82P33714_17 Datasheet, PDF (28/64 Pages) Integrated Device Technology – Synchronous Equipment Timing Source for Synchronous Ethernet

82P33714 Datasheet
3.2.3.3.2.2 Revertive and Non-Revertive Switching
For DPLL1, Revertive and Non-Revertive switchings are supported,
as selected by the REVERTIVE_MODE bit.
For DPLL2, only Revertive switching is supported.
GR-1244 defines Revertive and Non-Revertive Reference switching.
In Non-Revertive switching, a switch to an alternate reference is main-
tained even after the original reference has recovered from the failure
that caused the switch. In Revertive switching, the clock switches back
to the original reference after that reference recovers from the failure,
independent of the condition of the alternate reference. In Non-Revertive
switching, input clock switch is minimized.
In Revertive switching, the selected input clock is switched when
another qualified input clock with a higher priority than the current
selected input clock is available. Therefore, if REVERTIVE_MODE bit is
set to “1”, then the selected input clock is switched if any of the following
is satisfied:
• the selected input clock is disqualified;
• another qualified input clock with a higher priority than the
selected input clock is available.
A qualified input clock with the highest priority is selected by revertive
switching. If more than one qualified input clock INn is available, then it
is important to set appropriate priorities to the input clocks, two input
clocks must not have the same priority.
In Non-Revertive switching, the DPLL1 selected input clock is not
switched when another qualified input clock with a higher priority than
the current selected input clock becomes available. In this case, the
selected input clock is switched and a qualified input clock with the high-
est priority is selected only when the DPLL1 selected input clock is dis-
qualified. If more than one qualified input clock INn is available, then it is
important to set appropriate priorities to the input clocks, two input
clocks must not have the same priority.
3.2.3.3.3 Selected / Qualified Input Clocks Indication
The selected input clock is indicated by the CURRENTLY_SELECT-
ED_INPUT[3:0] bits.
When the device is configured in Automatic selection and Revertive
switching is enabled, the input clock indicated by the CURRENTLY_SE-
LECTED_INPUT[3:0] bits is the same as the one indicated by the HIGH-
EST_PRIORITY_VALIDATED[3:0] bits.
For proper operation COARSE_PH_LOS_LIMT_EN must be set to 0.
3.2.3.3.4 Input Clock Loss of Signal
There are 4 LOS input pins (LOS[3:0]) that can be used to disqualify
the input clock. If they are set high, then the associated input clock is
disqualified to be used as an input clock, and therefore the DPLLs will
not lock to that particular input clock.
The 4 LOS pins can be associated with any input clock by setting bits
LOS_EN in INn_LOS_SYNC_CNFG (1 < n < 6) register. By default, the
LOS pins are not associated with any input.
3.2.4 DPLL LOCKING PROCESS
The following events are always monitored for the DPLLs locking
process:
• Fast Loss;
• Fine Phase Loss;
• Hard Limit Exceeding.
For proper operation, COARSE_PH_LOS_LIMT_EN must be set to
0.
3.2.4.1 Fast Loss
A fast loss is triggered when the selected input clock misses 3 con-
secutive clock cycles. It is cleared once an active clock edge is detected.
For DPLL1 the occurrence of the fast loss will result in the DPLL to
unlock if the FAST_LOS_SW bit is ‘1’. For DPLL2, the occurrence of the
fast loss will result in the DPLL to unlock regardless of the
FAST_LOS_SW bit.
©2017 Integrated Device Technology, Inc.
28
Revision 6, January 23, 2017