Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 977

37.1.2 AAL0 Buffers
AAL0 buffers contain one raw cell. When the receiver or transmitter completes writing or reading the
buffer, it moves to the next BD in the AAL0 channel's BD table in preparation for the next transfer and
optionally issues an interrupt.
AAL0 buffers are 64 bytes. 52 bytes are used to hold the cell header and payload—the HEC is not included
in the receive or transmit buffers. The remaining 12 bytes of the buffer are not used and are available to
the user. The AAL0 buffer structure is shown in
Unlike other protocols, both the AAL0 transmit and receive buffers should be 16-byte aligned.
37.1.3 ATM Receive Buffer Descriptors (RxBDs)
The format of the ATM receive buffer descriptor (RxBD) applies to both UTOPIA and serial ATM modes.
ATM RxBDs are 12 bytes, as shown in
0
OFFSET + 0 E
OFFSET + 2
OFFSET + 4
OFFSET + 8
OFFSET + A
For UTOPIA operation, a global option to support expanded cells is available. ATM RxBDs in expanded
cell mode are 24 bytes, as shown in
Freescale Semiconductor
Figure
Cell Header (without HEC)
48-byte Payload
12-byte User Space
Figure 37-2. AAL0 Buffer Structure
Figure 37-3.
1 2 3 4 5
— W I L F/OAM
DATA LENGTH/CHANNEL CODE
RX DATA BUFFER POINTER
Figure 37-3. ATM RxBD
Figure 37-4.
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Buffer Descriptors and Connection Tables
37-2.
6 7 8 9 10
CM FMC HEC CLP CNG
CPCS-UU+CPI
—
11 12 13 14 15
ABT — — LN CR
37-3