AAL2 Implementation
MPC885 PowerQUICC Family Reference Manual, Rev. 2
44-26
Freescale Semiconductor
NOTE
Exceptions not generated by AAL2 may also occur on channels that run
AAL2. These exceptions will not set the entry’s bit 4 (the AAL2 bit). A
non-AAL2 entry will also have different field names and definitions. The
host should process the exception entry as defined in the MPC885
specification.
44.9.1
AAL2 Transmit Exceptions
After the AAL2 packs a transmit CPS-Packet, it generates an interrupt to the host and adds an entry to the
exception queue if TPD[INT] and the TPI bit in the channel’s AAL2_TCT are both set. The exception
entry will have the AAL2 bit and TXB bit set. AAL2 also clears TPD[R] (not ready to transmit) to make
the CPS-Packet available to the host.
44.9.2
AAL2 Receive Exceptions
After AAL2 retrieves a receive CPS-Packet, it generates an interrupt to the host and adds an entry to the
exception queue if RPD[INT] and the RPI bit in the channel’s AAL2_TCT are both set. The exception
entry will have the AAL2 bit and RXB bit set. AAL2 also clears RPD[E] (not empty) to make the
CPS-Packet available to the host.
44.10 Initialization of MPC885 for AAL2 Operation
This section describes the initialization process for ATM channels running AAL2 on the MPC885.
Because the AAL2 is built upon AAL0 operation, the AAL0 data structures have to be in place to support
AAL2 channels. The user also must configure the data structures specific to AAL2. The Timer CU
structure should also be initialized if any transmit channel uses the timer.
44.10.1 Initialization of AAL0 Structures for AAL2 Channels
During AAL2 operation, the host does not deal directly with the AAL0 BDs and buffers associated with
the AAL2 channels. However, the host does need to initialize these AAL0 structures for AAL2 to use.
Therefore, for each AAL2 channel, the host should do the following:
•
Allocate 64-byte buffers. The first 52 bytes of each buffer are used to hold the cell header and
payload. Each buffer should start on a 64-byte-aligned address.
•
Define TxBD and RxBD tables
— Point each BD to a 64-byte buffer
— Set the interrupt bit (I=1) in each BD to trigger the AAL2 operation
— For better performance, it is recommended to allocate 4–10 BDs for the RxBD table and
for the TxBD table
•
All TCT and RCT parameters should be programmed for AAL0 operation
•
The IMASK[AAL2] bit must be set for a channel to mark it as AAL2 channel
•
Set CPMCFG[AAL2] to enable AAL2
Содержание PowerQUICC MPC870
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