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PRELIMINARY
XRT79L71
556
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Figure 255 indicates that the complete LAPD/PMDL Message will consist of an information payload of either
76 or 82 bytes, along with a total of six overhead bytes consisting of the Flag Sequence, SAPI, TEI, Control
and the two FCS bytes.
NOTE: When receiving and processing the Standard 76 or 82-byte type of LAPD Messages, then the data residing within
the Receive LAPD Message will be exactly of the byte-format, as presented above in
Why determining the Size of the Incoming LAPD Message is important
If the Microprocessor has learned based upon reading out the values of the RxLAPDType[1:0] bit-fields that
the size of the total LAPD/PMDL Message is 88 bytes, then the Microprocessor knows when it comes time to
read out the contents of the Message, residing within the Receive LAPD Message Buffer that it can read out
and process the contents of 88-bytes out of 90 bytes within this Buffer. Conversely, if the Microprocessor has
learned that the size of the total LAPD/PMDL Message is only 82 bytes, then the Microprocessor must know
that it can only read out and process the first 82 bytes of data within the Receive LAPD Message Buffer. The
last eight bytes within the Buffer are simply junk bytes and have no value.
STEP 10 - Read out the contents of the Receive LAPD Message Buffer
The instructions that follow were written with the assumption that the user only wishes to extract out the
Information Payload bytes, from the complete LAPD Message that is residing within the Receive LAPD
Message Buffer.
Whenever the user wishes to read out the contents of newly received PMDL/LAPD
Messages from the Receive LAPD Message buffer, then the user MUST employ the Indirect Addressing
scheme that will be presented below.
In order to begin the process of reading out the contents of the incoming PMDL Message, then the user must
be aware of the following important Address Locations, within the XRT79L71 Address space.
1.
The LAPD Message Buffer - Indirect Address Register - Address = 0x11C0
2.
The LAPD Message Buffer - Indirect Data Register - Address = 0x11C1
By performing READ or WRITE operations to these two registers, the user can actually obtain READ/WRITE
access to both the Transmit LAPD Message Buffer and the Receive LAPD Message Buffer. This section will
describe the approach that one should use to access the Receive LAPD Message Buffer. The approach that
one should use to access the Transmit LAPD Message Buffer is presented in SEE”RECEIVING NON- The exact approach that one should use, when reading out the contents of the newly received PMDL Message
from the Receive LAPD Message buffer is presented in the flow-chart below.