XRT74L73 Datasheet, PDF (202/488 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 64 illustrates how one should interface the
Transmit Overhead Data Input Interface to the Termi-
nal Equipment, when using Method 1.
FIGURE 64. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA
INPUT INTERFACE (METHOD 1)
DS3_OH_Clock_In
DS3_OH_Out]
Tx_Start_of_Frame
Insert_OH
44.736 MHz Clock Source
TxInClk
TxOHClk
TxOH
TxOHFrame
TxOHIns
RxLineClk
44.736 MHz
Clock Source
Terminal Equipment
XRT72L5x DS3 Framer
Method 1 Operation of the Terminal Equipment
If the Terminal Equipment intends to insert any over-
head data into the outbound DS3 data stream, (via
the Transmit Overhead Data Input Interface), then it
is expected to do the following.
1. To sample the state of the TxOHFrame signal
(e.g., the Tx_Start_of_Frame input signal) on the
rising edge of the TxOHClk (e.g., the
DS3_OH_Clock_In signal).
2. To keep track of the number of rising clock edges
that have occurred, via the TxOHClk (e.g., the
DS3_OH_Clock_In signal) since the last time the
TxOHFrame signal was sampled "High". By
doing this the Terminal Equipment will be able to
keep track of which overhead bit is being pro-
cessed by the Transmit Overhead Data Input
Interface block at any given time. When the Ter-
minal Equipment knows which overhead bit is
being processed, at a given TxOHClk period, it
will know when to insert a desired overhead bit
value into the outbound DS3 data stream. From
this, the Terminal Equipment will know when it
should assert the TxOHIns input pin and place
the appropriate value on the TxOH input pin (of
the XRT74L73).
Table 37 relates the number of rising clock edges (in
the TxOHClk signal, since TxOHFrame was sampled
"High") to the DS3 Overhead Bit, that is being pro-
cessed.
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