XRT7250 datasheet(209/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

190

occur in a repeating 1001 pattern. The Receive DS3

Framer block will attempt to locate this F-bit pattern

by performing five (5) different searches in parallel.

The F-bit search has been declared successful if at

least 10 consecutive F-bits are detected. After the F-

bit match has been declared, the Receive DS3 Fram-

er block will then transition into the M-Bit Search state

within the DS3 Frame Acquisition/Maintenance algo-

rithm (per Figure 81). When the Receive DS3 Framer

block reaches this state, it will begin searching for val-

id M-bits. Recall from the discussion in Section 3.1

that each DS3 M-frame consists of three (3) M-bits

that occur in a repeating 010 pattern. The M-bit

search is declared successful if three consecutive M-

frames (or 21 F-frames) are detected correctly. Once

this occurs an M-frame lock is declared, and the Re-

ceive DS3 Framer block will then transition to the In-

Frame state. At this point, the Receive DS3 Framer

block will declare itself in the In-Frame condition, and

will begin Frame Maintenance operations. The Re-

ceive DS3 Framer block will then indicate that it has

transitioned from the OOF condition into the In-Frame

condition by doing the following.

• Generate a Change in OOF Condition interrupt to

the local µP.

• Negate the RxOOF output pin (e.g., toggle it

"Low").

• Negate the RxOOF bit-field (Bit 4) within the

Receive DS3 Configuration and Status Register.

The user can configure the Receive DS3 Framer to

operate such that 'valid parity' (P-bits) must also be

detected before the Receive DS3 Framer can declare

itself In Frame. The user can set this configuration by

writing the appropriate data to the Rx DS3 Configura-

tion and Status Register, as depicted below.

Table 37 relates the contents of this bit field to the

framing acquisition criteria.

Once the Receive DS3 Framer block is operating in

the In-Frame condition, normal data recovery and

processing of the DS3 data stream begins. The max-

imum average reframing time is less than 1.5 ms.

4.3.2.2

Frame Maintenance Mode Operation

When the Receive DS3 Framer block is operating in

the In-Frame state (per Figure 81), it will then begin to

perform Frame Maintenance operations, where it will

continue to verify that the F- and M-bits are present,

at their proper locations. While the Receive DS3

Framer block is operating in the Frame Maintenance

mode, it will declare an Out-of-Frame (OOF) condition

if 3 or 6 F-bits (depending upon user selection) out of

16 consecutive F-bits are in error. The user makes

this selection for the OOF Declaration criteria by writ-

ing the appropriate value to bit 1 (F-Sync Algo) of the

Rx DS3 Configuration and Status Register, as depict-

ed below.

RX DS3 CONFIGURATION AND STATUS REGISTER, (ADDRESS = 0X10)

BIT 7BIT 6BIT 5BIT 4BIT 3

BIT2BIT 1BIT 0

RxAIS

RxLOS

RxIdle

RxOOF

Int LOS

Disable

Framing on

Parity

F-Sync Algo

M-Sync Algo

R/W

XXX

TABLE 37: THE RELATIONSHIP BETWEEN THE CONTENTS OF BIT 2 (FRAMING ON PARITY) WITHIN THE RX DS3

CONFIGURATION AND STATUS REGISTER, AND THE RESULTING FRAMING ACQUISITION CRITERIA

FRAMING

PARITY

(BIT 2)

FRAMING ACQUISITION CRITERIA

The In-frame is declared after F-bit synchronization (10 F-bit matches) followed by M-bit synchronization (M-

bit matches for 3 DS3 M-frames)

The In-frame condition is declared after F-bit synchronization, followed by M-bit synchronization, with valid

parity over the frames. Also, the occurrence of parity errors in 2 or more out of 5 frames starts a frame search

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