Freescale Semiconductor MPC8260 - Manual (Page 1116)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
33-14 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.
33.3.2.3 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 33-6. Transmit Queue Normal Operating State
Figure 33-7. 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’
Normal ‘Wander zone’