AAL2 Implementation
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
44-5
If the MPC885 attempts to receive a cell, but there is no available AAL0 buffer (the next RxBD of the
channel is not empty), the MPC885 generates an exception entry with a BSY indication. Because this
indication is directly related to AAL0 operation, the AAL2 bit in the exception entry is not set. This type
of exception indicates that AAL2 is too slow to process the AAL0 buffers that were received previously
by the MPC885. The system load could be so great that AAL2 cannot maintain the data rate. Also, the
channel’s RxBD table could be too small; that is, the latency of in processing the active buffers is greater
than the delay tolerance provided by the RxBD table. The host should reconfigure the system to prevent
such exceptions.
44.3
AAL2 Interface with the Host
The host application interacts with AAL2 using packet descriptors (PDs) placed in queues. Inside each
queue, the PDs are processed first in first out (FIFO). Each channel has one transmit queue
(AAL2_Tx_Queue) and one receive queue (AAL2_Rx_Queue).
The transfer of control between the host and AAL2 is managed through the ready bit (R) in the TPD and
the empty bit (E) in the RPD. This is the same mechanism used for SCC TxBDs and RxBDs except that
the ATM exception queue is used instead of an event register to signal when the AAL2 is ready to transfer
control back to the host.
A TPD with R = 0 (not ready for transmission) is available to the host. A TPD and its packet are only
processed after the host has set TPD[R]. TPD[R] is cleared after AAL2 has finished processing the packet.
AAL2 can then signal the host by adding an entry in the exception queue.
An RPD with E = 0 (not empty) is available to the host. An RPD packet is retrieved only after the host has
set RPD[E], and then RPD[E] is immediately cleared. The host can be signaled by adding an entry in the
exception queue.
NOTE
The CPMCFG[AAL2] bit must be set to enable AAL2 operation, see
Section 18.5, “CPM Configuration Register (CPMCFG).”
44.4
AAL2 Data Structures for Transmit
This section describes the AAL2’s data structures used for transmitting AAL2 packets.
44.4.1
Transmit Packet Descriptor (TPD)
Similar to buffer descriptors, each transmit packet descriptor (TPD), shown in
Figure 44-1
, contains
control bits and the pointer to the CPS_Packet. However, the TPD does not contain status bits. The status
feedback from the CP is given through the exception queue; see
Section 44.9, “AAL2 Exceptions.”
The
host should program all TPD fields during initialization.
Summary of Contents for PowerQUICC MPC870
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