Freescale Semiconductor MPC8260 - Manual (Page 1069)

ATM AAL2
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 32-7
Figure 32-5. Fixed Priority Mode
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.
32.3.3 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.
The following figures provide examples of partial fill mode operation:
FirstQueue
TCT
NextQueue
TxQD
NextQueue
TxQD
0x0000 (null link)
TxQD
Fix=1
Highest priority queue
Lowest priority queue