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XRT79L71
PRELIMINARY
107
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
In Figure 45 the System-Side Terminal Equipment samples the TxOHFrame signal being "High" during Rising
Clock Edge # 0+ with the TxOHClk signal. At this point, the System-Side Terminal Equipment knows that the
XRT79L71 is just about to process the very first overhead bit within a given outbound DS3 frame. According to
Table 20, the very first overhead bit to be processed within a given DS3 frame is the first X bit. In order to
facilitate the transmission of the FERF indicator, the System-Side Terminal Equipment must set this particular
bit-field to "0". The System-Side Terminal Equipment begins this process by implementing the following two
tasks concurrently.
TASK 1 - The System-Side Terminal Equipment asserts the TxOHIns input pin by setting it "High".
TASK 2 - The System-Side Terminal Equipment sets the TxOH input pin to "0".
After the System-Side Terminal Equipment has executed these two tasks, the XRT79L71 will sample the
TxOHIns input pin being "High" and the TxOH input pin being set "Low" during the very next falling edge of
TxOHClk (Clock Edge # 0-, in Figure 45). Once the XRT79L71 has sampled these two signals, it will then
insert a "0" into the very first X bit-position, within the outbound DS3 frame.
Upon detection of the very next rising edge of the TxOHClk clock signal (designated as Clock Edge # 1, in
Figure 45), the System-Side Terminal Equipment will negate or de-assert the TxOHIns input signal (e.g.,
toggle it "Low") and cease inserting data into the Transmit Overhead Data Input Interface block until Rising
Clock Edge # 8.
According to Table 20, the occurrence of Rising Clock Edge # 8+ indicates that the
XRT79L71 is just about ready to process the second X bit, within the outbound DS3 frame. In order to facilitate
this transmission of the FERF indicator, this particular X bit must also be set to "0". The System-Side Terminal
Equipment begins this process by implementing the following two tasks concurrently.
TASK 1 - The System-Side Terminal Equipment asserts the TxOHIns input pin by setting it "High".
TASK 2 - The System-Side Terminal Equipment sets the TxOH input pin to "0".
After the System-Side Terminal Equipment has executed these two tasks, the XRT79L71 will sample the
TxOHIns' input pin being "High" and the TxOH input pin being set "Low" during the very next falling edge of
TxOHClk (Clock Edge # 8- in Figure 45). Once the XRT79L71 has sampled these two signals, it will then
insert a "0" into the second X bit-position within the outbound DS3 frame.
Upon detection of the very next rising edge of the TxOHClk clock signal, the System-Side Terminal Equipment
will negate or de-assert the TxOHIns input signal (e.g., toggle it "Low") and cease inserting data into the
Transmit Overhead Data Input Interface block until it samples the TxOHFrame output pin "High". Afterwards,
the System-Side Terminal Equipment will repeat all of the steps that have been outlined in this case study.
4.2.2.2
Operating the Transmit Overhead Data Input Interface block using Method 2 - The TxInClk/
TxOHEnable Method
This particular method is referred to as the TxInClk/TxOHEnable method for the following reasons.
a. The System-Side Terminal Equipment will use the TxOHEnable output pin from the Transmit Overhead
Data Input Interface block to keep track of which overhead bit is being processed by the Transmit
Overhead Data Input Interface at any given time.
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, then the System-Side Terminal Equipment will need to interface to the following Transmit
Overhead Data Input Interface pins.
TxOH
TxOHFrame
TxInClk
TxOHEnable
TxOHIns