Freescale Semiconductor MPC8260 - Manual (Page 1068)

ATM AAL2
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
32-6 Freescale Semiconductor
• Round robin (TCT[Fix]=0)
• Fixed priority (TCT[Fix]=1)
The following sections describe the priority options.
32.3.2.1 Round Robin Priority
In round robin priority mode, the Tx queues all have equal priority. The transmitter starts with the TxQD
pointed to by TCT[FirstQueue], as shown in
Figure 32-4 . The number of packets that the transmitter
services from each queue is determined by the one-packet bit (TCT[OneP]). If TCT[OneP]=0, the
transmitter tries to process as many packets in the queue as needed to fill up the cell. Only when the queue
is empty does the transmitter move on to the next queue (assuming the cell is not completed). If
TCT[OneP]=1, the transmitter attempts to take only one packet out of each queue. (Set TCT[OneP] for
implementations where each queue contains only one CID.)
Figure 32-4. Round Robin Priority
The transmitter steps from one TxQD to the next along the queue links. The TCT[MaxStep] parameter
limits the number of TX Queues that the transmitter visits during a cell time. If MaxStep is reached before
the cell has been completely filled, one of the following events takes place:
• TCT[ET]=0 (Timer CU disabled). The cell is padded with zeros and sent.
• TCT[ET]=1 (Timer CU enabled). If the timer has not expired, the cell is not sent. (The transmitter
attempts to fill the cell the next time this channel is scheduled.) If the timer has expired, the cell is
padded with zeros and sent
After the transmitter sends a cell, it saves the queue link of the last TxQD serviced in TCT[FirstQueue].
32.3.2.2 Fixed Priority
In fixed priority mode (TCT[Fix]=1), the transmitter, with each new cell, starts with searching the highest
priority queue and then moves on to the lower priority queues. The TCT[FirstQueue] points to the highest
priority queue, as shown in
Figure 32-5 , and remains unchanged by the CP.
FirstQueue
TCT
NextQueue
TxQD
NextQueue
TxQD
NextQueue
TxQD
Fix=0