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5.2.3 Ramped Input Switching
When switching between input clocks that are not synchronized to the same upstream clock source (i.e. are plesiochronous) there will
be differences in frequency between clocks. Ramped switching should be enabled in these cases to ensure a smooth frequency transi-
tion on the outputs. In this situation, it is also advisable to enable phase buildout, as discussed in the previous section to minimize the
input-to-output clock skew after the frequency ramp has completed.
When ramped clock switching is enabled, the Si5397/96 will enter into holdover and then exit from holdover when the exit ramp has
been calculated. This means that ramped switching behaves like an exit from holdover. This is particularly important when switching
between two input clocks that are not the same frequency so that the transition between the two frequencies will be smooth and linear.
Ramped switching is not needed for cases where the input clocks are locked to the same upstream clock source. The CBPro 'DSPLL
Configure' page defaults to enable 'Ramped Exit from Holdover', but the user needs to select the 'Ramped Input Switching & Exit from
Holdover' option when switching between non-synchronized input clocks.The same ramp rate settings are used for both exit from hold-
.
Table 5.6. Ramped Switching Decision Matrix
Frequency Difference be-
tween Input Frequencies
f
Pfd
> 500 kHz
f
Pfd
< 500 kHz
Zero PPM
Select "Ramped Exit from Holdover"
Non-Zero PPM
If difference is:
• Less than 10 ppm, select "Ramped Exit from Hold-
over".
• More than 10 ppm, select "Ramped input switching
and Ramped Exit from Holdover".
Select "Ramped input switching and Ramped
Exit from Holdover".
Table 5.7. Ramped Input Switching Control Registers
Setting Name
Hex Address [Bit Field]
Function
RAMP_SWITCH_EN_PLLA
0x04A6[3]
Enable frequency ramping on an input switch
RAMP_SWITCH_EN_PLLB
0x05A6[3]
RAMP_SWITCH_EN_PLLC
0x06A6[3]
RAMP_SWITCH_EN_PLLD
0x07A6[3]
HSW_MODE_PLLA
0x043A[1:0]
Input switching mode select
HSW_MODE_PLLB
0x053A[1:0]
HSW_MODE_PLLC
0x063A[1:0]
HSW_MODE_PLLD
0x073A[1:0]
5.2.4 Hitless Switching, LOL (Loss of Lock) and Fastlock
When doing a clock switch between clock inputs that are frequency locked, LOL may be momentarily asserted. In such cases, the as-
sertion of LOL will invoke Fastlock. Because Fastlock temporarily increases the loop BW by asynchronously inserting new filter parame-
ters into the DSPLL’s closed loop, there may be transients at the clock outputs when Fastlock is entered or exited. For this reason, it is
suggested that automatic entry into Fastlock be disabled by writing a zero to FASTLOCK_AUTO_EN_PLLx whenever a clock switch
might occur.
5.2.5 External Clock Switching
When applications require an external switch, it is difficult for the the PLL to predict when that switch will occur. The Si5397/96 will
temporarily go into holdover and then exit in a controlled manner to have a minimum phase/frequency transient. If expansion beyond
the maximum number of inputs is required, please see
AN1111: DSPLL Input Clock Expander
which describes how an external FPGA
can be used for this purpose.
Si5397/96 Reference Manual
Clock Inputs
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