![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_423.png)
PRELIMINARY
XRT79L71
408
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
5.3.4.1
Operating the Receive Overhead Data Output Interface block using Method 1 - The
"RxOHClk" Method
This particular method is referred to as the "RxOHClk" method for the following reasons.
a. The System-Side Terminal Equipment will use the RxOHClk clock output signal (from the Receive
Overhead Data Output Interface block) to sample and latch the "overhead" data via the "RxOH" output
pin.
b. b.The "Receive Overhead Data Output Interface" block will update the data (via the RxOH output pin)
upon the falling edge of the "RxOHClk" clock output signal.
If "Method 1" is used the "System-Side Terminal Equipment" will need to interface to the following "Receive
Overhead Data Output Interface" pins.
RxOH
RxOHClk
RxOHFrame
If "Method 1" 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 the manner as presented below in
Method 1 Operation of the Receive Overhead Data Output Interface Block
If the Receive Overhead Data Output Interface block is to be operated, then design/configure the "System-
Side" Terminal Equipment to continuously execute the following tasks.
TASK # 1: The "System-Side" Terminal Equipment must sample the state of the "RxOHFrame" output pin
(from the XRT79L71) upon the rising edge of the "RxOHClk" clock signal (which is also output from the
XRT79L71). Whenever the "System-Side Terminal Equipment" samples the "RxOHFrame" output pin "high",
then it will "know" 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 "RxOHFrame" output signal, it must also
keep track of the number of rising edges (within the RxOHClk signal) that have occurred since the last time
"RxOHFrame" was sampled "high". By doing this, the "System-Side" Terminal Equipment will be able to keep
track of which overhead bit is being output via the "RxOH" output pin, at any given "RxOHClk" period. When
the System-Side Terminal Equipment "knows" which overhead bit is being output (via the RxOH output pin)
during a particular RxOHClk clock period, then it can "make sense" of these overhead bits and process these
overhead bits as appropriate.
FIGURE 185. ILLUSTRATION OF HOW TO INTERFACE THE "SYSTEM-SIDE TERMINAL EQUIPMENT" TO THE "RECEIVE
OVERHEAD DATA OUTPUT INTERFACE" BLOCK WHEN USING "METHOD 1"
System-Side Terminal
Equipment
XRT779L71 DS3/E3 Framer
E3_OH_In
E3_OH_Clock_In
Rx_Start_of_Frame
RxOHClk
RxOHFrame
RxOH