Aal5 Buffers - Freescale Semiconductor MPC850 User Manual

ATM Buffer Descriptors (BDs)
Pointer from
TBD_BASE
Parameter RAM
TBASE
Pointers from
Ch1 TCT
TBD_PTR
TBASE
Pointers from
Ch4 TCT
TBD_PTR
Figure 37-1. Transmit Buffer and TxBD Table Example
In the example of Figure 37-1, when the transmitter encounters channel number 1 in the
transmit queue, it sends a cell using transmit buffer 4 because TxBD 4 is the active BD in
channel number 1's TxBD table. As the cell is sent, the transmit buffer pointer TB_PTR
tracks the current data position. After the cell is sent, the transmitter advances the channel's
TBD_PTR to the next BD in the table (unless the current buffer is a multiple-cell AAL5
frame in which case TBD_PTR is not advanced and the current data position is kept in
TB_PTR) and moves on to the next channel in the transmit queue. When the end of the BD
table is reached (TxBD[W] = 1), the transmitter returns to the head of the table by
re-initializing TBD_PTR to the channel's TBASE address.

37.1.1 AAL5 Buffers

Each AAL5 buffer can hold either a whole frame or part of it. During transmit or receive
operations, interrupts are optionally generated at the closing of each buffer or at the end of
a frame.
The last buffer of a frame is padded automatically by the transmitter to fit an AAL5 cell
payload according to ITU specification I.363. The transmit buffer data length for AAL5
transmit buffers must be greater than or equal to 48 bytes for all buffers except the first and
last buffer of a frame; the first and last buffers of a frame must have a data length greater
than zero. Note that AAL5 transmit buffers have no alignment restrictions.
Receive buffers, however, must start on a burst-aligned address (divisible by 16) and their
lengths should be a multiple of 48 bytes (that is, the value of SMRBLR in the SCC
BD Memory Space (up to 256 KByte)
Tx BD Table
of Ch1
Tx BD 1
Tx BD 2
Tx BD 3
Tx BD 4
Tx BD 5
Tx BD 6
Tx BD Table
of Ch4
Tx BD 1
Tx BD 2
Tx BD 3
Tx BD 4
Tx BD 5
Tx BD 6
Tx BD 7
Tx BD 8
Tx BD 9
MPC850 Family User's Manual
Buffer Memory Space (4 Gbyte)
Tx Buffer1 of
Channel1
Tx Buffer3 of
Channel1
Tx Buffer4 of
Ch1TB_PTR
Channel1
Tx Buffer1 of
Channel4
Tx Buffer2 of
Channel4
Tx Buffer3 of
Channel4
Ch4TB_PTR
Tx Buffer2 of
Channel1
Tx Buffer8 of
Channel4