MPC8280 datasheet(1112/1386 Pages) FREESCALE

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ATM AAL2

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

33-6

Freescale Semiconductor

33.3.2

Transmit Priority Mechanism

The transmit priority mechanism operates in two modes:

•

Round robin (TCT[Fix]=0)

•

Fixed priority (TCT[Fix]=1)

The following sections describe the priority options.

33.3.2.1

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 33-4. 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 33-4. 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].

FirstQueue

TCT

NextQueue

TxQD

NextQueue

TxQD

NextQueue

TxQD

Fix=0

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