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PRELIMINARY
XRT79L71
410
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
RxOHEnable
RxClk
If "Method 2" is used, the users system must be designed such that the "System-Side" Terminal Equipment
will be interfaced to the "Receive Overhead Data Output Interface" block, in a manner as presented below in
Method 2 Operation of the Receive Overhead Data Output Interface Block
If the Receive Overhead Data Output Interface block is to be operated per Method 2", then design/configure
the "System-Side" Terminal Equipment to continuously execute the following tasks.
TASK # 1: The "System-Side" Terminal Equipment must sample the states of the "RxOHFrame", "RxOH" and
the "RxOHEnable" output pins (from the XRT79L71) upon the falling edge of "RxClk" clock output signal. The
XRT79L71 will pulse the "RxOHEnable" output pin "high" for one "RxClk" period, coincident to whenever it is
"safe" (or OK) to sample the states of the "RxOH" and the "RxOHFrame" output pins. If the System-Side
Terminal Equipment samples both the "RxOHEnable" and the "RxOHFrame" output pin "high", then it "knows"
that the Receive Overhead Data Output Interface block is currently placing the very first overhead bit (within
the most recently received E3 frame) via the "RxOH" output pin.
TASK # 2: As the "System-Side" Terminal Equipment samples the "RxOHEnable" and "RxOHFrame" output
signals, it must also keep track of the number of times that the "RxOHEnable" output pin has been sampled
"high" since the last time that both the "RxOHEnable" and "RxOHFrame" output pins have been sampled
"high". By doing this, the "System-Side" Terminal Equipment will be able to keep track of which overhead bits
are being output via the "RxOH" output at any time. This will permit the "System-Side Terminal Equipment to
properly handle these overhead bits as appropriate.
Table 50 relates the number of "RxOHEnable" output pulses that have occurred since both the "RxOHFrame"
and the "RxOHEnable" output pins were sampled "high", to the E3 Overhead Bit that is being output via the
"RxOH" output pin. The user can use this table as a guide for extracting the appropriate overhead bits, via the
"Receive Overhead Data Output Interface" block, whenever "Method 2" is used.
FIGURE 186. ILLUSTRATION OF HOW TO INTERFACE THE "SYSTEM-SIDE TERMINAL EQUIPMENT" TO THE "RECEIVE
OVERHEAD DATA OUTPUT INTERFACE" BLOCK WHEN USING "METHOD 2"
System-Side Terminal
Equipment
XRT79L71 DS3/E3 Framer
E3_OH_In
E3_OH_Enable
Rx_Start_of_Frame
RxOHEnable
RxOHFrame
RxClk
RxOH
E3_OH_Clock_In