6 CLOCK MANAGEMENT UNIT (CMU)
6-8
Seiko Epson Corporation
S1C33L26 TECHNICAL MANUAL
Notes: • A stabilized clock must be supplied to the SSCG module when turning the SSCG on and off.
The following shows the operation procedure.
To turn the SSCG on
1. Turn the PLL on.
2. Wait for the PLL stabilization time to elapse at the minimum.
3. Turn the SSCG on.
To turn the SSCG off
1. Turn the SSCG off.
2. Turn the PLL off.
• SS modulation is conducted on the PLL output clock signal. SS modulation is not applicable to
the signals other than the PLL clock signal. When the PLL output clock is not used for the sys-
tem clock, turn the SSCG off.
SS Modulation Parameter Settings
6.5.2
As described in “About spectrum spread (SS modulation)” above, it is necessary to set the upper-limit value of the
maximum frequency change width and the frequency change cycle.
The maximum frequency change width should be set to the appropriate value according to the PLL output clock
frequency as shown in the table below using SSMCIDT[3:0]/CMU_SSCG1 register. The maximum frequency
change width will be about
±
2% of the PLL output clock by the above setting.
5.2.1 Maximum Frequency Change Width Settings
Table 6.
PLL output clock frequency f [MHz]
SSMCIDT[3:0]
f
≤
19.8
0xf
19.8 < f
≤
21.2
0xe
21.2 < f
≤
22.5
0xd
22.5 < f
≤
24.2
0xc
24.2 < f
≤
25.9
0xb
25.9 < f
≤
28.4
0xa
28.4 < f
≤
30.8
0x9
30.8 < f
≤
34.2
0x8
34.2 < f
≤
37.8
0x7
37.8 < f
≤
43.1
0x6
43.1 < f
≤
48.9
0x5
48.9 < f
≤
58.5
0x4
58.5 < f
≤
69.7
0x3
69.7 < f
≤
90.0
0x2
–
0x1
–
0x0
(Default: undefined)
SSMCITM[3:0]/CMU_SSCG1 register is used to set the frequency change cycle. However, always set it to 0x1.
Notes: • SSMCIDT[3:0] must be set according to the PLL output clock frequency as shown in Table
6.5.2.1. Using the SSCG with an improper setting may cause a malfunction of the IC.
• When the PLL is off, the initial values and the written values cannot be read correctly from
SSMCIDT[3:0] and SSMCITM[3:0] since the source clock is not supplied from the PLL (different
values are read out). The correct values can be read out when the PLL is on.