XRT74L73 Datasheet, PDF (206/488 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
sion of the Yellow Alarm this X-Bit must also be set to
0. Hence, the Terminal Equipment will (once again)
implement the following steps, concurrently.
a. Assert the TxOHIns input pin by setting it "High".
b. Set the TxOH input to 0.
Once again, after the Terminal Equipment has ap-
plied these signals, the XRT74L73 will sample the da-
ta on both the TxOHIns and TxOH signal upon the
very next falling edge of TxOHClk (designated as 8-
in Figure 65 ). Once the XRT74L73 has sampled this
data, it will then insert a "0" into the second X bit posi-
tion, in the outbound DS3 frame.
4.2.2.2 Method 2 - Using the TxInClk and TxO-
HEnable Signals
Method 1 requires the use of an additional clock sig-
nal, TxOHClk. However, there may be a situation in
which the user does not wish to accommodate and
process this extra clock signal to their design, in order
to use the Transmit Overhead Data Input Interface.
Hence, Method 2 is available. When using Method 2,
either the TxInClk or RxOutClk signal is used to sam-
ple the overhead bits and signals which are input to
the Transmit Overhead Data Input Interface. Method
2 involves the use of the following signals:
• TxOH
• TxInClk
• TxOHFrame
• TxOHEnable
Each of these signals are listed and described in
Table 38 .
TABLE 38: DESCRIPTION OF METHOD 2 TRANSMIT OVERHEAD INPUT INTERFACE SIGNALS
NAME
TxOHEnable
TxOHFrame
TxOHIns
TxOH
TYPE
DESCRIPTION
Output Transmit Overhead Data Enable Output pin
The XRT74L73 will assert this signal, for one TxInClk period, just prior to the instant that
the Transmit Overhead Data Input Interface is processing an overhead bit.
Output Transmit Overhead Input Interface Frame Boundary Indicator Output:
This output signal pulses "High" when the XRT74L73 is processing the last bit within a
given DS3 frame.
Input Transmit Overhead Data Insert Enable input pin.
Asserting this input signal (e.g., setting it "High") enables the Transmit Overhead Data Input Inter-
face to accept overhead data from the Terminal Equipment. In other words, while this input pin is
"High", the Transmit Overhead Data Input Interface will sample the data at the TxOH input pin, on
the falling edge of the TxInClk output signal.
Conversely, setting this pin "Low" configures the Transmit Overhead Data Input Interface to NOT
sample (e.g., ignore) the data at the TxOH input pin, on the falling edge of the TxOHClk output
signal.
NOTE: If the Terminal Equipment attempts to insert an overhead bit that cannot be accepted by
the Transmit Overhead Data Input Interface (e.g., if the Terminal Equipment asserts the TxOHIns
signal, at a time when one of these non-insertable overhead bits are being processed), that par-
ticular insertion effort will be ignored.
Input Transmit Overhead Data Input pin:
The Transmit Overhead Data Input Interface accepts the overhead data via this input
pin, and inserts into the overhead bit position within the very next outbound DS3 frame.
If the TxOHIns pin is pulled "High", the Transmit Overhead Data Input Interface will
sample the data at this input pin (TxOH), on the falling edge of the TxOHClk output pin.
Conversely, if the TxOHIns pin is pulled "Low", then the Transmit Overhead Data Input
Interface will NOT sample the data at this input pin (TxOH). Consequently, this data will
be ignored.
Interfacing the Transmit Overhead Data Input Interface to
the Terminal Equipment
Figure 66 illustrates how one should interface the
Transmit Overhead Data Input Interface to the Termi-
nal Equipment when using Method 2.
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