6 CLOCK MANAGEMENT UNIT (CMU)
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
6-7
Note: The PLL can only be set up when the PLL is turned off (PLLPOWR/CMU_PLLCTL0 register = 0)
and the clock source is other than the PLL (CLKSEL[1:0]/CMU_OSCSEL register is not 0x2). If
settings are changed while the system is operating with the PLL clock, the system may operate
erratically.
Power Supply for PLL
6.4.5
In order to prevent undesirable effects of noise, the PLLV
DD
and PLLV
SS
pins are provided, in addition to the core
power supply, to feed power to the PLL. Make sure that the following voltages are supplied to the respective pins.
PLLV
DD
pin: Supply LV
DD
level voltage.
PLLV
SS
pin: Set to V
SS
level.
For pin assignments, see the “Pin Descriptions” section.
SSCG
6.5
The SSCG (Spread Spectrum Clock Generator) is a circuit used to reduce EMI (Electromagnetic Interference) noise
by conducting spread spectrum (or SS modulation) on the PLL output clock signal. SSCG conducts SS modulation
on the PLL output clock signal. Thus SS modulation contributes in reducing noise when the PLL output clock is
selected as the system clock source since, in this case, the SS modulation is effective for all the operating clocks for
the core and peripheral circuits (except the RTC that uses the OSC1 clock).
Note: When the OSC3 or OSC1 clock is selected as the system clock source, SS modulation is not per-
formed for the operating clock (system clock).
*
About spectrum spread (SS modulation)
The SSCG performs SS modulation by adjusting the width of the high section of the input clock. This adjust-
ment 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
Input clock cycle
Maximum frequency change width
+
–
Frequency change cycle
5.1 SS Modulation
Figure 6.
SSCG On/Off Control
6.5.1
The SSCG can be turned on or off using SSMCON/CMU_SSCG0 register. Setting SSMCON to 1 causes the SSCG
to start operating. When initially reset, SSMCON is initialized to 0, with the SSCG turned off (bypassed).