Motorola PowerQUICC II MPC8280 Series Reference Manual page 1176

AAL2 Transmitter
TCT
Fix=1
FirstQueue
The TCT[OneP] determines the number of packets that the transmitter attempts to take
from each queue (see the explanation in round robin mode).
The NextQueue field of the lowest priority TxQD should be cleared (null link), and
TCT[MaxStep] should be programmed to the total number of TX queues in the channel.
When the transmitter reaches the null link and the cell is still not complete, the transmitter
checks the TIMER CU mode as described above for the round robin mode.
33.3.3 Partial Fill Mode (PFM)
The partial fill mode (TCT[PFM]=1) allows the user to specify a partial fill threshold
(TCT[PFT]), which limits the number of data octets sent with each ATM cell. Partial fill
mode assures that packets are not split over two cells, unless the first packet of the cell is
greater than 47 bytes.
In partial fill mode, the transmitter starts by filling the ATM cell with the first packet. After
the first packet, the CP determines if including the next packet in the cell would exceed the
PFT limit. If so, the second packet is not inserted into the current cell, the unused payload
is padded with zeros, and the cell is sent. If the second packet does not exceed PFT, the CP
inserts it into the CPS-PDU and moves on to the third packet, and so on.
By programming PFT = 1, the partial fill mode can also serve to assure that only one packet
is sent per cell.
33-8
Freescale Semiconductor, Inc.
0x0000 (null link)
Figure 33-5. Fixed Priority Mode
MPC8280 PowerQUICC II Family Reference Manual
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TxQD
Highest priority queue
NextQueue
TxQD
NextQueue
TxQD
Lowest priority queue
MOTOROLA