MPC8280 datasheet(1220/1386 Pages) FREESCALE

1220 / 1386 page

ATM Transmission Convergence Layer

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

35-4

Freescale Semiconductor

cell header. The cell delineation algorithm assumes that repetitive correct HEC calculations over

consecutive cells indicate valid ATM cell boundaries.

The RCF performs a sequential bit by bit hunt for a correct HEC sequence. While performing this hunt,

the cell delineation state machine is in HUNT state. When a correct HEC is found, the RCF locks on the

particular cell boundary and enters the PRESYNCH state, which indicates that the previously detected

HEC pattern is not a false indication. If a correct HEC pattern is false, an incorrect HEC is received within

the next DELTA cells. If an incorrect cell is detected, then a transition to the HUNT state is made. If an

incorrect HEC is not detected in the PRESYNCH state, a transition to the SYNCH state is made. In the

SYNCH state, the TC is assumed to be synchronized so that other functions can be applied to the received

cell. A transition back to the HUNT state is made only after ALPHA consecutive incorrect HEC patterns

are detected.

The cell delineation state machine is shown in Figure 35-3. The ALPHA and DELTA parameters

determine the robustness of the delineation method. ALPHA determines the robustness against false

misalignment due to bit errors. DELTA determines the robustness against false delineation in the

synchronization. Both parameters are programmable for each TC block and are provided to help tune the

system according to the line error characteristics of a specific application.

Figure 35-3. TC Cell Delineation State Machine

The RCF descrambles (programmable) the cell payload using the self-synchronizing descrambler with a

polynomial of x43 + 1.

The HEC calculation is a CRC-8 calculation over the first four octets of the ATM cell header. The RCF

verifies the received HEC using the accumulation polynomial, x8 + x2 + x + 1. The coset polynomial

x6 +x4 + x2 + 1 is added (modulo 2) to the received HEC octet before comparison with the calculated

result (programmable).

The RCF can perform single bit error correction on the header. If multiple bit errors are found in the HEC,

the cell is discarded. If the single bit error correction mode is not enabled (TCMODE[SBC] = 1), the cell

is also discarded when a single bit error is found in the header.

HUNT

PRESYNCH

SYNCH

DELTA Consecutive

Correct HEC

Incorrect HEC

ALPHA

Incorrect HEC

Consecutive

Bit-by-Bit

Cell-by-Cell

MPC8280 Datasheet(PDF) 1220 Page - Freescale Semiconductor, Inc