Motorola PowerQUICC II MPC8280 Series Reference Manual page 1174

AAL2 Transmitter
buffer is completely sent, an optional interrupt event is issued to the host. Also, if an SSSAR
TX queue is empty an optional interrupt event is issued to the host.
In case of CPS TX Queue, the transmitter fetches the packet header out of a buffer
descriptor and the packet payload out of a CPS buffer (see Section 33.3.5.3, "CPS Buffer
Structure"). The HEC in the packet header is calculated by the CP or taken from the buffer
descriptor based on the user configuration. After a CPS packet is sent, an optional interrupt
event is issued to the host. Also, if the CPS TX queue is empty an optional interrupt event
is issued to the host.
The optional partial filled mode (see Section 33.3.3, "Partial Fill Mode (PFM)") limits the
number of data octets per cell. This can be used to ensure that a cell does not contain a split
packet or to limit transmission to one packet per TX cell by setting a low partial fill
threshold (PFT).
The no-STF (no start of frame) mode (see Section 33.3.4, "No STF Mode") enables the
transmission of cells that do not include the STF byte, thus allowing for 48-byte packets.
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.)
33-6
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