7.1 Reference Input Selection (IN0, IN1, IN2, XA/XB)
The active clock input is selected using the IN_SEL1,0 pins or by register control. The register bit IN_SEL_REGCTRL determines input
selection as pin or register selectable.
Note:
If the selected input does not have a clock, all output clocks will be shut off (squelched) until a valid input clock is present.
Table 7.1. Manual Input Selection Using IN_SEL[1:0] Pins
IN_SEL[1:0]
Selected Input
0
0
IN0
0
1
IN1
1
0
IN2
1
1
XA/XB
Table 7.2. Input Control Registers
Register Name
Hex Address [Bit Field]
Function
Si5391
0202[7:0]–0205[7:0]
Note:
1. Do NOT use this register on any version of the Si5391.
XAXB_EXTCLK_EN
090E[0]
Selects between the XTAL or external reference clock on the
XA/XB pins. Default is 0, XTAL. Set to 1 to use an external ref-
erence oscillator. It must always be set to 0 (default) for Si5391P.
IN_SEL_REGCTRL
0021[0]
Determines pin or register clock input selection.
IN_SEL
0021[2:1]
Selects the input when in register input selection mode.
IN_EN
0949[3:0]
Allows enabling/disabling IN0, IN1, IN2 and FB_IN when not in
use.
Table 7.3. XAXB Pre-Scale Divide Ratio Register
Setting Name
Hex Address [Bit Field]
Function
Si5391/Si5391P
PXAXB
0x0206[1:0]
Sets the XAXB input divider value according to the table below.
The following table lists the values, along with the corresponding divider ratio.
Table 7.4. XAXB Pre-Scale Divide Values
Value (Decimal)
PXAXB Divider Value
0
1
1
2
2
4
3
8
Si5391 Reference Manual • Clock Inputs
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.5 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • January 11, 2022
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