DE1-S
O
C C
OMPUTER
S
YSTEM WITH
N
IOS
II
For Quartus II 15.0
/* start interval timer, enable its interrupts */
movi
r15, 0b0111
/* START = 1, CONT = 1, ITO = 1 */
sthio
r15, 4(r16)
/* write to the pushbutton port interrupt mask register */
movia
r15, 0xFF200050
/* pushbutton key base address */
movi
r7, 0b1111
/* set interrupt mask bits */
stwio
r7, 8(r15)
/* interrupt mask register is (base + 8) */
/* enable Nios II processor interrupts */
movi
r7, 0b011
/* set interrupt mask bits for levels 0 (interval */
wrctl
ienable, r7
/* timer) and level 1 (pushbuttons) */
movi
r7, 1
wrctl
status, r7
/* turn on Nios II interrupt processing */
IDLE:
br
IDLE
/* main program simply idles */
.
data
/* The global variables used by the interrupt service routines for the interval timer and the
* pushbutton keys are declared below */
.
global
PATTERN
PATTERN:
.
word
0x0000000F
/* pattern to show on the HEX displays */
.
global
KEY_PRESSED
KEY_PRESSED:
.
word
KEY2
/* stores code representing pushbutton key pressed */
.
global
SHIFT_DIR
SHIFT_DIR:
.
word
2
/* default shift direction (2 == right) */
.
end
Figure
16
. An example of assembly language code that uses interrupts (Part
b
).
The reset and exception handlers for the main program in Figure
16
are given in Figure
17
. The reset handler simply
jumps to the
_start
symbol in the main program. The exception handler first checks if the exception that has occurred
is an external interrupt or an internal one. In the case of an internal exception, such as an illegal instruction opcode
or a trap instruction, the handler simply exits, because it does not handle these cases. For external exceptions, it
calls either the interval timer interrupt service routine, for a level 0 interrupt, or the pushbutton key interrupt service
routine for level 1. These routines are shown in Figures
18
and
19
, respectively.
Altera Corporation - University Program
2015
17