XRT7250 datasheet(311/463 Pages) EXAR

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XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

292

Hence, the purpose of the FAS Pattern Verification

state.

When the Receive E3 Framer block enters this state,

it will then quit performing its bit-by-bit search for the

Frame Alignment Signaling bits. Instead, the Receive

E3 Framer block will read in the 10 bits that occur

1536 bit (e.g., one E3 frame period later) after the

candidate FAS pattern was first detected. If these ten

bits match the assigned values for the FAS Pattern

octets, then the Receive E3 Framer block will con-

clude that it has found the FAS pattern and will then

transition to the In-Frame state. However, if these two

bytes do not match the assigned values for the FAS

pattern then the Receive E3 Framer block will con-

cluded that it has been fooled by data mimicking the

Frame Alignment bytes, and will transition back to the

FAS Pattern Search state.

In Frame State

Once the Receive E3 Framer block enters the In-

Frame state, then it will cease performing Frame Ac-

quisition functions, and will proceed to perform Fram-

ing Maintenance functions. Therefore, the operation

of the Receive E3 Framer block, while operating in

the In-Frame state, can be found in Section 4.3.2.2

(The Framing Maintenance Mode).

OOF (Out of Frame) Condition State

If the Receive E3 Framer while operating in the In-

Frame state detects four (4) consecutive frames,

which do not have the valid Frame Alignment Signal-

ing (FAS) patterns, then it will transition into the OOF

Condition State. The Receive E3 Framer block’s op-

eration, while in the OOF condition state is a unique

mix of Framing Maintenance and Framing Acquisition

operation. The Receive E3 Framer block will exhibit

some Framing Acquisition characteristics by attempt-

ing to locate (once again) the FAS pattern. However,

the Receive E3 Framer block will also exhibit some

Frame Maintenance behavior by still using the most

recent frame synchronization for its overhead bits and

payload bits processing.

The Receive E3 Framer block will inform the Micro-

processor/Microcontroller of its transition from the In-

Frame state to the OOF Condition state, by generat-

ing a Change in OOF Condition Interrupt. When this

occurs, Bit 3 (OOF Interrupt Status), within the Rx E3

Interrupt Status Register - 1, will be set to “1”, as de-

picted below.

The Receive E3 Framer block will also inform the ex-

ternal circuitry of its transition into the OOF Condition

state, by toggling the RxOOF output pin "High”.

If the Receive E3 Framer block is capable of finding

the FAS pattern within a user-selectable number of

E3 frame periods, then it will transition back into the

In-Frame state. The Receive E3 Framer block will

then inform the Microprocessor/Microcontroller of its

transition back into the In-Frame state by generating

the Change in OOF Condition Interrupt.

However, if the Receive E3 Framer block resides in

the OOF Condition state for more than this user-se-

lectable number of E3 frame periods, then it will auto-

matically transition to the LOF (Loss of Frame) Condi-

tion state.

The user can select this user-selectable number of

E3 frame periods that the Receive E3 Framer block

will remain in the OOF Condition state by writing the

appropriate value into Bit 7 (RxLOF Algo) within the

Rx E3 Configuration & Status Register, as depicted

below.

RXE3 INTERRUPT STATUS REGISTER - 1 (ADDRESS = 0X14)

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

Not Used

COFA

Interrupt

Status

OOF

Interrupt

Status

LOF

Interrupt

Status

LOS

Interrupt

Status

AIS

Interrupt

Status

RUR

000

XRT7250 Datasheet(PDF) 311 Page - Exar Corporation

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