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CHAPTER 5 SAR FUNCTION
194
(a) Free buffer underflow
After reception of an AAL-5 packet, the
μ
PD98405 transfers the segment obtained from a received cell
to system memory. If the free buffer does not contain at least 48 bytes of free space, however, the
μ
PD98405 discards the cell. If the discarded cell is an intermediate cell, or the last cell, of the packet,
the
μ
PD98405 suspends packet reception then issues a receive indication for reporting a free buffer
underflow error. If a free buffer underflow occurs, an RQU interrupt for the relevant pool occurs
simultaneously (see
Section 7.2 (4)
). The
μ
PD98405 discards all remaining cells, up to the last cell in
this packet.
If the discarded cell is the first cell of the AAL-5 packet, no receive indication is issued to report free
buffer underflow. Only an RQU interrupt is issued to report that no free buffer is available in the pool.
To enable reception, the host must add, using the Add_Batches command, a batch to the pool for
which an RQU interrupt has been issued.
(b) Receive FIFO overrun
During the reception of an AAL-5 packet in DROP mode (GMR register DR bit = 0), if the receive
FIFO, which can store 96 cells, is full upon the reception of a cell of the packet, the
μ
PD98405
discards that cell. The
μ
PD98405 discards all remaining cells, up to the last cell in this packet.
If a receive FIFO overrun occurs, the corresponding receive indication is issued in the following two
ways:
If FIFO overrun is cleared while the cells of a packet that has an error are arriving, the
μ
PD98405
issues an indication to report the receive FIFO overrun once the last cell of the packet has arrived.
If a FIFO overrun is not cleared before the last cell of the erroneous packet arrives, the
μ
PD98405
does not issue an indication to report the receive FIFO overrun.
(c) "MAX. NUMBER OF SEGMENTS" violation
This error occurs if the last cell of the packet has not yet been received, despite the number of
received cells reaching the value of "MAX. NUMBER OF SEGMENTS," set by the user in the VC table.
The data received prior to "MAX. NUMBER OF SEGMENTS" being reached is stored in system
memory. A receive indication is issued upon the reception of the cell next to that at which "MAX.
NUMBER OF SEGMENTS" was reached. The
μ
PD98405 discards all remaining cells, up to the last
cell in this packet.
If, for example, the user has set 100 for "MAX. NUMBER OF SEGMENTS," the last cell must be
received before 100 cells are received. If the last cell has not yet been received when 100 cells have
been received, a receive indication for reporting a "MAX. NUMBER OF SEGMENTS" violation is
issued upon the reception of the 101st cell. The
μ
PD98405 discards the subsequently received cells,
from the 101st to the last cells.