5. Dynamic PLL Changes
It is possible for the PLL to become unresponsive (i.e., lose lock indefinitely) when it is dynamically reprogrammed or changed via the
serial port. Any change that causes the VCO frequency to change by more than 250 ppm since Power-up, a NVM download, assertion
of SOFT_RST, or changes to any of the following list of registers will require the special PLL re-initialization sequence below
Registers:
• XAXB_FREQ_OFFSET
• PXAXB
• MXAXB_NUM
• MXAXB_DEN
• M_NUM
• M_DEN
PLL Re-Initialization Sequence:
1. First, the preamble:
Write 0x0B24 = 0xC0
Write 0x0B25 = 0x00
2. Wait 300 ms for Grade A/B/C/D, Wait 625ms for Grade P
3. Then, perform the desired register modifications.
4. Write SOFT_RST - 0x001C[0] = 1
5. Write the post-amble:
Write 0x0B24 = 0xC3
Write 0x0B25 = 0x02
Note:
This programming sequence applies only to Rev D and later revisions. The preamble and postamble values for updating certain
registers during device operation are different for earlier revisions. Either the new or old values below may be written to revision D or
later devices without issue. No system software changes are necessary for legacy systems. When writing old values, note that reading
back these registers will not give the written old values, but will reflect the new values. Skyworks recommends using the new values
for all revision D (described above) and later designs, since the write and read values will match. Please contact Skyworks if you need
information about an earlier revision. Please always ensure to use the correct sequence for the correct revision of the device. Also
check for the latest information online. This information is updated from time to time. The latest information is always posted online.
5.1 Dynamic Changes to Output Frequencies without Changing PLL Settings
This section applies to the following scenario:
1. A CBPro generated register map "was" used to program either the volatile or the non-volatile memory of a Si5391. Changes to
output frequencies without changing the PLL settings are desired.
2. The CBPro project file can be used to look for the VCO frequency (FVCO), Ry, Nx values for each OUTy in the design report
and/or the datasheet addendum.
OUTy = FVCO/(Nx * Ry)
Solve for Nx based on the desired OUTy. The Nx dividers can be digitally controlled to so that all outputs connected to the Nx divider
change frequency in real time without any transition glitches. There are two ways to control the Nx divider to accomplish this task:
1. Use the Frequency Increment/Decrement Pins or register bits.
2. Write directly to the numerator or denominator of the Nx divider.
The details of both methods are covered in
10.1 Using the N Dividers for DCO Applications
Si5391 Reference Manual • Dynamic PLL Changes
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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