XRT7250 datasheet(427/463 Pages) EXAR

427 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

408

ABORT Sequence, then the LAPD Receiver will as-

sert the RxABORT bit-field (Bit 6) within the Rx E3

LAPD Status Register, as depicted below.

However, if this next octet is Octet #2 of an incoming

LAPD Message frame, then the LAPD Receiver is be-

ginning to receive a LAPD Message frame.

As the LAPD Receiver receives this LAPD Message

frame, it is reading in the LAPD Message frame oc-

tets, from either the “GC” or the “NR” byte-fields with-

in each incoming E3 frame. Secondly, it is reassem-

bling these octets into a LAPD Message frame.

Once the LAPD Receiver has received the complete

LAPD Message frame, then it will proceed to perform

the following five (5) steps.

1. PMDL Message Extraction

The LAPD Receiver will extract out the PMDL Mes-

sage, from the newly received LAPD Message frame.

The LAPD Receiver will then write this PMDL Mes-

sage into the Receive LAPD Message buffer within

the Framer IC.

NOTE: As the LAPD Receiver is extracting the PMDL Mes-

sage, from the newly received LAPD Message frame, the

LAPD Receiver will also check the PMDL data for the

occurrence of stuff bits (e.g., “0s” that were inserted into the

PMDL Message by the Remote LAPD Transmitter, in order

to prevent this data from mimicking the Flag Sequence byte

or an ABORT Sequence), and remove them prior to writing

the PMDL Message into the Receive LAPD Message

Buffer. Specifically, the LAPD Receiver will search through

the PMDL Message data and will remove any “0” that

immediately follows a string of 5 consecutive “1’s”.

For more information on how the LAPD Transmitter

inserted these stuff bits, please see Section 5.2.3.1.

2. FCS (Frame Check Sequence) Word Verification

The LAPD Receiver will compute the CRC-16 value

of the header octets and the PMDL Message octets,

within this LAPD Message frame and will compare it

with the value of the two octets, residing in the FCS

word-field of this LAPD Message frame. If the FCS

value of the newly received LAPD Message frame

matches the locally-computed CRC-16 value, then

the LAPD Receiver will conclude that it has received

this LAPD Message frame in an error-free manner.

However, if the FCS value does not match the locally-

computed CRC-16 value, then the LAPD Receiver

will conclude that this LAPD Message frame is erred.

The LAPD Receiver will indicate the results of this

FCS Verification process by setting Bit 2 (RxFCS Er-

ror) within the Rx E3 LAPD Status Register, to the ap-

propriate value as tabulated below.

If the LAPD Receiver detects an error in the FCS val-

ue, then it will set the RxFCS Error bit-field to “1”.

Conversely, if the LAPD Receiver does not detect an

error in the FCS value, the it will clear the RxFCS Er-

ror bit-field to “0”.

NOTE: The LAPD Receiver will extract and write the PMDL

Message into the Receive LAPD Message buffer indepen-

dent of the results of FCS Verification. Hence, the user is

urged to validate each PMDL Message that is read in from

the Receive LAPD Message buffer, by first checking the

state of this bit-field.

3. Check and Report the State of the “C/R” Bit-field

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

Rx ABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

000

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

Rx ABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

000

100

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