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PRELIMINARY
XRT79L71
454
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
"System-Side Terminal Equipment" should "hold" (or NOT advance the very next payload bit) to the "TxSer"
input pin (of the XRT79L71) until it samples the "TxOH_Ind" output "LOW" once again.
In this particular approach, the user must design in the appropriate State Machine circuitry within the "System-
Side" Terminal Equipment in order to properly respond to the state of the "TxOH_Ind" output pin, while
providing the payload data to the Transmit Payload Data Input Interface.
While designing such a "State
Machine" into a CPLD or ASIC design is not very difficult, the user can take advantage of an easier approach
by configuring the Transmit Payload Data Input Interface block to operate in the "Gapped-Clock" Mode.
If the Transmit Payload Data Input Interface block has been configured to operate in the "Gapped-
Clock" Mode
If the Transmit Payload Data Input Interface block is configured to operate in the "Gapped-Clock" Mode, then
the role of the "TxOH_Ind" output pin will change from being the "Overhead Indicator" output pin, to now being
a "demand-clock" output pin. In other words, If the Transmit Payload Data Input Interface block is configured to
operate in the "Gapped-Clock" Mode, then it (the Transmit Payload Data Input Interface block) will generate a
clock pulse (via the "TxOH_Ind" output pin) if and only if it is ready to accept and process a payload bit. If the
Transmit Payload Data Input Interface block is about to process an overhead bit, then it will not generate a
clock pulse via the "TxOH_Ind" output pin. This action will result in the Transmit Payload Data Input Interface
block generating a "gapped" clock signal via the "TxOH_Ind" output pin (hence the term "Gapped-Clock"
Mode).
If the Transmit Payload Data Input Interface block is configured to operate in the "Gapped-Clock" Mode, then
the "System-Side" Terminal Equipment will be expected to update the data on the "TxSer" input pin (of the
XRT79L71) upon the rising edge of the "TxOH_Ind" output signal. The XRT79L71 will sample and latch the
"TxSer" data, upon the falling edge of the "TxOH_Ind" output signal. In this case, there is no need to "check
the state" of a certain output pin, and then "gate" the placement of the next payload bit (on the TxSer input pin)
with the sampled state of this particular signal. The System-Side Terminal Equipment only needs to respond to
the rising edge of this particular "Gapped-Clock" signal.
If the XRT79L71 has been configured to operate in the "Loop-Timing" Mode (e.g., Mode 1), then this "Gapped-
Clock" signal (from the TxOH_Ind output pin) will be derived from the "LIU Recovered Clock signal (from the
Receive DS3 LIU Block). Similarly, if the XRT79L71 has been configured to operate in the "Local-Timing"
Mode, then this "Gapped-Clock" signal will be derived form the "TxInClk" input signal.
Configuring the Transmit Payload Data Input Interface block to operate in the Gapped-Clock Mode
If the Transmit Payload Data Input Interface block is to be configured to operate in the "Gapped-Clock" Mode,
then do all of the following.
STEP 1 - Interface the "System-Side" Terminal Equipment to the Transmit Payload Data Input Interface