AAL2 Implementation
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
44-11
A description of the data flow shown in Figure 44-4. is as follows:
•
The first part of the CPS-Packet (CID=1) has already been packed into the previous AAL0 buffer.
•
The tail of CPS-Packet (CID=1) is packed into the active AAL0 buffer of this channel. TPD 2 is
marked available (R=0).
•
CPS-Packet (CID=19) is copied to the AAL0 buffer pointed to by TPD 3. TPD 3 is marked
available.
•
Only the first part of CPS-Packet (CID=56) has room in the active AAL0 buffer. The rest of the
CPS-Packet remains in external memory to be placed in the next AAL0 buffer when AAL2 is
activated again for this channel.
•
The TxBD (pointed to by AAL2_TACT_PTR) that points to the active buffer is closed (R bit is
set). (The MPC885 transmits this buffer later according to the APC or host scheduling.)
•
Note that while AAL2 fills the active AAL0 buffer, the MPC885 (AAL0 implementation) uses
TBD_PTR to actually transmit the AAL0 buffers prepared earlier.
44.4.5
Built-in Timer CU Support
The Timer CU mechanism is designed to guarantee a minimum packet throughput for an AAL2 channel.
It ensures that transmit CPS-Packets are not delayed beyond the quality of service requirements for a
channel.
If a channel does not have enough CPS-Packets to fill a partially filled active buffer, the AAL2 will wait
to send the buffer. If the active buffer is not filled with new CPS-Packets the next time this channel is
activated, the buffer is still delayed in hopes of achieving a better utilization of the transmission line. The
Timer CU mechanism limits this delay.
The Timer CU mechanism is implemented using a wait table (AAL2_TxWait_table allocated by the host)
and the internal CP RISC timer software table. The wait table holds the remaining wait time for all partially
filled active buffers. Each “tick” of the RISC timer activates AAL2 to pad (fill with zeros) and send all
partially filled buffers which have expired. The duration between ticks is defined in the TIMEP field of the
RISC controller configuration register (RCCR). The RISC timers are enabled in RCCR[TIME]. AAL2
Timer CU processing needs to be enabled by setting CPMCFG[AAL2], see
Section 18.5, “CPM
Configuration Register (CPMCFG).”
Once AAL2 timer processing is enabled, CPM RISC timers can be
used by setting the RTE bit in the AAL2 parameter RAM. See
Section 18.6.1, “RISC Controller
Configuration Register (RCCR).”
For each AAL2 channel using the Timer CU mechanism, the host should set AAL2_TCT[ET] and
initialize AAL2_TCT[TCU]. Once initialized, the Timer CU mechanism does not require host
intervention.
44.4.5.1
Algorithm Description
This section describes the algorithm used to implement the Timer CU mechanism for timer-enabled
channels.
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