4.2 Dynamic PLL Changes
ClockBuilder Pro generates all necessary control register writes to update settings for the entire device, including the ones described
below. This is the case for both “Export” generated files as well as when using the GUI. This is sufficient to cover most applications.
However, in some applications it is desirable to modify only certain sections of the device while maintaining unaffected clocks on the
remaining outputs. If this is the case CBPro provides some frequency changes on the fly examples.
If certain registers are changed while the device is in operation, it is possible for the PLL to become unresponsive (i.e. lose lock indefi-
nitely). Additionally, making single frequency step changes greater than ±350 ppm, either by using the DCO or by directly updating the
M dividers, may also cause the PLL to become unresponsive. Changes to the following registers require this special sequence of
writes:
Control
Register(s)
PXAXB
0x0206[1:0]
MXAXB_NUM
0x0235 – 0x023A
MXAXB_DEN
0x023B – 0x023E
PLL lockup can easily be avoided by using the following the preamble and postamble write sequence below when one of these regis-
ters is modified or large frequency steps are made. Clockbuilder Pro software adds these writes to the output file by default when Ex-
porting Register Files.
To start, write the preamble by updating the following control bits using Read/Modify/Write sequences:
Address
Value
0x0B24
0xC0
0x0B25
0x00
0x0B4E
0x1A
Wait 300 ms for the device state to stabilize.
Then, modify all desired control registers.
Write 0x01 to Register 0x001C (SOFT_RST_ALL) to perform a Soft Reset once modifications are complete.
Write the postamble by updating the following control bits using Read/Modify/Write sequences:
Address
Value
0x0B24
0xC3
0x0B25
0x02
Note, however, that this procedure affects all DSPLLs and outputs on the device.
Si5397/96 Reference Manual
Modes of Operation
silabs.com
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