MPC8280 datasheet(1081/1386 Pages) FREESCALE

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ATM AAL1 Circuit Emulation Service

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

32-21

Figure 32-19. 3-Step-SN-Algorithm

32.7

Pointer Verification Mechanism

After the 3-step-SN algorithm processes the incoming cells, the cells status (Valid, Tag, Drop or Dummy)

is delivered to the pointer verification mechanism. This state machine calculates the expected received

pointer. If the current cell is valid and supposed to deliver a pointer, the received pointer is compared with

the expected one. In the case of pointer mismatch or pointer error (i.e parity error), the pointer state

machine switches to “Pre-Hunt-Mode”. If the cell status is not valid (i.e Tag, Drop or Dummy) and it

supposed to carry a pointer, the pointer declared a mismatch and the pointer state machine switches to

“Pre-Hunt-Mode”. The receiver stays in this transient state for only one AAL1 cell cycle (8 successive

cells). When the pointer received in this state is different from the expected one or had a non-valid status,

the receiver switches to hunt mode where it stays until a valid cell with a “start” structured pointer is

received to regain synchronization.

Note that a “start” pointer is a valid pointer with a value not equal to 93 or 127.

Hunt mode

Sync fail

Sync mode

No valid SN

Else

Valid SN

Invalid SN or

out of sequence

Valid SN

ESC=RSC–1/0/1

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