ACS8525T View Datasheet(PDF) - Semtech Corporation
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Description
MFG CO.
'ACS8525T' PDF : 112 Pages View PDF
ACS8525 LC/P
ADVANCED COMMUNICATIONS
FINAL
DATASHEET
z An Early/Late phase detector for fine resolution
z The coarse phase lock detector, which monitors whole
z A multi-cycle phase detector for large input jitter
tolerance (up to 8191 UI), which captures and
cycle slips
z Detection that the DPLL is at min. or max. frequency
remembers phase differences of many cycles
between input and feedback clocks.
The phase detectors can be configured to be immune to
occasional missing input clock pulses by using nearest
edge detection (±180°capture) or the normal
± 360° phase capture range which gives frequency
locking. The device will automatically switch to nearest
edge locking when the multi-UI phase detector is not
enabled, and the other phase detectors have detected
that phase lock has been achieved. It is possible to
disable the selection of nearest edge locking via Reg. 03
Bit 6 (set to 1). In this setting, frequency locking will
always be enabled.
The balance between the first two types of phase detector
employed can be adjusted via Reg. 6A to 6D. The default
settings should be sufficient for all modes. Adjustment of
these settings affects only small signal overshoot and
bandwidth.
z Detection of no activity on the input
Each of these sources of phase loss indication is
individually enabled via register bits (see Reg. 73 and 74).
Phase lock or lost is used to determine whether to switch
to nearest edge locking and whether to use acquisition or
normal bandwidth settings for the DPLL. Acquisition
bandwidth is used for faster pull-in from an unlocked
state.
The coarse phase lock detector detects phase differences
of n cycles between input and feedback clocks, where n is
set by Reg. 74 Bits [3:0]; the same register that is used for
the coarse phase detector range, since these functions go
hand in hand. This detector may be used in the case
where it is required that a phase loss indication is not
given for reasonable amounts of input jitter and so the
fine phase loss detector is disabled and the coarse
detector is used instead.
The multi-cycle phase detector (wide-range) is enabled via Phase Compensation Functions
Reg. 74, Bit 6 set to 1 and the range is set in exponentially
increasing steps from ±1 UI up to 8191 UI via Reg. 74,
Bits [3:0].
When this detector is enabled it keeps a track of the
correct phase position over many cycles of phase
difference to give excellent jitter tolerance. This provides
an alternative to switching to Lock8k mode as a method
of achieving high jitter tolerance.
An additional control (Reg. 74 Bit 5) enables the
multi-phase detector value to be used in the final phase
value as part of the DPLL loop. When enabled by setting
High, the multi cycle phase value will be used in the loop
and gives faster pull-in (but more overshoot). The
characteristics of the loop will be similar to Lock8k mode
where again large input phase differences contribute to
the loop dynamics. Setting the bit Low only uses a max
figure of 360° in the loop and will give slower pull-in but
gives less overshoot. The final phase position that the
loop has to pull in to is still tracked and remembered by
the multi-cycle phase detector in either case.
Phase Lock/Loss Detectors
Phase lock detection is handled in several ways. Phase
The ACS8525 has the following phase compensation
functions and controls:
z Phase Build-out (PBO)
z PBO Phase Offset
z Input-to-Output Phase Adjustment
Phase Build-out
Phase Build-out (PBO) is the function to minimize phase
transients on the output SEC clock during input reference
switching. If the currently selected input reference clock
source is lost (due to a short interruption or complete loss
of reference), the next highest priority SEC will be
selected, and a PBO event triggered. When a PBO event is
triggered, the device enters a temporary Holdover state.
When in this temporary state, the phase of the input
reference is measured, relative to the output. The device
then automatically accounts for any measured phase
difference and adds the appropriate phase offset into the
DPLL to compensate.
Following a PBO event, whatever the phase difference on
change of input, the output phase transient is minimized
to be typically less than ±2.5 ns (in digital feedback
loss can be triggered from:
mode).
z The fine phase lock detector, which measures the
phase between input and feedback clock
On the ACS8525, PBO can be enabled, disabled or frozen
using the Serial interface. By default, it is enabled. When
Revision 3.01/August 2005 © Semtech Corp.
Page 19
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