Motorola PowerQUICC II MPC8280 Series Reference Manual page 1020

Freescale Semiconductor, Inc.
ATM Controller Overview
when no buffer is ready to transmit. In this case, a new command is needed
ATM TRANSMIT
for transmission of the VC to resume.
31.2.1.1 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 31.10.2.3,
"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. Note that a buffer-not-ready indication during frame
transmission aborts the frame transfer.
31.2.1.2 AAL1 Transmitter Overview
The MPC8280 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
generated and inserted into the cell. The MPC8280 supports synchronous residual time
stamp (SRTS) generation using external PLL. If this mode is enabled, the MPC8280 reads
the SRTS code from the external logic and inserts it into four outgoing cells. See
Section 31.16, "SRTS Generation and Clock Recovery Using External Logic."
For the structured format, the transmitter reads 47 or 46 bytes from the external buffer and
inserts them into the AAL1 user data field. The CP generates the AAL1 PDU header and
inserts it into the cell. The header consists of the SN, SNP, and the structured pointer.
The MPC8280 supports partially filled cells configured on a per-VC basis. In this mode
(TCT[PFM] = 1), only valid octets are copied from the buffer to the ATM cell; the rest of
the cell is filled with padding octets.
31.2.1.2.1 AAL1 CES Transmitter Overview
Refer to Section 32.2, "AAL1 CES Transmitter Overview."
31-6 MPC8280 PowerQUICC II Family Reference Manual MOTOROLA
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