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XRT79L71
PRELIMINARY
461
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
There are two methods that can be used to insert the overhead data into the Transmit Overhead Data Input
Interface Block. One method is referred to as Method 1 or the TxOHClk Method, and the other method is
referred to as Method 2 or the TxInClk Method. Each of these methods is described in considerable detail
below.
6.2.2.1
Operating the Transmit Overhead Data Input Interface block using Method 1 - The
TxOHClk Method
This particular method is referred to as the TxOHClk method for the following reasons.
a. The System-Side Terminal Equipment will use the TxOHClk clock output signal from the Transmit
Overhead Data Input Interface block to clock overhead data onto the TxOH input pin.
b. The Transmit Overhead Data Input Interface block will use the falling edge of the TxOHClk clock output
signal to sample and latch the data residing on the TxOH input pin.
To use Method 1, the System-Side Terminal Equipment will need to interface to the following Transmit
Overheads Data Input Interface pins.
TxOH
TxOHClk
TxOHFrame
TxOHIns
If Method 1 is used, the user must design their system such that the System-Side Terminal Equipment will be
interfaced to the Transmit Overhead Data Input Interface block, in a manner as presented below in Figure 221.
Method 1 Operation of the Transmit Overhead Data Input Interface Block
To operate the Transmit Overhead Data Input Interface block, design/configure the System-Side Terminal
Equipment to continuously and repeatedly execute the following tasks.
TASK # 1: The System-Side Terminal Equipment must sample the state of the TxOHFrame output pin from
the XRT79L71 upon the rising edge of the TxOHClk clock signal which is also output from the XRT79L71.
Whenever the System-Side Terminal Equipment samples the TxOHFrame output pin "High" then it will know
that the Transmit Overhead Data Input Interface block is ready to accept overhead bits for a new E3 frame.
TASK # 2: As the System-Side Terminal Equipment samples the TxOHFrame signal, it must also keep track of
the number of rising edges within the TxOHClk signal that have occurred since the last time TxOHFrame was
sampled "High". By doing this, the System-Side Terminal Equipment will be able to keep track of which
FIGURE 221. ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT TO THE TRANSMIT
OVERHEAD DATA INPUT INTERFACE BLOCK WHEN USING METHOD 1
System-Side Terminal
Equipment
XRT779L71 DS3/E3 Framer
E3_OH_Out
E3_OH_Clock_In
Tx_Start_of_Frame
TxOHClk
TxOHFrame
TxOHIns
34.368 MHz Clock Source
TxInClk
TxOH
Insert_OH