ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
40-10
Freescale Semiconductor
channels (using the
TRANSMIT
ACTIVATE
CHANNEL
command) at random intervals, such that they are not
all written to the same APC scheduling table entry.
40.2
Direct Scheduling of Cells
The ATM controller implements an
APC
BYPASS
command to allow the user to insert a channel number
directly into the transmit queue on a cell-by-cell basis. This command can be used at any time in either
serial mode or UTOPIA mode to insert a single cell for a channel into the transmit queue, with no direct
transmit queue pointer manipulation required.
40.3
Using the APC with Multiple ATM Ports
The APC algorithm always begins in page 4 (SCC4) of the dual-port RAM. Thus, normally, when
configured with a single ATM port (in either UTOPIA or serial mode), the ATM controller operates in page
4 of the dual-port RAM. Although it is possible to run a single serial ATM port on SCC1, SCC2, or SCC3,
this requires that the page-4 APCST parameter to define the APC of page 4 as ‘inactive,’ and point to the
active APC scheduling tables. The following discussion assumes that the APC for page 4 is active;
however, the same general concepts apply to a system in which the APC for page 4 is inactive.
All the APCs are strobed by the same the APC timer request, which provides the common basic pace from
which all the APCs (one for each ATM port configured) are scheduled. The APC timer activates several
independent sub-timers in each APC implemented in the APCNT field of the APC parameters. Scheduling
of cells from a particular APC will occur only when APCNT exceeds one. The APC timer should be
programmed to supply an optimized common pace for all the APCs. This pace can then be divided or
multiplied by setting NCITS appropriately for each APC.
For example, for a 50-MHz system implementing two SARs one of which (in page3) is a 25-Mbps SAR
and the other is a T1 serial SAR, the APC timer is programmed to generate an APC request every 4 cell
times of the 25-Mbps SAR (CPM timer 4 should be programmed to 50 MHz/25 Mbps*53*8*4 = 3392).
An NCITS value of 4 is selected for the 25-Mbps SAR and an NCITS value of 0.24 for the 1.544-Mbps
SAR so that the 25-Mbps APC will schedule 4 cells per APC timer timeout, and the T1 APC will schedule
one cell per 4.166 APC timer timeout (an average of 0.24) for the T1 SAR.
In the example shown in
Figure 40-4
, the APCST[NSER] field in page 4 points to page 1, APCST[NSER]
of page 1 points to page 4. When an APC timer timeout occurs, the NCITS timer of page 1 is updated.
Once per 4.166 requests the APCNT timer exceeds one, and APC1 schedules a channel for transmission.
After the APCNT timer of page 1 is updated, APC2 is selected and schedules up to four (NCITS=4)
channels.
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