XRT72L50 Datasheet, PDF (230/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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TABLE 40: LISTING AND DESCRIPTION OF THE PIN ASSOCIATED WITH THE RECEIVE OVERHEAD DATA OUTPUT
INTERFACE BLOCK (METHOD 2)
SIGNAL NAME TYPE
DESCRIPTION
RxOHFrame
Output Receive Overhead Data Output Interface - Start of Frame Indicator:
The XRT72L50 will drive this output pin "High" (for one period of the RxOH signal), whenever
the first overhead bit, within a given DS3 frame is being driven onto the RxOH output pin.
RxOutClk
Output
Receive Section Output Clock Signal:
This clock signal is derived from the RxLineClk signal (from the LIU) for loop-timing applica-
tions, and the TxInClk signal (from a local oscillator) for local-timing applications. For DS3
applications, this clock signal will operate at 44.736MHz.
The user is advised to design the Terminal Equipment to latch the contents of the RxOH pin,
anytime the RxOHEnable output signal is sampled "High" on the falling edge of this clock sig-
nal.
Interfacing the Receive Overhead Data Output Interface block to the Terminal Equipment (Method 2)
Figure 78 illustrates how one should interface the Receive Overhead Data Output Interface block to the
Terminal Equipment, when using Method 2 to sample and process the overhead bits from the inbound DS3
data stream.
FIGURE 78. THE TERMINAL EQUIPMENT BEING INTERFACED TO THE RECEIVE OVERHEAD DATA OUTPUT INTERFACE
(METHOD 2)
DS3_OH_In
DS3_OH_Enable_In
DS3_Clk_In
Rx_Start_of_Frame
RxOH
RxOHEnable
RxOutClk
RxOHFrame
Terminal Equipment
DS3 Framer
Method 2 Operation of the Terminal Equipment
If the Terminal Equipment intends to sample any overhead data from the inbound DS3 data stream (via the
Receive Overhead Data Output Interface), then it is expected to do the following.
1. Sample the state of the RxOHFrame signal (e.g., the Rx_Start_of_Frame input) on the falling edge of the
RxOutClk clock signal, whenever the RxOHEnable output signal is also sampled "High".
2. Keep track of the number of times that the RxOHEnable signal has been sampled "High" since the last time
the RxOHFrame was also sampled "High". By doing this, the Terminal Equipment will be able to keep track
of which overhead bit is being output via the RxOH output pin. Based upon this information, the Terminal
Equipment will be able to derive some meaning from these overhead bits.
3. Table 41 relates the number of RxOHEnable output pulses (that have occurred since both the RxOHFrame
and the RxOHEnable pins were both sampled "High") to the DS3 overhead bit that is being output via the
RxOH output pin.
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