Interrupts
Reset Interrupt Sequence
MC68HC(7)05H12
—
Rev. 1.0
General Release Specification
MOTOROLA
Interrupts
63
L
G
R
4.4 Reset Interrupt Sequence
The reset function is not in the strictest sense an interrupt; however, it is
acted upon in a similar manner as shown in
Figure 4-1
. A low level input
on the RESET pin or internally generated RST signal causes the
program to vector to its starting address which is specified by the
contents of memory locations $3FFE and $3FFF. The I-bit in the
condition code register is also set. The MCU is configured to a known
state during this type of reset as described in
Section 5 Resets
.
4.5 Software Interrupt (SWI)
The SWI is an executable instruction and a non-maskable interrupt since
it is executed regardless of the state of the I-bit in the CCR. If the I-bit is
zero (interrupts enabled), the SWI instruction executes after interrupts
which were pending before the SWI was fetched, or before interrupts
generated after the SWI was fetched. The interrupt service routine
address is specified by the contents of memory locations $3FFC and
$3FFD.
4.6 Hardware Interrupts
All hardware interrupts except reset are maskable by the I-bit in the
CCR. If the I-bit is set, all hardware interrupts (internal and external) are
disabled. Clearing the I-bit enables the hardware interrupts. There are
two types of hardware interrupts (external, internal) which are explained
in the following sections.
4.7 External Interrupt (IRQ/Keyboard)
If the interrupt mask bit (I bit) of the CCR is set, all maskable interrupts
(internal and external) are disabled. Clearing the I bit enables interrupts.
The interrupt request is latched immediately following the falling edge of
IRQ. It is then synchronized internally and serviced by the interrupt
service routine located at the address specified by the contents of
$3FFA and $3FFB.