Datasheet

 2011-2013 Microchip Technology Inc. DS30001037C-page 65
PIC24F16KL402 FAMILY
8.0 INTERRUPT CONTROLLER
The PIC24F interrupt controller reduces the numerous
peripheral interrupt request signals to a single interrupt
request signal to the CPU. It has the following features:
• Up to eight processor exceptions and
software traps
• Seven user-selectable priority levels
• Interrupt Vector Table (IVT) with up to 118 vectors
• Unique vector for each interrupt or exception
source
• Fixed priority within a specified user priority level
• Alternate Interrupt Vector Table (AIVT) for debug
support
• Fixed interrupt entry and return latencies
8.1 Interrupt Vector Table (IVT)
The IVT is shown in Figure 8-1. The IVT resides in the
program memory, starting at location, 000004h. The
IVT contains 126 vectors, consisting of eight non-mas-
kable trap vectors, plus up to 118 sources of interrupt.
In general, each interrupt source has its own vector.
Each interrupt vector contains a 24-bit wide address.
The value programmed into each interrupt vector loca-
tion is the starting address of the associated Interrupt
Service Routine (ISR).
Interrupt vectors are prioritized in terms of their natural
priority; this is linked to their position in the vector table.
All other things being equal, lower addresses have a
higher natural priority. For example, the interrupt
associated with vector 0 will take priority over interrupts
at any other vector address.
PIC24F16KL402 family devices implement
32 non-maskable traps and unique interrupts; these
are summarized in Ta bl e 8- 1 and Tab le 8- 2.
8.1.1 ALTERNATE INTERRUPT VECTOR
TAB LE (AIV T)
The Alternate Interrupt Vector Table (AIVT) is located
after the IVT, as shown in Figure 8-1. Access to the AIVT
is provided by the ALTIVT control bit (INTCON2<15>). If
the ALTIVT bit is set, all interrupt and exception
processes will use the alternate vectors instead of the
default vectors. The alternate vectors are organized in
the same manner as the default vectors.
The AIVT supports emulation and debugging efforts by
providing a means to switch between an application
and a support environment without requiring the
interrupt vectors to be reprogrammed. This feature also
enables switching between applications for evaluation
of different software algorithms at run time. If the AIVT
is not needed, the AIVT should be programmed with
the same addresses used in the IVT.
8.2 Reset Sequence
A device Reset is not a true exception, because the
interrupt controller is not involved in the Reset process.
The PIC24F devices clear their registers in response to
a Reset, which forces the Program Counter (PC) to
zero. The microcontroller then begins program
execution at location, 000000h. The user programs a
GOTO instruction at the Reset address, which redirects
the program execution to the appropriate start-up
routine.
Note: This data sheet summarizes the features of
this group of PIC24F devices. It is not
intended to be a comprehensive reference
source. For more information on the Inter-
rupt Controller, refer to the “dsPIC33/PIC24
Family Reference Manual”, “Interrupts”
(DS39707).
Note: Any unimplemented or unused vector
locations in the IVT and AIVT should be
programmed with the address of a default
interrupt handler routine that contains a
RESET instruction.