/csr/atlas7cb/clock.c stay Uboot Of Clock.c Open... In file USE_SSC_RAM Macro definition ：

 

recompile Uboot, And burn it into the locomotive , Frequency spreading function can be realized ：

devmem 0x18620084

You can find 0x18620084（CLKC_SYS3_SSCPLL_AB_SSRAM_DATA） The value of the register is no longer a fixed value , The spread frequency effect can be seen by measuring with an oscilloscope .

 

Real case sharing ：

chart 6 Is off by default SSC EMC Experimental chart , chart 7 Yes open SSC Functional testing is EMC Experimental chart

                                                                          chart 6 SSC OFF EMC Experimental chart

 

 

 

                                                                                      chart 7 SSC ON EMC Experimental chart