
XRT79L71
PRELIMINARY
255
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
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 DS3 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.
4.3.6.1.2
Operating the Receive Payload Data Output Interface block in the 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.