ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
40-14
Freescale Semiconductor
Note that the scheduling tables need not be the same length; the length of the table affects only the
minimum programmable transmit rate, which might be different for high-priority or low-priority channels.
40.7
PHY Transmit Queues
An APC schedules up to NCITS channels in a time slot. It writes each channel number into a dedicated
PHY transmit queue in the position indicated by TQAPTR. The transmitter’s pointer TQTPTR, lagging
behind the APC pointer, is used to read channel numbers. For each channel number read, the transmitter
sends a single cell. The channels waiting to be sent lie between TQTPTR and TQAPTR.
Figure 40-7
shows
the organization of a PHY transmit queue.
Figure 40-7. PHY Transmit Queue
A transmit queue never overflows because the TQAPTR pointer never wraps to point to the TQTPTR
pointer. If the transmit queue is full, the APC does not insert more channels, and the APCT_SPTR stalls
until space is available in the transmit queue. The depth of the transmit queue is equal to the number of
entries minus 1.
In single PHY mode, TQBASE, TQEND, TQTPTR, and TQAPTR are located in the parameter RAM. In
multi-PHY master mode (indicated by the MPHY and ESAR bits in the APCST), each PHY has its own
dedicated transmit queue. In this case, the TQBASE, TQEND, TQAPTR and TQTPTR pointers in the
parameter RAM are reserved. In their place, each PHY has its own dedicated ETQBASE, ETQEND,
ETQAPTR and ETQTPTR located in the first level of the PHY associated APC priority levels.
40.8
MPHY Pointing Table (Master Only)
The MPHY pointing table is needed only in multi-PHY mode, in which case APCPTR is used as a pointer
to the MPHY pointing table (instead of pointing directly to the APC priority levels). The pointing table
contains pointers to the APC priority levels of each PHY; see Figure 40-8.
Half word
TQBASE
TQEND
TQTPTR
TQAPTR
Scheduled channels
Содержание PowerQUICC MPC870
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