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PRELIMINARY
XRT79L71
466
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
b. The Transmit Overhead Data Input Interface block will use the rising edge of TxInClk in order to sample
and latch the data residing on the TxOH input pin.
If Method 2 is used, the System-Side Terminal Equipment will need to interface to the following Transmit
Overhead Data Input Interface pins.
TxOH
TxOHFrame
TxInClk
TxOHEnable
TxOHIns
To use Method 2 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 223.
Method 2 Operation of the Transmit Overhead Data Input Interface Block
Tto operate the Transmit Overhead Data Input Interface block per Method 2, 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 states of both the TxOHFrame and the
TxOHEnable output pins from the XRT79L71 upon the falling edge of the TxInClk clock input signal. The
XRT79L71 will pulse the TxOHEnable output pin "High" for one TxInClk period, just prior to the instant that the
Transmit Overhead Data Input Interface block is processing an overhead bit. Therefore, if the System-Side
Terminal Equipment samples the TxOHEnable output pin "High", then it knows that an overhead bit insertion
opportunity via the Transmit Overhead Data Input Interface block is just about to occur. If the System-Side
Terminal Equipment samples both the TxOHEnable and the TxOHFrame output pins "High" at the same time,
then it knows that the Transmit Overhead Data Input Interface block is just about to process the very first
overhead bit within a new E3 frame.
TASK # 2: As the System-Side Terminal Equipment samples the TxOHEnable and TxOHFrame signals, it
must also keep track of the number of times that the TxOHEnable output pin has been sampled "High" since
the last time both the TxOHEnable and the TxOHFrame 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 processed by
the Transmit Overhead Data Input Interface block at any give TxOHEnable assertion. When the System-Side
Terminal Equipment knows which overhead bit is currently being processed within a given TxOHEnable
FIGURE 223. ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT TO THE TRANSMIT
OVERHEAD DATA INPUT INTERFACE BLOCK WHEN USING METHOD 2
System-Side Terminal
Equipment
XRT79L71 DS3/E3 Framer
E3_OH_Out
E3_OH_Enable
Tx_Start_of_Frame
TxOHEnable
TxOHFrame
TxOHIns
34.368 MHz Clock Source
TxInClk
TxOH
Insert_OH