XRT74L73 Datasheet, PDF (270/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
FIGURE 100. ILLUSTRATION OF THE XRT74L73 DS3/E3 FRAMER IC BEING INTERFACED TO THE RECEIVE TERMINAL
EQUIPMENT (SERIAL MODE OPERATION)
Rx_DS3_Clock_In
DS3_Data_In
Rx_Start_of_Frame
Rx_DS3_OH_Ind
44.736 MHz Clock Signal
RxClk
RxSer
RxFrame
RxOHIns
44.736 MHz
Clock Source
RxLineClk
Terminal Equipment
(Receive Payload Section)
XRT72L5x DS3 Framer
Required Operation of the Terminal Equipment
The XRT74L73 will update the data on the RxSer out-
put pin, upon the rising edge of RxClk. However, be-
cause the rising edge of RxClk to data delay is be-
tween 14ns to 16ns, the Terminal Equipment should
sample the data on the RxSer output pin (or the
DS3_Data_In pin at the Terminal Equipment) upon
the rising edge of RxClk. This will still permit the Ter-
minal Equipment with a RxSer to RxClk set-up time of
approximately 6ns and a hold time of 14 to 16ns. As
the Terminal Equipment samples RxSer with each ris-
ing edge of RxClk it should also be sampling the fol-
lowing signals.
• RxFrame
• RxOHInd
The Need for sampling RxFrame
The XRT74L73 will pulse the RxFrame output pin
"High" coincident with it driving the very first bit of a
given DS3 frame onto the RxSer output pin. If knowl-
edge of the DS3 Frame Boundaries is important for
the operation of the Terminal Equipment, then this is
a very important signal for it to sample.
The Need for sampling RxOHInd
The XRT74L73 will indicate that it is currently driving
an overhead bit onto the RxSer output pin, by pulsing
the RxOHInd output pin "High". If the Terminal Equip-
ment samples this signal "High", then it should know
that the bit, that it is currently sampling via the RxSer
pin is an overhead bit and should not be processed.
The Behavior of the Signals between the Receive
Payload Data Output Interface block and the Ter-
minal Equipment
The behavior of the signals between the XRT74L73
and the Terminal Equipment for DS3 Serial Mode Op-
eration is illustrated in Figure 101 .
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