Motorola PowerQUICC II MPC8280 Series Reference Manual page 1158

Freescale Semiconductor, Inc.
Buffer Descriptors
32.11 Buffer Descriptors
The AAL1 CES controller operates as a multi-channel protocol, performing simultaneous
segmenting and reassembling of transmit and receive data, to and from different sets of
memory buffers for all channels. This behavior makes it necessary to have a separate list of
BDs for each channel. Each channel is configured with two BD lists: one for transmit and
the other for receive operations. The amount of BDs' in the table is defined by the user.
The BD table is a circular list, the last BD in the table holds a wrap indication. When the
MPC8280 reaches the last BD, it returns to the head of the list. Each BD in the TxBD table
points to a buffer to send. At the receive side, the user allocates dedicated buffers to each
channel (that is, one BD for each buffer). When the receiver or transmitter completes
writing or reading the buffer, it moves to the next buffer in the list and optionally issues an
interrupt.
32.11.1 Transmit Buffer Operation
The user prepares a table of BDs pointing to the buffers to be sent. The address of the first
BD is put in the channel's TCT[TBD_BASE]. The transmit process starts when the core
issues an atm transmit command. The CP reads the first TxBD in the table and sends its
associated buffer. When the current buffer is finished, the CP increments TBD_OFFSET,
which holds the offset from TBD_BASE to the current BD. It then reads the next BD in the
table. If the BD is ready (TxBD[R] = 1), the CP continues sending. If the current BD is not
ready, the CP polls the ready bit at the channel rate unless TCT[AVCF] = 1, in which case
the CP removes the channel from the APC and clears TCT[VCON]. The core must issue a
new atm transmit command to restart transmission.
Note that when the ATM transmitter is in CES mode, the buffer-not-ready state is ignored
by the ATM controller; see Section 32.4.1.2, "TDM-to-ATM."
32-38 MPC8280 PowerQUICC II Family Reference Manual MOTOROLA
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