ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
40-6
Freescale Semiconductor
40.1.4
Defining APC Slot Time
The APC defines the maximum bit rate of the cell scheduler through the period of the APC timer tick and
the number of cells scheduled per APC timer tick (NCITS). The period of the APC timer is referred to as
an APC time slot.
Defining:
P = Cell scheduling bit rate (usually equal to the PHY bit rate)
APC_timer_per = CPM timer 4 period, programmed in TRR4 and TMR4
Then:
Using the previous example, if CLKOUT=50MHz, P=51.84Mbps, 53 bytes per cell, and NCITS=4, then
APC_timer_per is approximately 1635.8. The idea now is to set up the timer 4 in any way that best matches
the calculated period value. This involves setting up the prescaler TMR4[PS], clock source TMR4[ICLK],
and timer reference register TRR4 appropriately. In the following example we arbitrarily chose to use the
configuration bits in TMR4 exclusively by using a clock divider of 16 and a high prescaler value. The
closest value available using TMR4 only (TRR4 is programmed to 0x1) is 103*16 = 1648, by
programming TMR4[PS] = 0x67 and TMR4[ICLK]=0b10. Obviously, in general, it is possible to use any
value combination which leads to the desired period. Be sure to choose the next largest period value from
that which was calculated; otherwise, the transmit queue would eventually overrun as the APC scheduler
would provide slightly more traffic than the physical layer can transmit.
Ensure that you do not define an arbitrarily short timer 4 period. On each timer tick, the APC has to service
all configured PHYs, creating CPM loading. If the configured timer 4 period is too short, the CPM load
may be so great that the APC may behave erratically, and may not be able to pace bit rate as configured.
40.1.5
Programming Rates for CBR Channels
The bit rate for a particular ATM channel is defined by that channel’s APC pace parameter which represent
the peak cell rate (PCR) value of the CBR channel in its TCT. The APC pace value (APCP +
(APCPF/65536)) for any channel must fall between 1 and (APC scheduling table size -1). Values outside
this range result in erratic pace and/or scheduling table overflows (APCO interrupts). The maximum rate
for transmission of a particular channel is achieved when the APC pace is equal to one.
Defining:
P = cell scheduling bit rate (usually equal to the PHY bit rate)
des_rate = desired bit rate for this channel
Then:
P
CLKOUT
APC_timer_per
(
)
------------------------------------------
NCITS
(
)
number_of_bits_per_cell
(
)
=
APC_Pace
P
(NCIST)(des_rate)
---------------------------------------------
=
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