XRT7250 datasheet(222/463 Pages) EXAR

222 / 463 page

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

203

The receipt of the Flag Sequence octet can mean

one of two things.

1. The Flag Sequence byte marks the beginning or

end of an incoming LAPD Message.

2. The received Flag Sequence octet could be just

one of many Flag Sequence octets that are trans-

mitted via the DS3 Transport Medium, during idle

periods between the transmission of LAPD Mes-

sages.

The LAPD Receiver will clear the Flag Present bit as

soon as it has received an octet that is something

other than the Flag Sequence octet. At this point, 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 1s). If

this next set of data is an abort sequence, then the

LAPD Receiver will assert the RxAbort bit (Bit 6) with-

in the Rx DS3 LAPD Status Register. However, if this

next octet is Octet #2 of an incoming LAPD Message,

then the Rx DS3 LAPD Status Register will begin to

present some additional status information on this in-

coming message. Each of these indicators is pre-

sented below in sequential order.

Bit 3 - RxCR Type - C/R (Command/Response)

Type

This bit-field reflects the contents of the C/R bit-field

within octet #2 of the LAPD Frame Header. When

this bit is "0" it means that this message is originating

from a customer installation. When this bit is "1" it

means that this message is originating from a net-

work terminal.

Bit 4,5 - RxLAPD Type[1, 0] - LAPD Message Type

The combination of these two bit fields indicate the

Message Type and the Message Size of the incoming

LAPD Message frame. Table 40 relates the values of

Bits 4 and 5 to the Incoming LAPD Message Type/

Size.

NOTE: The Message Size pertains to the size of the Infor-

mation portion of the LAPD Message Frame (as presented

in Figure 85).

Bit 3 - Flag Present

The LAPD Receiver should receive another Flag Se-

quence octet, which marks the End of the Message.

Therefore, this bit field should be asserted once

again.

Bit 1 - EndOfMessage - End of LAPD Message

Frame

Upon receipt of the closing Flag Sequence octet, this

bit-field should be asserted. The assertion of this bit-

field indicates that a LAPD Message Frame has been

completely received. Additionally, if this newly re-

ceived LAPD Message is different from the previous

message, then the LAPD Receiver will inform the µC/

µP of the EndOfMessage event by generating an in-

terrupt.

Bit 2 - RxFCSErr - Frame Check Sequence Error

Indicator

The LAPD Receiver will take the incoming LAPD

Message and compute its own version of the Frame

Check Sequence (FCS) word. Afterwards, the LAPD

Receiver will compare its computed value with that it

has received from the remote LAPD Transmitter. If

these two values match, then the LAPD Receiver will

presume that the LAPD Message has been properly

received and the contents of the Received LAPD

Message (payload portion) will be retained at loca-

tions 0xDE through 0x135 in on-chip RAM. The

LAPD Receiver will indicate an error-free reception of

the LAPD Message by keeping this bit field negated

(Bit 2 = 0). However, if these two FCS values do not

match, then the received LAPD Message is corrupted

and the user is advised not to process this erroneous

information. The LAPD Receiver will indicate an erred

receipt of this message by setting this bit-field to 1.

NOTE: The Receive DS3 HDLC Controller block will not

generate an interrupt to the

µP due to the detection of an

FCS error. Therefore, the user is advised to validate each

and every received LAPD message by checking this bit-

field prior to processing the LAPD message.

Removal of Stuff Bits from the Payload Portion of

the incoming LAPD Message

While the LAPD Receiver is receiving a LAPD Mes-

sage, it has the responsibility of removing all of the

"0" stuff bits from the Payload Portion of the incoming

LAPD Message Frame. Recall that the text in Section

3.2.3.2 indicated that the LAPD Transmitter (at the re-

TABLE 40: THE RELATIONSHIP BETWEEN RXLAPDTYPE[1:0] AND THE RESULTING LAPD MESSAGE TYPE AND SIZE

RXLAPD TYPE[1, 0]

MESSAGE TYPE

MESSAGE SIZE

Test Signal Identification

76 bytes

Idle Signal Identification

76 bytes

CL Path Identification

76 bytes

TU-T Path Identification

82 bytes

XRT7250 Datasheet(PDF) 222 Page - Exar Corporation

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