Freescale Semiconductor MPC8260 - Manual (Page 988)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
30-68 Freescale Semiconductor
Figure 30-42. Receive Static Buffer Allocation Example
30.10.5.2.2 Global Buffer Allocation
The user prepares a table of BDs without assigning buffers to them (no buffer pointers). The address of the
first BD is put into the channel’s RCT[RBD_BASE]. The user also prepares sets of free buffers (of size
RCT[MRBLR]) in up to four free buffer pools (chosen in RCT[BPOOL]); see
Section 30.10.5.2.3, “Free
Buffer Pools.”
When an ATM cell arrives, the CP opens the first BD in the table, fetches a buffer pointer from the free
buffer pool associated with this channel, and writes the pointer to RxBD[RXDBPTR], the receive data
buffer pointer field in the BD. When the current buffer is full, the CP increments RBD_Offset, which is
the offset from the RBD_BASE to the current BD, and reads the next BD in the table. If the BD is empty
(RxBD[E] = 1), the CP fetches another buffer pointer from the free buffer pool and reception continues. If
the BD is not empty, a busy condition occurs and a busy interrupt is sent to the event queue specifying the
ATM channel code. As software then processes each full buffer (RxBD[E] = 0), it sets RxBD[E] and copies
the buffer pointer back to the free buffer pool.
Figure 30-43 shows two ATM channels’ BD tables and one free buffer pool. Both channels are associated
with free buffer pool 1. The CP allocates the first two buffers of buffer pool 1 to channel 1 and the third to
channel 4.
Rx Buffer 1 of Channel 1
Rx Buffer 2 of Channel 1
Rx Buffer 3 of Channel 1
Rx Buffer 4 of Channel 1
Rx Buffer 5 of Channel 1
Ch1 RxBD Table
0 BD 1
1 BD 2
1 BD 3
0 BD 4
0 BD 5
RBD_BASE
RBD_Offset
Note: The shaded buffers are empty; unshaded buffers are waiting to be processed.
Ch1 RxBD Table
Pointers in the RCT
Rx Buffer 1 of Channel 4
Rx Buffer 2 of Channel 4
Rx Buffer 3 of Channel 4
Rx Buffer 4 of Channel 4
Rx Buffer 5 of Channel 4
Ch4 RxBD Table
1 BD 1
0 BD 2
0 BD 3
0 BD 4
0 BD 5
RBD_BASE
RBD_Offset
Ch4 RxBD Table
Pointers in the RCT
1 BD 6
BD 7
Rx Buffer 6 of Channel 4
Rx Buffer 7 of Channel 4
1