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Si3035
Rev. 1.2
23
The main design consideration is the generation of a
base frequency, defined as the following:
N1 (Register 7) and M1 (Register 8) are 8-bit unsigned
values. F
MCLK
is the clock provided to the MCLK pin.
Table 18 lists several standard crystal oscillator rates
that could be supplied to MCLK. This list simply
represents a sample of MCLK frequency choices. Many
more are possible.
After the first PLL has been setup, the second PLL can
be programmed easily. The values for N2 and M2
(Register 9) are shown in Table 17. N2 and M2 are 4-bit
unsigned values.
When programming the registers of the clock generator,
the order of register writes is important. For PLL1
updates, N1 (Register 7) must always be written first,
immediately followed by a write to M1 (Register 8). For
PLL2, the CGM bit must be set as desired prior to
writing N2/M2 (Register 9). Changes to the CGM bit
only take effect when N2/M2 are written.
Note:
The values shown in Table 17 and Table 18 satisfy the
equations above. However, when programming the
registers for N1, M1, N2, and M2, the value placed in
these registers must be one less than the value calcu-
lated from the equations. For example, for CGM = 0
with a MCLK of 48.0 MHz, the values placed in the N1
and M1 registers would be 0x7C and 0x5F, respec-
tively. If CGM = 1, a non-zero value must be pro-
grammed to Register 9 in order for the 16/25 ratio to
take effect.
PLL Lock Times
The Si3035 changes sample rates very quickly.
However, lock time will vary based on the programming
of the clock generator. The major factor contributing to
PLL lock time is the CGM bit. When the CGM bit is used
(set to 1), PLL2 will lock slower than when CGM is 0.
The following relationships describe the boundaries on
PLL locking time:
PLL1 lock time < 1 ms (CGM = 0,1)
PLL2 lock time: 100 us to 1 ms (CGM = 0)
PLL2 lock time <1 ms (CGM = 1)
For modem designs, it is recommended that PLL1 be
programmed
during
initialization.
programming of PLL1 is necessary. The CGM bit and
PLL2 can be programmed for the desired initial sample
No
further
Table 17. N2, M2 Values (CGM = 0, 1)
Fs (Hz)
N2
M2
7200
2
2
8000
9
10
8229
7
8
8400
6
7
9000
4
5
9600
3
4
10286
7
10
FBase
FMCLK
M1
N1
36.864MHz CGM
,
0
=
=
=
FBase
N1 25
16
FMCLKM1
36.864MHz CGM
,
1
=
=
=
Table 18. MCLK Examples
MCLK (MHz)
N1
M1
CGM
1.8432
1
20
0
4.0000
5
72
1
4.0960
1
9
0
5.0688
11
80
0
6.0000
5
48
1
6.1440
1
6
0
8.1920
32
225
1
9.2160
1
4
0
10.0000
25
144
1
10.3680
9
32
0
11.0592
3
10
0
12.288
1
3
0
14.7456
2
5
0
16.0000
5
18
1
18.4320
1
2
0
24.5760
32
75
1
25.8048
7
10
0
33.8688
147
160
0
44.2368
96
125
1
46.0800
5
4
0
47.9232
13
10
0
48.0000
125
96
0
56.0000
35
36
1
60.0000
25
24
1