MPC8280 datasheet(1113/1386 Pages) FREESCALE

1113 / 1386 page

ATM AAL2

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

33-7

33.3.2.2

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 33-5, and remains unchanged by the CP.

Figure 33-5. 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.

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.

FirstQueue

TCT

NextQueue

TxQD

NextQueue

TxQD

0x0000 (null link)

TxQD

Fix=1

Highest priority queue

Lowest priority queue

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