![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_433.png)
PRELIMINARY
XRT79L71
418
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
If the Receive Payload Data Output Interface block (within the XRT79L71) has been configured to operate in
the "Serial" Mode, then we have recommended that the user design or configure their "System-Side Terminal
Equipment" to do the following, when receiving/accepting E3 data via the "RxSer" output pin.
Check the state of the "RxOH_Ind" output pin (from the XRT79L71) upon the falling edge of the "RxClk"
signal.
Perform either of the following actions, depending upon the sampled state of the "RxOH_Ind" output pin, as
described below.
If RxOH_Ind is sampled "LOW"
Then the "System-Side Terminal Equipment" should accept this particular data bit (that is being sampled from
the RxSer output pin) and treat it as a payload bit.
If RxOH_Ind is sampled "HIGH"
Then the "System-Side Terminal Equipment" should NOT accept this particular data bit (that is being sampled
from the RxSer output pin) and should definitely NOT treat it as a payload bit.
In this particular approach, the user must "gate" the acceptance of a particular data bit (being sampled via the
RxSer output pin) based upon the corresponding sampled state of the "RxOH_Ind" output pin.
While
implementing such a design into a CPLD or ASIC design is not very difficult, the user can take advantage of an
easier approach by configuring the Receive Payload Data Output Interface block to operate in the "Gapped-
Clock" Mode.
5.3.5.1.2
Operating the Receive Payload Data Output Interface block has been configured to operate in
the "Gapped-Clock" Mode
As mentioned above, in order to simplify the task of interfacing the Receive Payload Data Output Interface
block to certain devices, the XRT79L71 permits the user to configure the Transmit Payload Data Input Interface
and the Receive Payload Data Output Interface blocks to operate in the "Gapped-Clock" Mode. If the Receive
Payload Data Output Interface block is configured to operate in the "Gapped-Clock" Mode, then the role of the
"RxOH_Ind" output pin will change from being the "Overhead Indicator" output pin, to now being a "payload
data clock" output pin. In other words, If the Receive Payload Data Output Interface block is configured to
operate in the "Gapped-Clock" Mode, then it (the Receive Payload Data Output Interface block) will only
generate a clock pulse (via the "RxOH_Ind" output pin) coincident to whenever it outputs a payload bit via the
"RxSer" output pin. Whenever the Receive Payload Data Output Interface block outputs an overhead bit (via
the "RxSer" output pin) then it will NOT generate a clock pulse via the "RxOH_Ind" output pin. This action will
result in the Receive Payload Data Output Interface block generating a "gapped" clock signal via the
"RxOH_Ind" output pin (hence the term, "Gapped-Clock" Mode).
If the Receive Payload Data Output Interface block is configured to operate in the "Gapped-Clock" Mode, then
the "System-Side" Terminal Equipment will be expected to sample the data (that is being output via the RxSer
output pin) upon the rising edge of RxClk. In this case, there is no need to "check the state" of the certain
output pin, then "gate" the acceptance/treatment of the next bit (output via the RxSer output pin) based upon
the state of this particular output pin. The System-Side Terminal Equipment only needs to sample the "RxSer"
output signal upon the rising edge of this particular "Gapped-Clock" signal.
Configuring the Receive Payload Data Input Interface block to operate in the Gapped-Clock Mode
To configure the Receive Payload Interface block to operate in the "Gapped-Clock" Mode, do all of the
following.
STEP 1 - Interface the "System-Side" Terminal Equipment to the Receive Payload Data Input Interface
block, in a manner as indicated below.