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XRT79L71
PRELIMINARY
399
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
Figure 181 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 9 - 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 reading 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 is 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”TRANSMITTING The approach to 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.
FIGURE 181. LAPD MESSAGE FRAME FORMAT
Flag Sequence (8 bits)
SAPI (6-bits)
C/R
EA
TEI (7 bits)
EA
Control (8-bits)
76 or 82 or Any-size Bytes of Information (Payload)
FCS - MSB
FCS - LSB
Flag Sequence (8-bits)