SI5325C-C-GM Datasheet, PDF (86/178 Pages) Silicon Laboratories – ANY-FREQUENCY PRECISION CLOCKS

Si53xx-RM
FOS reference. When the XA/XB input is used as the FOS reference, there is only one reference frequency band
that is allowed: from 37 MHz to 41 MHz.
Table 41. FOS Reference Clock Selection
FOSREFSEL[2:0]
000
001
010
011
100
all others
FOS Reference
Si5326
XA/XB
CKIN1
CKIN2 (default)
Reserved
Reserved
Reserved
Si5368
XA/XB
CKIN1
CKIN2 (default)
CKIN3
CKIN4
Reserved
Both the FOS reference and the FOS monitored clock must be divided down to the same clock rate and this clock
rate must be between 10 MHz and 27 MHz. As can be seen in Figure 31, the values for P and Q must be selected
so that the FOS comparison occurs at the same frequency. The registers that contain the values for P and Q are
the CKINnRATE[2:0] registers.
CKIN
P
FOS
Compare
FOS_REF
Q
10 MHz min,
27 MHz max
Figure 31. FOS Compare
The frequency band of each input clock must be specified to use the FOS feature. The CLKNRATE registers
specify the frequency of the device input clocks as shown in Table 42.
When the FOS reference is the XA/XB oscillator (either internal or external), the value of Q in Figure 31 is always
2, for an effective CLKINnRATE of 1, as shown in Table 42.
Table 42. CLKnRATE Registers
CLKnRATE
0
1
2
3
4
5
Divisor, P or Q
1
2
4
8
16
32
Min Frequency, MHz
10
25
50
95
190
375
Max Frequency, MHz
27
54
105
215
435
710
86
Rev. 1.2