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XRT79L71
PRELIMINARY
91
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
4.2.1.3), then we have recommended that the user design or configure their System-Side Terminal Equipment
to perform the following procedure when supplying payload data to the TxSer input pin.
Check the state of the TxOH_Ind output pin from the XRT79L71 upon the falling edge of either the TxInClk or
the RxOutClk signal.
Perform either of the following actions, depending upon the sampled state of the TxOH_Ind output pin, as
described below.
If TxOH_Ind is sampled "Low"
Then the System-Side Terminal Equipment should proceed to place the very next payload bit on the TxSer
input pin upon the very next rising edge of either the TxInClk or RxOutClk signal.
If TxOH_Ind is sampled "High"
Then the System-Side Terminal Equipment should NOT proceed to place the very next payload bit on the
TxSer input pin, upon the very next rising edge of either the TxInClk or RxOutClk signal. In this case, the
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 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 from the TxInClk input signal.
Configuring the Transmit Payload Data Input Interface block to operate in the Gapped-Clock Mode
To configure the Transmit Payload Data Input Interface block to operate in the Gapped-Clock Mode do all of
the following.
STEP 1 - The user should interface the System-Side Terminal Equipment to the Transmit Payload Data
Input Interface block, in a manner as indicated below in Figure 42.