MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
41-1
Chapter 41
ATM Exceptions
Interrupt handling for ATM channels involves two principal data structures: an event register (SCCE or
IDSR1) and a circular ATM interrupt queue. The interrupt queue (one per controller) is shown in
Figure 41-1
.
Figure 41-1. ATM Interrupt Queue
The INTBASE and INTPTR pointers are host-initialized global ATM parameters that respectively point to
the starting location of the queue structure in external memory and to the current empty position available
to the CP. The end of the queue is defined by the entry containing the wrap (W) bit set.
When an ATM channel generates an interrupt request, the CP writes a new entry to the queue consisting
of the channel’s code number (CHNUM_INDEX) and a description of the exception. The valid (V) bit is
then set and INTPTR is advanced. After the CP writes the last entry in the queue (W bit set), it re-initializes
INTPTR to point to the base of the queue (INTBASE).
For each event sent to an interrupt queue, the CP decrements a down counter which has been initialized to
a threshold number of interrupts. When the counter reaches zero, the controller’s global interrupt
(SCCEx[GINT] or IDSR1[GINT]) is set. The user controls how often the host application is interrupted to
service new entries in the queue by programming the interrupt threshold (INT_ICNT) in the parameter
RAM; see
Table 38-1
.
After an interrupt request, the host’s interrupt service routine polls the controllers’ event registers
(SCCEx[GINT] and/or IDSR1[GINT]) to determine which controller is requesting service. After clearing
Service Pointer
X
V = 0
W = 0
X
V = 0
W = 0
X
V = 0
W = 0
Interrupt Flags
V = 1
W = 0
Interrupt Flags
V = 1
W = 0
Interrupt Flags
V = 1
W = 0
Interrupt Flags
V = 1
W = 0
X
V = 0
W = 0
X
V = 0
W = 0
X
V = 0
W = 1
32 Bits
INTBASE
INTPTR
(maintained by host software)
CHNUM_INDEX
CHNUM_INDEX
CHNUM_INDEX
CHNUM_INDEX
X
X
X
X
X
X
Summary of Contents for PowerQUICC MPC870
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