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XRT79L71
PRELIMINARY
569
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
This particular method is referred to as the "RxClk/RxOHEnable" method for the following reasons.
a. The System-Side Terminal Equipment will use the "RxOHEnable" output pin (form the Receive
Overhead Data Output Interface block) to keep track of which overhead bit is being processed by the
Receive Overhead Data Output Interface (via the RxOH output pin) as any given time.
b. The System-Side Terminal Equipment must use the falling edge of "RxClk" in order to sample and latch
the data residing on the "RxOH" output pin.
If "Method 2" is used then the "System-Side" Terminal Equipment will need to interface to the following
"Receive Overhead Data Output Interface" pins.
RxOH
RxOHFrame
RxOHEnable
RxClk
If "Method 2" is used then design your system such that the "System-Side" Terminal Equipment will be
interfaced to the "Receive Overhead Data Output Interface" block as shown below in Figure 260.
Method 2 Operation of the Receive Overhead Data Output Interface Block
If the Receive Overhead Data Output Interface block is operated per Method 2", 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 DS3 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.
FIGURE 260. 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