Freescale Semiconductor PowerQUICC MPC870, Reference Manual

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ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor 40-9
2. Use TCT[TSERVICE] = 11 with a low out-of-buffers rate (APCP << OOBR < APC_table_size).
OOBR should be low enough so as not to consume too much CPM power when polling yet high
enough not to create cell delay. In this case when data is not ready, the APC keeps serving the
channel at a much lower rate and uses the APCP value only when buffers are ready.
NOTE
After the ACT bit has been cleared by the CPM, the CPU should wait at least
one APC timer interval and afterwards check the corresponding PHY
transmit queue until the TQ_APTR has reached or passed the TQ_TPTR. In
other words, until the channel number has been safely handled and also
cleared out of the transmit queue. Only then is it safe to access or reuse the
TCT.
40.1.8 APC Initialization and Operating Considerations
As long as the APC timer (CPM timer 4) is not active, the APC parameters may be initialized in any order.
The APC timer must be initialized last, following the initialization of the APC priority levels of all the
ATM ports in the system. Failure to initialize the APC timer last will allow the scheduling algorithm to
start prematurely, resulting in unpredictable behavior.
Furthermore, the APC scheduling table parameters must be initialized before any
TRANSMIT ACTIVATE
CHANNEL
commands are issued. However, these commands may be issued at any time, whether the APC
timer is active or inactive. For more information, see the description of the
TRANSMIT ACTIVATE CHANNEL
command Section 39.7, “ATM Commands.”
Also, note that the physical interface (serial or UTOPIA) must be enabled and its associated clocks and
synchronization signals must be active before the APC timer is activated. Otherwise, the transmit queue
will simply overflow (causing an APCO interrupt).
40.1.9 Modifying Channel Transmit Pace
The APC pace parameter in the TCT of any channel can be manipulated at any time. Software can use this
capability to support various dynamic traffic types, such as ABR.
40.1.10 Minimizing Cell Delay Variation
The number of cells sent in each time slot (determined by NCITS) has an effect on the maximum cell delay
variation (CDV) since the order of the cells in a given time slot is not controlled. Therefore, the CDV
increases as NCITS increases.
Also, as described before, the entries in the APC scheduling table are actually the heads of linked lists,
which allows the APC to schedule multiple channels for the same time slot. These linked lists can be of
unlimited depth, and although only NCITS cells are actually written to the transmit queue each time the
APCT_PTRx advances, no cells are lost because the lagging APCT_SPTRx service pointer keeps track of
the pending channels for next time. However, deeper linked lists result in more cell delay variation.
Therefore, if cell delay variation is a concern, schedule channels such that they are distributed as uniformly
as possible throughout the APC scheduling table. This can effectively be implemented by activating