XRT74L73 Datasheet, PDF (360/488 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
The E3, ITU-T G.751 frame consists of 1536 bits. Of
these bytes, 1524 bits are payload bits and the re-
maining 12 bits are overhead bits. The XRT74L73
has been designed to handle and process both the
payload type and overhead type bits for each E3
frame.
Within the Receive Section of the XRT74L73, the Re-
ceive Payload Data Output Interface block has been
designed to handle the payload bits. Likewise, the
Receive Overhead Data Output Interface block has
been designed to handle and process the overhead
bits.
The Receive Overhead Data Output Interface block
unconditionally outputs the contents of all overhead
bits. The XRT74L73 does not offer the user a means
to shut off this transmission of data. However, the
Receive Overhead Output Interface block does pro-
vide the user with the appropriate output signals for
external Data Link Layer equipment to sample and
process these overhead bits, via the following two
methods.
• Method 1- Using the RxOHClk clock signal.
• Method 2 - Using the RxClk and RxOHEnable out-
put signals.
Each of these methods are described below.
5.3.4.1 Method 1 - Using the RxOHClk Clock
signal
The Receive Overhead Data Output Interface block
consists of four (4) signals. Of these four signals, the
following three signals are to be used when sampling
the E3 overhead bits via Method 1.
• RxOH
• RxOHClk
• RxOHFrame
Each of these signals are listed and described below
in Table 79 .
Interfacing the Receive Overhead Data Output In-
terface block to the Terminal Equipment (Method
1)
Figure 152 illustrates how one should interface the
Receive Overhead Data Output Interface block to the
Terminal Equipment when using Method 1 to sample
and process the overhead bits from the inbound E3
data stream.
FIGURE 152. HOW TO INTERFACE THE TERMINAL EQUIPMENT TO THE RECEIVE OVERHEAD DATA OUTPUT INTER-
FACE BLOCK FOR METHOD 1
E3_OH_Clock_In
RxOHClk
E3_OH_In
RxOH
Rx_Start_of_Frame
RxOHFrame
Terminal Equipment
XRT74L73 E3 Framer
Method 1 Operation of the Terminal Equipment
If the Terminal Equipment intends to sample any
overhead data from the inbound E3 data stream (via
the Receive Overhead Data Output Interface block)
then it is expected to do the following:
1. Sample the state of the RxOHFrame signal (e.g.,
the Rx_Start_of_Frame input signal) on the rising
edge of the RxOHClk (e.g., the
E3_OH_Clock_In) signal.
2. Keep track of the number of rising clock edges
that have occurred in the RxOHClk (e.g., the
E3_OH_Clock_In) signal, since the last time the
RxOHFrame signal was sampled "High”. By
doing this, the Terminal Equipment will be able to
keep track of which overhead byte is being output
via the RxOH output pin. Based upon this infor-
mation, the Terminal Equipment will be able to
derive some meaning from these overhead bits.
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