Si5345-44-42-D-RM
22
Rev. 1.0
5.1.2. Automatic Input Selection
In automatic mode CLK_SWITCH_MODE = 0x01 (non-revertive) or 0x02 (revertive)
An automatic input selection is available in addition to the above mentioned manual switching option described in
“5.1.1. Manual Input Switching”. In automatic mode, the selection criteria is based on input clock qualification, input
priority and the revertive option. The IN_SEL[1:0] pins or IN_SEL[2:1] register bits are not used in automatic input
selection. Also, only input clocks that are valid (i.e., with no active alarms) can be selected by the automatic clock
selection. If there are no valid input clocks available the DSPLL will enter the holdover mode. With revertive
switching enabled, the highest priority input with a valid input clock is always selected. If an input with a higher
priority becomes valid then an automatic switchover to that input will be initiated. With non-revertive switching, the
active input will always remain selected while it is valid. If it becomes invalid an automatic switchover to a valid
input with the highest priority will be initiated.
When in zero delay mode (ZDM_EN (0x0487[0]) the phase difference between the output, which is connected to
the selected input, will be nulled to zero. However the IO delay variation will substantially increase in ZDM mode if
the Fpfd is much below 64 kHz. When in zero delay mode, the DSPLL must have the phase buildout turned off for
input switching or else the IO delay can change on each input switch.
Manual control of the input clock selection is
by either pin or register and also depends upon the device being in zero delay mode or not. See Table
11.
Table 10. Registers for Automatic Input Selection
Register Name
Hex
Address
[Bit Field]
Function
CLK_SWITCH_MODE 0x0536[1:0] Selects manual or automatic switching modes. Automatic mode can be
revertive or non-revertive. Selections are the following: 00 Manual,01
Automatic non-revertive 02 Automatic revertive, 03 Reserved
ZDM_EN
0x0487[0]
0: disable zero delay mode
1: enable zero delay mode
ZDM_IN_SEL
0x0487[2:1] Selects the input when in manual register controlled mode when zero
delay mode is enabled. Selections are IN0,IN1,IN2. A register value of 3
is not allowed.
ZDM_AUTOSW_EN
0x0487[4]
0: automatic switching disabled for zero-delay mode
1: automatic input switching enabled and input clock selection governed
by automatic input switching engine
IN0_PRIORITY
0x0538[2:0] IN0, IN1, IN2, IN3 priority select for the automatic selection state machine.
Priority selections are 1,2,3,4, or zero for never selected.
IN1_PRIORITY
0x0538[6:4]
IN2_PRIORITY
0x0539[2:0]
IN3_PRIORITY
0x0539[6:4]
IN_LOS_MSK
0x0537[3:0] Determines the LOS status for IN3,2,1,0 and is used in determining a
valid clock for automatic input selection
0 to use LOS in clock selection logic, 1 to mask LOS from the clock selec-
tion logic
IN_OOF_MSK
0x0537[7:4] Determines the OOF status for IN3,2,1,0 and is used in determining a
valid clock for the automatic input selection
0 to use OOF in the clock selection logic, 1 to mask the OOF from the
clock selection logic