- Silicon Laboratories, Inc. Clock User Manual

Si53xx-RM
Rev. 0.5 85
7.5. Si5319, Si5324, Si5326, Si5327, Si5368, Si5369, Si5374 and Si5375 Free Run Mode
Figure 29. Free Run Mode Block Diagram
 CKIN2 has an extra mux with a path to the crystal oscillator output.
 When in Free Run mode, CKIN2 is sacrificed (Si5326, Si5368, Si5369, Si5374).
 Switching between the crystal oscillator and CLKIN1 is hitless.
 Either a crystal or an external oscillator can be used.
 External oscillator connection can be either single ended or differential.
 All other features and specifications remain the same.
7.5.1. Free Run Mode Programming Procedure
 Using DSPLLsim, determine the frequency plan:
Write to the internal dividers, including N31 and N32.
 Enable Free Run Mode (the mux select line), FREE_RUN.
 Select CKIN1 as the higher priority clock.
 Establish revertive and autoselect modes.
 Once properly programmed, the part will:
Initially lock to either the XA/XB (OSC_P and OSC_N for the Si5374/75) or to CKIN1.
Automatically select CKIN1, if it is available.
Automatically and hitlessly switch to XA/XB if CKIN1 fails.
Automatically and hitlessly switch back to CKIN1 when it subsequently returns.
 For the Si5319:
Clock selection is manual using an input pin.
Clock switching is not hitless.
CKIN2 is not available.
7.5.2. Clock Control Logic in Free Run Mode
Noting that the mux that selects CKIN2 versus the XA/XB oscillator is located before the clock selection and control
logic, when in Free Run mode operation, all such logic will be driven by the XA/XB oscillator, not the CKIN2 pins.
For example, when in Free Run mode, the CK2B pin will reflect the status of the XA/XB oscillator and not the status
of the CKIN2 pins.
Crystal or an external oscillator
(external oscillator only for the Si5374/75)
Si5319,
Si5324,
Si5326,
Si5327,
Si5368,
Si5369,
Si5374,
Si5375
CKOUT1
CKOUT2
Xtal osc
DSPLL
Core
CKIN1
I
2
C/SPI
CKIN2
Control
XA-XB

N31

N31

N32

N32
XAXB