XRT7250 Datasheet, PDF (238/463 Pages) Exar Corporation – DS3/E3 FRAMER IC

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DS3/E3 FRAMER IC
XRT7250
REV. 1.1.1
The XRT7250 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 XRT7250 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 XRT7250
and the Terminal Equipment for DS3 Serial Mode Op-
eration is illustrated in Figure 94.
FIGURE 94. AN ILLUSTRATION OF THE BEHAVIOR OF THE SIGNALS BETWEEN THE RECEIVE PAYLOAD DATA OUTPUT
INTERFACE BLOCK OF THE XRT7250 AND THE TERMINAL EQUIPMENT (SERIAL MODE OPERATION)
Terminal Equipment Signals
DS3_Clock_In
DS3_Data_In
Rx_Start_of_Frame
DS3_Overhead_Ind
Payload[4702] Payload[4703]
X-Bit
Payload[0]
XRT7250 Receive Payload Data I/F Signals
RxClk
RxSer
Payload[4702] Payload[4703]
RxFrame
RxOH_Ind
X-Bit
Payload[0]
DS3 Frame Number N
Note: RxFrame pulses high to denote
DS3 Frame Boundary.
Note: RxOH_Ind pulses high to
denote Overhead Data
(e.g., the X-bit).
DS3 Frame Number N + 1
Note: X-Bit will not be processed by the
Transmit Payload Data Input Interface.
4.3.5.2 Nibble-Parallel Mode Operation
Behavior of the XRT7250
If the XRT7250 has been configured to operate in the
Nibble-Parallel Mode, then the XRT7250 will behave
as follows.
Payload Data Output
The XRT7250 will output the payload data of the in-
coming DS3 frames, via the RxNib[3:0] output pins,
upon the falling edge of RxClk.
NOTES:
1. In this case, RxClk will function as the Nibble Clock
signal between the XRT7250 the Terminal Equip-
ment. The XRT7250 will pulse the RxClk output
signal "High" 1176 times, for each inbound DS3
frame.
2. Unlike Serial Mode operation, the duty cycle of
RxClk, in Nibble-Parallel Mode operation is approx-
imately 25%.
Delineation of Inbound DS3 Frames
The XRT7250 will pulse the RxFrame output pin
"High" for one nibble-period coincident with it driving
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