XRT74L73 Datasheet, PDF (367/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
Operation of the Receive Payload Data Output In-
terface block
The Receive Payload Data Output Interface permits
the user to read out the payload data of inbound E3
frames, via either of the following modes.
• Serial Mode
• Nibble-Parallel Mode
Each of these modes are described in detail, below.
5.3.5.1 Serial Mode Operation Behavior of the
XRT74L73
If the XRT74L73 has been configured to operate in
this mode, then the XRT74L73 will behave as follows.
Payload Data Output
The XRT74L73 will output the payload data, of the in-
coming E3 frames via the RxSer output pin, upon the
rising edge of RxClk.
Delineation of inbound E3 Frames
The XRT74L73 will pulse the RxFrame output pin
"High" for one bit-period coincident with it driving the
first bit within a given E3 frame, via the RxSer output
pin.
Interfacing the XRT74L73 to the Receive Terminal
Equipment
Figure 157 presents a simple illustration as how the
user should interface the XRT74L73 to that terminal
equipment which processes Receive Direction pay-
load data.
FIGURE 157. THE TERMINAL EQUIPMENT BEING INTERFACED TO THE RECEIVE PAYLOAD DATA INPUT INTERFACE
BLOCK OF THE XRT74L73 FRAMER IC (SERIAL MODE OPERATION)
Rx_E3_Clock_In
E3_Data_In
Rx_Start_of_Frame
Rx_E3_OH_Ind
34.368MHz
Clock Signal
RxClk
RxSer
RxLineClk
RxFrame
RxOHInd
34.368MHz
Clock Source
Terminal Equipment
Receive Payload Section
XRT74L73 E3 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. Hence, the
Terminal Equipment should sample the data on the
RxSer output pin (or the E3_Data_In pin at the Termi-
nal Equipment) upon the rising edge of RxClk. As the
Terminal Equipment samples RxSer with each rising
edge of RxClk it should also be sampling the follow-
ing 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 E3 frame onto the RxSer output pin. If knowl-
edge of the E3 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 E3 Serial Mode Op-
eration is illustrated in Figure 158 .
368