23©2016 Inte grate d Dev ice T echno logy, Inc . Revision 8, Apr il 1 2, 2 016
8V39307 D atash eet
3.8 INPUT CLOCK SELECTION
If the input clock is selected by Forced selection, it can be switched
by setting the related registers (refer to Chapter 3.6.1 Forced Selection)
any time. In this case, whether the input clock is qualified for DPLL lock-
ing does not affect the clock switch.
When the input clock is selected by Automatic selection, the input
clock switch depends on its validity, priority and locking allowance con-
figuration. If the current selected input clock is disqualified, a new quali-
fied input clock may be switched to.
3.8.1 INPUT CLOCK VALIDITY
For all the input clocks, the validity depends on the results of input
clock quality monitoring (refer to Chapter 3.5 Input Clock Quality Moni-
toring). When all of the following conditions are satisfied, the input clock
is valid; otherwise, it is invali d.
•No no-activity alarm (the INn_NO_ACTIVITY_ALARM bit is ‘0’);
•No frequency hard alarm (the INn_FREQ_HARD_ALARM bit is
‘0’);
•If the IN_NOISE_WINDOW bit is ‘1’, all the edges of the input
clock of 2 kHz, 4 kHz or 8 kHz drift inside ±5%; if the
IN_NOISE_WINDOW bit is ‘0’, this condition is ignored.
•No phase lock alarm, i.e., the INn_PH_LOCK_ALARM bit is ‘0’;
•If the ULTR_FAST_SW bit is ‘1’, the DPLL selected input clock
misses less than (<) 2 consecutive clock cycles; if the ULTR_-
FAST_SW bit is ‘0’, this condition is ignored.
The validities of all the input clocks are indicated by the INn 1 bit (3
n 1). When the input clock validity changes (from ‘valid’ to ‘invalid’ or
from ‘invalid’ to ‘valid’), the INn 2 bit will be set. If the INn 3 bit is ‘1’, an
interrupt will be generated.
When the selected input clock has failed, i.e., the validity of the
selected input clock changes from ‘valid’ to ‘invalid’, the MAIN_REF_-
FAILED 1 bit will be set. If the MAIN_REF_FAILED 2 bit is ‘1’, an inter-
rupt will be generated. This interrupt can also be indicated by hardware -
the TDO pin, as determined by the LOS_FLAG_TO_TDO bit. When the
TDO pin is used to indicate this interrupt, it will be set high when this
interrupt is generated and will remain high until this interrupt is cleared.
3.8.2 INPUT CLOCK SELECTION
When the device is configured as Automatic input clock selection,
Revertive and Non-Revertive switchings are supported, as selected by
the REVERTIVE_MODE bit.
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.
Conditions of the qualified input clocks available for selection are:
• Valid, i.e., the INn 1 bit is ‘1’;
• Priority enabled, i.e., the corresponding INn_SEL_PRIORITY[3:0]
bits are not ‘0000’;
• Locking to the input clock is allowed, i.e., the corresponding
INn_VALID bit is ‘0’.
The input clock is disqualified if any of the above conditions is not
satisfied.
In summary, the selected input clock can be switched by:
•Forced selection;
•Revertive switching;
•Non-Revertive switching.
3.8.2.1 Revertive Switching
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.
The selected input clock is switched if any of the following is satis-
fied: •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 and has
the same priority, the input clock with the smallest ‘n’ is selected.
3.8.2.2 Non-Revertive Switching
In Non-Revertive switching, the DPLL selected input clock is not
switched when another qualified input clock with a higher priority than
the current selected input clock is available. In this case, the selected
input clock is switched and a qualified input clock with the highest prior-
ity is selected only when the DPLL selected input clock is disqualified. If
more than one qualified input clock is available and has the same prior-
ity, the input clock with the smallest ‘n’ is selected.
3.8.3 SELECTED / QUALIFIED INPUT CLOCKS INDICATION
The selected input clock is indicated by the CURRENTLY_SELECT-
ED_INPUT[3:0] bits.
The qualified input clocks with the three highest priorities are indi-
cated by HIGHEST_PRIORITY_VALIDATED[3:0] bits, the SECOND_
PRIORITY_VALIDATED[3:0] bits and the THIRD_PRIORITY _VALI-
DATED[3:0] bits respectively. If more than one input clock INn has the
same priority, the input clock with the smallest ‘n’ is indicated by the
HIGHEST_PRIORITY_VALIDATED[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.