XRT72L56 Datasheet, PDF (450/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
Within the Receive Section of the XRT72L56, 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 XRT72L56 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.
6.3.4.1 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 88.
Interfacing the Receive Overhead Data Output In-
terface block to the Terminal Equipment (Method
1)
Figure 204 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 204. ILLUSTRATION OF HOW TO INTERFACE THE TERMINAL EQUIPMENT TO THE RECEIVE OVERHEAD DATA
OUTPUT INTERFACE BLOCK (FOR METHOD 1).
E3_OH_Clock_In
E3_OH_In
Rx_Start_of_Frame
RxOHClk
RxOH
RxOHFrame
Terminal Equipment
XRT72L5x 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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