
29-5
MPC8260 PowerQUICC II User’s Manual
MOTOROLA
Part IV. Communications Processor Module
29.2.1 Transmitter Overview
Before the transmitter is enabled, the host must initialize the MPC8260 and create the
transmit data structure, described in Section 29.10, “ATM Memory Structure.” When data
is ready for transmission, the host arranges the BD table and writes the pointer of the first
BD in the transmit connection table (TCT). The host issues an
ATM
TRANSMIT
command,
which inserts the current channel to the ATM pace control (APC) unit. The APC unit
controls the ATM traffic of the transmitter. It reads the traffic parameters of each channel
and divides the total bandwidth among them. The APC unit can pace the peak cell rate,
peak-and-sustain cell rate (GCRA traffic) or peak-and-minimum cell rate traffic. The APC
implements up to eight priority levels for servicing real-time channels before non-real-time
channels.
The transmitter ATM cell is 53–65 bytes and includes 4 bytes of ATM cell header, a 1-byte
HEC, and 48 bytes of payload. The HEC is a constant taken from FDSR
x
[0–15] when using
UTOPIA 16 and from FDSR
x
[8–15] when using UTOPIA 8; see Section 28.4, “FCC Data
Synchronization Registers (FDSRx).” User-defined cells (UDC mode) include an extra
header of 1–12 bytes with an optional HEC octet. Cell transfers use the UTOPIA level II,
cell-level handshake.
Transmission starts when the APC schedules a channel. According to the channel code, the
ATM controller reads the channel’s entry in the TCT and opens the first BD for
transmission.
29.2.1.1 AAL5 Transmitter Overview
The transmitter reads 48 bytes from the external buffer, adds the cell header, and sends the
cell through the UTOPIA interface. The transmitter adds any padding needed and appends
the AAL5 trailer in the last cell of the AAL5 frame. The trailer consists of CPCS-UU+CPI,
data length, and CRC-32 as defined in ITU I.363. The CPCS-UU+CPI (2-byte entry) can
be specified by the user or optionally cleared by the transmitter; see Section 29.10.2.3,
“Transmit Connection Table (TCT).” The transmitter identifies the last cell of the AAL5
message by setting the last (L) indication bit in the PTI field of the cell header. An interrupt
may be generated to indicate the end of the frame.
When the transmission of the current frame ends and no additional valid buffers are in the
BD table, the transmit process ends. The transmitter keeps polling the BD table every time
this channel is scheduled to transmit. In auto-VC-off mode, the APC automatically
deactivates the current channel when no buffer is ready to transmit. In this case, a new
ATM
TRANSMIT
command is needed for transmission of the VC to resume. Note that a buffer-not-
ready indication during frame transmission aborts the frame transfer.
29.2.1.2 AAL1 Transmitter Overview
The MPC8260 supports both structured and unstructured AAL1 formats. For the
unstructured format, the transmitter reads 47 bytes from the external buffer and inserts them
into the AAL1 user data field. The AAL1 PDU header, which consists of the sequence
number (SN) and the sequence number protection (SNP) (CRC-3 and parity bit), is