PM73121ꢀAAL1gator II
Data Sheet
PMC-Sierra, Inc.
PMC-980620
,VVXHꢀꢁ
AAL1 SAR Processor
Figure 46 shows the state machine that checks for valid pointers and structures.
No pointer
found
Underrun
Non-dummy pointer
Underrun or
force-underrun
found and parity
good (or not
Pointer does not
checking parity)
match prediction or
underrun or
force-underrun (or
checking parity and
Pointer Found
Underrun or
force-underrun
Structure
found
Pointer does not
match prediction
(or parity bad and
checking parity) or
out-of-sequence or
invalid cell received
Pointer matches
prediction and parity
good (or not
Structure Found
checking parity)
One Mismatch
(Signaling is not
updated)
Pointer matches prediction
and parity good (or not
checking parity)
Figure 46. Pointer/Structure State Machine
3.6.4 Overrun
Overrun occurs when the data in the buffer is removed at a slower rate than it is filled. However,
because the AAL1gator II buffers are quite large, 16 kBytes per line, by the time this happens, all
data in the buffer can be quite old. Therefore, the buffer size is adjustable, which regulates how
much data can be stored in the buffer before an overrun occurs. The R_MAX_BUF field in the R_
MAX_BUF register controls the maximum size of the receive buffer. The value of R_MAX_BUF
should be equal to or greater than two times CDVT, or two times the number of frames required
per cell, whichever is greater.
The overrun condition is declared when the data in the receive buffer exceeds the maximum spec-
ified buffer size (refer to “R_MAX_BUF Word Format” on page 157). When a cell is received
that causes the maximum buffer depth to be exceeded, the OVERRUN sticky bit is set and the
AAL1gator II enters the forced underrun condition. The incoming cells for the queue are dropped
until underrun occurs. Each time a cell is received and dropped in the forced underrun condition,
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PMC [ PMC-SIERRA, INC ]
