GS2985 Datasheet, PDF (21/45 Pages) Gennum Corporation – Uses standard 27MHz crystal

4.5 Crystal Buffer
The GS2985 features a crystal buffer supporting a Gennum recommended external 27MHz
crystal. The GS2985 requires an external 27MHz reference clock for correct operation. This
reference clock is generated by connecting a crystal to the XTAL- and XTAL+ pins of the device.
Alternately, a 27MHz external clock source can be connected to the XTAL- pin of the device,
while the XTAL+ pin should be left floating.
4.6 LOS (Loss Of Signal) Detection
The LOS (Loss Of Signal) status pin is an active-high output that indicates when the serial digital
input signal selected at the 4:1 input mux is invalid. In order for this output to be asserted,
transitions must not be present for a period of t_LA = 5 - 10μs. After this output has been asserted,
LOS will de-assert within t_LD = 0 - 5μs after the appearance of a transition at the DDIx input. See
Figure 4-1.
This signal is HIGH (signal lost), when the number of data edges within a window is below a
defined threshold. The output is automatically muted when LOS is detected.
This signal is LOW (signal valid), when the number of data edges within a window is above a
defined threshold. See Table 4-4.
Table 4-4: LOS Operation
LOS
HIGH
LOW
Signal
Invalid
Valid
The LOS function is operational for all operating modes of the device.
t _LA
t _LD
DATA
LOS
Figure 4-1: LOS Signal Timing
The LOS detector has two major modes. In legacy mode, a simple edge-based detector is used to
monitor the received signal at the output of the data slicer. Since the incoming signal has
undergone considerable gain by this point, the legacy detector can be more susceptible to false
de-assertion of LOS for unused channels which experience significant cross-talk from adjacent
active channels.
GS2985 Multi-Rate SDI Reclocker with Equalization &
De-emphasis
Data Sheet
36663 - 5
July 2012
21 of 45
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