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5.0 Reassembly Coprocessor
5.4 Buffer Management
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
5.4.8.6 AAL3/4 Early
Packet Discard
Processing
The RS8234 performs EPD for AAL3/4 CPCS-PDUs with these steps:
Sets the appropriate error bit in the PDU_CHECKS field for a new Rsm
Status Queue entry.
Sets the CPCS_LENGTH field to zero.
Sets the BD_PNTR field to point to the partially reassembled PDU.
Writes the Status Queue entry.
Discards all cells of that PDU up to and including the EOM cell for that
PDU.
5.4.8.7 Error Conditions
Partially reassembled CPCS-PDUs will be recovered for the following error con-
ditions:
Non-EOM Max PDU Length exceeded
Free buffer queue underflow
Status queue overflow
5.4.9 Hardware PDU Time-out
The RS8234 automatically detects active CPCS-PDU time-out for reassembly
channels. A PDU time-out occurs when a partially received PDU does not com-
plete within a set time period. When it detects this time-out condition, the
RS8234 provides a status queue indication to the host. This indication allows the
host to recover the buffers held by the partially completed PDU. The RS8234 sup-
ports up to eight reassembly time-out periods.
5.4.9.1 Reassembly
Time-out Process
A background hardware process performs the reassembly time-out function. The
process is activated at a user-selected interval. The process is globally enabled by
setting the GTO_EN bit in the RSM_CTRL0 register. The RSM_TO register con-
trols the process activity once enabled. The process is activated every
RSM_TO_PER rising edges of SYSCLK on cell boundaries. Note that GTO_EN
set to zero resets the internal time-out interrupt counter.
Each time the process is activated, it examines a single VCC, identified by
TO_VCC_INDEX. This is a 16-bit variable located at address 0x1350, in internal
SRAM. The Host should initialize this register to zero at system initialization.
To enable hardware time-out on an individual VCC, the Host must set TO_EN
in the VCC Table entry. The Host also assigns one of eight time-out periods to
each VCC by initializing the TO_INDEX field in the VCC Table entry.
The RS8234 checks the TO_EN bit and the active PDU indicator bit,
ACT_PDU, to see if time-out processing is enabled and necessary, respectively,
for the current connection. If either bit is zero, then TO_VCC_INDEX is incre-
mented by one and compared to RSM_TO_CNT in the RSM_TO register. If
TO_VCC_INDEX = RSM_TO_CNT, then TO_VCC_INDEX is reset to zero and
the time-out search is restarted at the beginning of the VCC Table.
If both bits are set, the RS8234 increments CUR_TOCNT in the Rsm VCC
Table entry. Then it compares CUR_TOCNT to the time-out value selected,
TERM_TOCNTx,
where
x
=
TO_INDEX.
TERM_TOCNT0
through
TERM_TOCNT7 are located at address 0x1340 through 0x134c in internal
SRAM. They must be initialized to appropriate values during system initializa-
tion.