XRT74L73 Datasheet, PDF (454/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
Bit 1 (DL from NR) within the Rx E3 LAPD Control
Register, as depicted below.
RXE3 LAPD CONTROL REGISTER (ADDRESS = 0X18)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
Not Used
DL from NR
BIT 2
RxLAPD
Enable
RO
RO
RO
RO
R/W
R/W
0
0
0
0
0
0
BIT 1
RxLAPD
Interrupt
Enable
R/W
1
BIT 0
RxLAPD
Interrupt
Status
RUR
0
Writing a “0” into this bit-field causes the LAPD Re-
ceiver to read in the octets from the GC byte-field of
each E3 frame and with these octets, reassembling
the LAPD Message frame. Writing a “1” into this bit-
field causes the LAPD Receiver to receive the LAPD
Message frame octets from the NR byte-field of each
E3 frame.
2. Enabling the LAPD Receiver
The LAPD Receiver must be enabled before it can
begin receiving and processing any LAPD Message
frames. The LAPD Receiver can be enabled by writ-
ing a “1” to Bit 2 (RxLAPD Enable) of the Rx E3 LAPD
Control Register, as indicated below.
RXE3 LAPD CONTROL REGISTER (ADDRESS = 0X18)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Not Used
DL from NR
RxLAPD
Enable
RxLAPD
Interrupt
Enable
RxLAPD
Interrupt
Status
RO
RO
RO
RO
R/W
R/W
R/W
RUR
0
0
0
0
0
1
1
0
Once the LAPD Receiver has been enabled, it will
begin searching for the Flag Sequence octet (0x7E),
in either the “GC” or the “NR” byte-fields within each
incoming E3 frame. When the LAPD Receiver finds
the flag sequence byte, it will assert the Flag Present
bit (Bit 0) within the Rx E3 LAPD Status Register, as
depicted below.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
1
The receipt of the Flag Sequence octet can mean
one of two things.
1. This Flag Sequence byte may be marking the
beginning or end of an incoming LAPD Message
frame.
2. The Received Flag Sequence octet could be just
one of many Flag Sequence octets that are trans-
mitted via the E3 Transport Medium, during idle
periods between the transmission of LAPD Mes-
sage frames.
The LAPD Receiver will negate the Flag Present bit
as soon as it has received an octet that is something
other than the Flag Sequence octet. Once this hap-
pens, the LAPD Receiver should be receiving either
octet # 2 of the incoming LAPD Message, or an
ABORT Sequence (e.g., a string of seven or more
consecutive “1’s”). If this next set of data is an
455