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PRELIMINARY
XRT79L71
246
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
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 the Method 2 is to be used then the System-Side Terminal Equipment will need to interface to the following
Receive Overhead Data Output Interface pins.
RxOH
RxOHFrame
RxOHEnable
RxClk
To use Method 2 then the user must design their system 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
To operate the Receive Overhead Data Output Interface block 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
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.
Table 33 relates the number of RxOHEnable output pulses that have occurred since both the RxOHFrame and
the RxOHEnable output pins were sampled "High", to the DS3 Overhead Bit that is being output via the RxOH
FIGURE 105. ILLUSTRATION ON 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
DS3_OH_In
DS3_OH_Enable
Rx_Start_of_Frame
RxOHEnable
RxOHFrame
RxClk
RxOH
DS3_OH_Clock_In