Motorola PowerQUICC II MPC8280 Series Reference Manual page 1224

IMA Microcode Architecture
34.3.2.2 Non-TRL Operation
A request from a non-TRL PHY does not trigger any scheduling task. The cells for
non-TRL links will already be supplied (by the TRL task) in its associated transmit queue.
The TRL will simply read a cell out of its transmit queue and update the queue pointers.
If the transmit queue becomes too shallow (because this link's request rate is faster than the
TRL), the link will flag that a stuff event is imminent. The link will signal an upcoming stuff
event in the LSI field of its next ICP cell and will then flag that a stuff event is due. Having
flagged this stuff event, the link will continue sending cells from its queue as normal until
it reaches its next ICP cell, upon which it indicates a stuff event in the ICP cell and transmits
it, but does not update the transmit queue pointers. When the link next requests a cell, the
previous ICP cell is repeated (since the queue pointers were not updated). This process
causes the transmit queue to deepen to the intended level.
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.
Queue insertion pointer
(Filled by TRL task)
Floats between these
two positions
Depth averages at 3.x
Figure 34-6. Transmit Queue Normal Operating State
34-14
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Normal 'Wander zone'
Queue extraction pointer
(Transmitted by non-TRL task)
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