Silicon Laboratories SI5375 - Manual (Page 73)

Si53xx-RM
Rev. 1.2 73
6.5. Si5319, Si5324, Si5326, Si5327, Si5328, Si5368, Si5369, Si5374, Si5375, and Si5376
Free Run Mode
Figure 27. 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, and Si5376).
 Switching between the crystal oscillator and CLKIN1 is graceful and well-behaved.
 Either a crystal or an external oscillator can be used, except for the Si5374/75/76.
 External oscillator connection can be either single ended or differential.
 All other features and specifications remain the same.
6.5.1. 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 Si5374/75/76) 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.
6.5.2. 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 external oscillator (external
oscillator only for the Si5374/75/76,
TCXO/OCXO only for the Si5328)
Si5319,
Si5324,
Si5326,
Si5327,
Si5368,
Si5369,
Si5374,
Si5375,
Si5376
CKOUT1
CKOUT2
Xtal osc
DSPLL
Core
CKIN1
I 2 C/SPI
CKIN2
Control
XA-XB

N31

N31

N32

N32
XA XB