5.2.2 Hitless Input Switching with Phase Buildout
Phase buildout, also referred to as hitless switching, prevents a phase change from propagating to the output when switching between
two clock inputs with an integer related frequency and a fixed phase relationship (i.e., they are phase/frequency locked, but with a non-
zero phase difference). When phase buildout is enabled, the DSPLL absorbs the phase difference between the two input clocks during
a clock switch. When phase buildout is disabled, the phase difference between the two inputs is propagated to the output at a rate
determined by the DSPLL loop bandwidth. Lower PLL loop bandwidth provides more filtering.
Hitless Switching with Phase Buildout should be used for applications where the input clocks are all locked to a common upstream
clock, as in most synchronization systems. Hitless switching is supported for input frequencies down to 8 kHz. Gapped clocks are not
recommended for use with Hitless Switching, as this may increase the phase transient on the outputs.
Table 5.5. Hitless Switching Enable Bit
Setting Name
Hex Address [Bit Field]
Function
Si5397
Si5396
HSW_EN_PLLA
0436[2]
0436[2]
Phase Buildout Switching Enable/Disable for DSPLL A,
B, C, D. Phase Buildout Switching is enabled by default.
HSW_EN_PLLB
0536[2]
0536[2]
HSW_EN_PLLC
0636[2]
—
HSW_EN_PLLD
0737[2]
—
Si5397/96 Reference Manual
Clock Inputs
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