Motorola MPC8260 - Manual (Page 845)

29-65 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
Figure 29-41. Transmit Buffers and BD Table Example
29.10.5.2 Receive Buffers Operation
For AAL5 channels, the user should choose to operate in static buffer allocation or in global
buffer allocation by writing to RCT[BUFM]. AAL1 and AAL0 channels must use static
buffer allocation.
29.10.5.2.1 Static Buffer Allocation
The user prepares a table of BDs pointing to the receive buffers. The address of the Þrst BD
is put in the channelÕs RCT[RBD_BASE]. When an ATM cell arrives, the CP opens the Þrst
BD in the table and starts Þlling its associated buffer with received data. When the current
buffer is full, the CP increments RBD_Offset, which is the offset to the current BD from
RBD_BASE, and reads the next BD in the table. If the BD is empty (RxBD[E] = 1), the CP
continues receiving. If the BD is not empty, a busy condition has occurred and a busy
interrupt is sent to the event queue.
Figure 29-42 shows the empty bit in the RxBD tables and their associated buffers for two
example ATM channels.
Tx Buffer 1 of Channel 1
Tx Buffer 2 of Channel 1
Tx Buffer 3 of Channel 1
Tx Buffer 4 of Channel 1
Tx Buffer 5 of Channel 1
Ch1 TxBD Table
0 BD 1
1 BD 2
1 BD 3
0 BD 4
0 BD 5
TBD_BASE
TBD_Offset
Note: The shaded buffers are ready to be sent; unshaded buffers are waiting to be prepared.
Ch1 TxBD Table
Pointers in the TCT
Tx Buffer 1 of Channel 4
Tx Buffer 2 of Channel 4
Tx Buffer 3 of Channel 4
Tx Buffer 4 of Channel 4
Tx Buffer 5 of Channel 4
Ch4 TxBD Table
1 BD 1
0 BD 2
0 BD 3
0 BD 4
0 BD 5
TBD_BASE
TBD_Offset
Ch4 TxBD Table
Pointers in the TCT
1 BD 6
BD 7
Tx Buffer 6 of Channel 4
Tx Buffer 7 of Channel 4
1