Programming Rates For Channels - Freescale Semiconductor MPC850 User Manual

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:
------------------------------------------ NCITS
P =
APC_timer_per
(
Using the previous example, if CLKOUT=50MHz, P=51.84Mbps, and NCITS=4, then
APC_timer_per is approximately 1635.8. 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. 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.

40.1.5 Programming Rates for Channels

The bit rate for a particular ATM channel is defined by that channel's APC pace parameter
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:
For example, again using the previous example with P=51.84Mbps and NCIST=4, assume
that a channel with a bit rate of 100kbps is desired. APC_Pace should therefore be
programmed to 51.84Mbps / (4 * 100kbps) = 129.6. This can be approximated by
programming APCP=129 and APCPF=39322.
CLKOUT
(
)
APC_Pace =
Chapter 40. ATM Pace Control
) number_of_bits_per_cell
(
P
-------------------------------------------- -
(NCIST)(des_rate)
APC Algorithm
)