XR72L52 Datasheet, PDF (438/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L52
REV. 1.0.1
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
The LAPD Receiver can receive the LAPD Message frame octets via either the GC-byte-field or the NR-byte-
field, within each incoming E3 frame. The user makes this selection by writing the appropriate bit to Bit 3 (DL
from NR) within the Rx E3 LAPD Control Register, as depicted below.
RXE3 LAPD CONTROL REGISTER (ADDRESS = 0X18)
BIT 7
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
DL from NR
BIT 2
RxLAPD
Enable
R/W
R/W
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 Receiver 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 writing 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
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
DL from NR
BIT 2
RxLAPD
Enable
R/W
R/W
0
1
BIT 1
RxLAPD
Interrupt
Enable
R/W
1
BIT 0
RxLAPD
Interrupt
Status
RUR
0
Once the LAPD Receiver has been enabled, it will begin searching for the Flag Sequence octet (0x7E), in ei-
ther the GC or the NR byte-fields within each incoming E3 frame. When the LAPD Receiver finds the flag se-
quence byte, it will assert the Flag Present bit (Bit 0) within the Rx E3 LAPD Status Register, as depicted be-
low.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
Rx ABORT
RO
RO
0
0
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
0
0
BIT 3
RxCR
Type
RO
0
BIT 2
RxFCS
Error
RO
0
BIT 1
End of
Message
RO
0
BIT 0
Flag
Present
RO
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 transmitted
via the E3 Transport Medium, during idle periods between the transmission of LAPD Message 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 happens, the LAPD Receiver should be receiving either octet # 2 of
422