
MOTOROLA
Chapter 40. ATM Exceptions
40-5
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Interrupt Queue Entry
Table 40-3 describes the elds of an interrupt queue entry.
0
1
2
3
4
7
8
9
10
11
12
13
14
15
16
31
v
w
—
CNG
—
APCO
—
TQF
UN
RXF BSY TXB RXB CHNUM_INDEX
Figure 40-4. Interrupt Queue Entry
Table 40-3. Interrupt Queue Entry Field Descriptions
Bit
Name
Description
0
V
Valid bit. Indicates that this entry contains valid interrupt information. The CP sets this bit when
generating a new entry. The V bit and all event bits should be cleared by the host service routine
immediately after reading the entry.
1
W
Wrap bit. Indicates the last entry in the interrupt queue. After the CP writes to this entry, it moves to
the beginning of the queue for the next event; that is, INTPTR is re-initialized to INTBASE. After the
host services this entry, it should move to the beginning of the queue for the next entry to be
processed; that is, the service pointer should be re-initialized to INTBASE. During initialization, the
host should set the W bit only for the last entry of the queue.
2
—
Reserved
3
CNG
Congestion. Set by the CP when a congestion indication on a received cell (the middle bit of the PTI
eld is set). This interrupt applies only to channels whose RCT[CNGI] is set.
4–7
—
Reserved
8
APCO
APC overrun. Set by the CP if an APC scheduling table overruns. The address of the affected APC
level is placed in the CHNUM_INDEX eld. Indicates that the total programmed cell rate for this APC
priority level is greater than the maximum cell rate capability of the transmitter. That is, the real-time
scheduling pointer APCT_PTR has wrapped around to the service pointer APCT_SPTR position,
causing an entire APC scheduling pass to be lost because the scheduling pointer begins overwriting
the time slots with new channel numbers. Note however that no cells are lost, only the cell rates of
the channels belonging to this scheduling table have been diminished.
A scheduling table overrun occurs when (1) the programmed pace is greater than NCITS (that is, if
NCITS = 1, and 1/APCP1 + 1/APCP2 + 1/APCPn > 1), (2) the APCT_SPTR has stalled for some
reason, such as a full transmit queue, or (3) this level’s APC_MI is too low relative to higher APC
priority levels.
9
—
Reserved
10
TQF
Transmit queue full. Indicates an APC BYPASS command has failed because the transmit queue is full.
Occurs when the total APC scheduling rate is greater than the outgoing rate. The CHNUM_INDEX
eld contains the number of the channel attempting the APC bypass.
11
UN
Transmit underrun. Occurs for both AAL0 and AAL5 channels when scheduled to transmit without
sufcient data to form a complete cell.
No other action is taken and the channel remains enabled. If more data is not supplied, another UN
exception is generated the next time the channel is scheduled. When an underrun occurs, an idle
cell is sent, either generated by the MPC866 (in serial mode) or by the UTOPIA PHY (in UTOPIA
mode).
12
RXF
Receive frame. Indicates that a complete AAL5 frame has been received.
13
BSY
Busy. Indicates that a cell was received but discarded due to lack of empty buffers.
For AAL0, the receiver attempts again to open the same BD when the next cell for this channel
arrives. For AAL5, the remaining cells of the current frame are discarded. After the last cell of the
frame (PTI[1]=1) is received (but not stored), the receiver attempts again to open the same BD when
the rst cell of the next frame arrives.