National CP3BT26 - Manual (Page 47)

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CP3
BT26
10.0 Interrupts
The Interrupt Control Unit (ICU) receives interrupt requests
from internal and external sources and generates interrupts
to the CPU. Interrupts from the timers, UARTs, Microwire/
SPI interface, and Multi-Input Wake-Up module are all
maskable interrupts. The highest-priority interrupt is the
Non-Maskable Interrupt (NMI), which is triggered by a falling
edge received on the NMI input pin.
The priorities of the maskable interrupts are hardwired and
therefore fixed. The implemented interrupts are named
IRQ0 through IRQ47, in which IRQ0 has the lowest priority
and IRQ47 has the highest priority. (IRQ0 is not implement-
ed, so IRQ1 is the lowest priority interrupt that normally may
occur.)
10.1 NON-MASKABLE INTERRUPTS
The Interrupt Control Unit (ICU) receives the external NMI
input and generates the NMI signal driven to the CPU. The
NMI input is an asynchronous input with Schmitt trigger
characteristics and an internal synchronization circuit,
therefore no external synchronizing circuit is needed. The
NMI pin triggers an exception on its falling edge.
10.1.1 Non-Maskable Interrupt Processing
The CPU performs an interrupt acknowledge bus cycle
when beginning to process a non-maskable interrupt.
At reset, NMI interrupts are disabled and must remain dis-
abled until software initializes the interrupt table, interrupt
base register (INTBASE), and the interrupt mode. The ex-
ternal NMI interrupt is enabled by setting the EXNMI.EN-
LCK bit and will remain enabled until a reset occurs.
Alternatively, the external NMI interrupt can be enabled by
setting the EXNMI.EN bit and will remain enabled until an in-
terrupt event or a reset occurs.
10.2 MASKABLE INTERRUPTS
The ICU receives level-triggered interrupt request signals
from 47 sources and generates a vectored interrupt to the
CPU when required. Priority among the implemented inter-
rupt sources (named IRQ1 through IRQ47) is fixed.
The maskable interrupts are globally enabled and disabled
by the E bit in the PSR register. The EI and DI instructions
are used to set (enable) and clear (disable) this bit. The glo-
bal maskable interrupt enable bit (I bit in the PSR) must also
be set before any maskable interrupts are taken.
Each interrupt source can be individually enabled or dis-
abled under software control through the ICU interrupt en-
able registers and also through interrupt enable bits in the
peripherals that request the interrupts. The ICU supports
IRQ0, but in the CP3BT26 it is not connected to any inter-
rupt source.
10.2.1 Maskable Interrupt Processing
Interrupt vector numbers are always positive, in the range
10h to 3Fh. The IVCT register contains the interrupt vector
of the enabled and pending interrupt with the highest priori-
ty. The interrupt vector 10h corresponds to IRQ0 and the
lowest priority, while the vector 3Fh corresponds to IRQ47
and the highest priority. The CPU performs an interrupt ac-
knowledge bus cycle on receiving a maskable interrupt re-
quest from the ICU. During the interrupt acknowledge cycle,
a byte is read from address FF FE00h (IVCT register). The
byte is used as an index into the Dispatch Table to deter-
mine the address of the interrupt handler.
Because IRQ0 is not connected to any interrupt source, it
would seem that the interrupt vector would never return the
value 10h. If it does return a value of 10h, the entry in the
dispatch table should point to a default interrupt handler that
handles this error condition. One possible condition for this
to occur is deassertion of the interrupt before the interrupt
acknowledge cycle.
10.3 INTERRUPT CONTROLLER REGISTERS
Table 19 lists the ICU registers.
Table 19 Interrupt Controller Registers
Name Address Description
IVCT FF FE00h
Interrupt Vector
Register
NMISTAT FF FE02h
Non-Maskable
Interrupt Status
Register
EXNMI FF FE04h
External NMI Trap
Control and Status
Register
ISTAT0 FF FE0Ah
Interrupt Status
Register 0
ISTAT1 FF FE0Ch
Interrupt Status
Register 1
ISTAT2 FF FE20h
Interrupt Status
Register 2
IENAM0 FF FE0Eh
Interrupt Enable and
Mask Register 0
IENAM1 FF FE10h
Interrupt Enable and
Mask Register 1
IENAM2 FF FE22h
Interrupt Enable and
Mask Register 2