
iiiPeriPheraLModuLes1(systeM):CLoCkManageMentunit(CMu)
iii-1-14
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s1C33L17teChniCaLManuaL
iii.1.7ControlofthessCg
The Spread Spectrum Clock Generator (SSCG) is a circuit used to reduce Electromagnetic Interference (EMI) noise
by spreading the spectrum (or performing SS modulation) for the PLL output clock signal. The SS modulation is
effective for all operating clocks for the core and peripheral circuits (except the RTC that uses the OSC1 clock)
when the PLL output clock has been selected as the system clock source and only this case has the effect of
reducing noise.
note: When the OSC3 or OSC1 clock is selected as the system clock source, SS modulation is not
performedfortheoperatingclock(systemclock).
About spectrum spread (SS modulation)
The SSCG performs SS modulation by adjusting the width of the high section of the input clock. This
adjustment is made by increasing or reducing the set value of the internal delay adjust circuit of the SSCG. The
maximum width within which the set value is changed constitutes the maximum frequency change width. The
relevant control register is used to set the upper-limit value of this width. In the SSCG, an interval timer adjusts
the interval at which the set value changes. The relevant control register is also used to set this interval (frequency
change cycle).
±0
Inputclockcycle
Maximumfrequencychangewidth
+
–
Frequencychangecycle
FigureIII.1.7.1SSModulation
iii.1.7.1turningthessCgon/off
The SSCG can be turned on or off by using SSMCON (D0/0x301B10).
ssMCon:SSCGMacroOn/OffControlBitintheSSCGMacroControlRegister(D0/0x301B10)
Setting SSMCON (D0/0x301B10) to 1 causes the SSCG to start operating. When initially reset, SSMCON (D0/
0x301B10) is initialized to 0, with the SSCG turned off (bypassed).
notes: AstabilizedclockmustbesuppliedtotheSSCGmodulewhenturningtheSSCGonandoff.
Thefollowingshowstheoperationprocedure.
ToturntheSSCGon
1.TurnthePLLon.
2.WaitmorethanthePLLstabilizationtime.
3.TurntheSSCGon.
ToturntheSSCGoff
1.TurntheSSCGoff.
2.TurnthePLLoff.
TheSSmodulationiseffectiveonlyforthePLLoutputclock,andisnotperformedforother
sourceclocks.WhenthePLLoutputclockisnotusedforthesystemclock,turntheSSCGoff.