ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
40-12
Freescale Semiconductor
If SCC4 or the UTOPIA port are not used, the APCST parameter on parameter page 4 must indicate that
the APC on page 4 is disabled and must also point to the active APC page(s). This is accomplished using
the APCST[NSER,CSER] mechanism described in
Section 40.3, “Using the APC with Multiple ATM
Ports.”
For example, if only SCC1 is used in ATM mode, then APCST on page 4 should be programmed to
0x0004, which sets APCST[NSER]=APCST[CSER]=00 (indicating SCC1). For the APCST on page 1,
APCST[NSER] and APCST[CSER] should be programmed to 11 so that the APC algorithm points back
to page 4 to terminate.
Note, however, that by programming APCST4, the use of most other protocols on SCC4 is precluded
because of parameter RAM conflicts with this parameter. The only protocol which does not conflict with
this parameter is transparent mode. Therefore, if the APC is used, parameter RAM page 4 can only be used
with SCC4 in transparent mode, SCC4 in serial ATM mode, or the UTOPIA port.
40.5
APC Flux Compensation
Under peak load ATM UTOPIA MPHY master applications the receiver may have to handle more
incoming cells than possible on average either as burst or as receive traffic over multiple PHYs. Normally,
this would degrade the available CPM performance for the APC as the receiver has higher priority and
therefore degrades the paced bandwidth, if overloaded.
An optional feature has been integrated to detect and correct temporary overload conditions that would
otherwise result in lost APC bandwidth. APC Flux Compensation will detect the condition and keep the
APC active as required to compensate for the previously lost bandwidth. A loss of APC bandwidth due to
a temporary APC overload can no longer occur then, and the overload condition on the receiver will only
result in APC jitter.
If APC Flux Compensation is desired, hardware timer 3 is used as reference timer and will not be available
for other purposes.
40.5.1
Enabling APC Flux Compensation
Before the APC is enabled, i.e., before timer 4 is enabled several fields must be initialized. In the SCC4
parameter page, APCTIMERREF must be set to 0 initially. The APCTIMERADDR field must be set to
the absolute memory address of TCN4, i.e., the upper 16 bits must match the respective IMMR content
and the lower 16 bits must be set to 0x09AE, which references TCN4.
The timer 3 registers need to be initialized as follows. TMR3,TCN3, and TRR3 should be set to 0x0000.
APC flux compensation needs to be enabled by setting bit 31 in the CPMCFG register. See
Section 18.5,
“CPM Configuration Register (CPMCFG),”
for a description of CPMCFG.
The APCT_REAL value in SCC4 ATM parameter RAM at offset 0x60 must be cleared. Finally, timer 4
and timer 3 should be enabled at the same time by setting the appropriate two RST bits in the TGCR.
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Страница 1068: ...ATM Pace Control MPC885 PowerQUICC Family Reference Manual Rev 2 40 22 Freescale Semiconductor ...
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