MPC8280 datasheet(1158/1386 Pages) FREESCALE

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Inverse Multiplexing for ATM (IMA)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

34-12

Freescale Semiconductor

At group start-up, instead of accessing its transmit queue, the link will send filler cells. This is to allow the

transmit queues to reach their target steady-state depth. After the group start-up flag is cleared, normal

operation as described above will commence.

34.3.2.3

Transmit Queue Operation Examples (ITC mode)

The following diagrams demonstrate the different cases of queue operation, and consequently justify the

queue depth of 5 cells.

•

The extraction pointer points to the queue entry that is currently being supplied to the PHY. This

cell must be entirely ready when the PHY requests it.

•

The insertion pointer points to the queue entry which will be filled next by the TRL process.

In the figures, note that the pointers and filled queue locations are just shown with respect to the overall

queue depth available with the extraction pointer always shown at the bottom of the queue. This is done

only for the purpose of ease of illustration. In reality, the transmit queues are circular queues in which the

insertion and extraction pointers are continually rotating through the queue.

Figure 34-6. Transmit Queue Normal Operating State

Figure 34-7. Transmit Queue Behavior: Link Clock Rate Same as TRL

Queue extraction pointer

Tx Queue

Queue insertion pointer

(Filled by TRL task)

(Transmitted by non-TRL task)

Normal ‘Wander zone’

Floats between these

two positions

Depth averages at 3.x

Queue extraction pointer

Tx Queue

TRL stuff event occurs

(Transmitted by non-TRL task)

a cell during a round

robin pass.

Depth decreases to 2.x

and this link is not given

Queue extraction pointer

Tx Queue

Queue insertion pointer

Stuff event is performed

(Transmitted by non-TRL task)

(ICP cell is left in queue

after transmission to be

Depth increases back to 3.x

“Imminent stuff” flagged

sent one extra time.)

Normal ‘Wander zone’

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