Altera DE1-SoC, Manual, Page: 23 / 45

DE1-S
O
C C
OMPUTER
S
YSTEM WITH
N
IOS
II
For Quartus II 15.0
DO_KEY3:
movi r12, KEY3
stw r12, 0(r10) /* return KEY3 value */
END_PUSHBUTTON_ISR:
ldw ra, 0(sp) /* Restore all used register to previous values */
ldw r10, 4(sp)
ldw r11, 8(sp)
ldw r12, 12(sp)
ldw r13, 16(sp)
addi sp, sp, 20
ret
. end
Figure 19 . Interrupt service routine for the pushbutton keys (Part b ).
3.5 Using Interrupts with C Language Code
An example of C language code for the DE1-SoC Computer that uses interrupts is shown in Figure 20 . This code
performs exactly the same operations as the code described in Figure 16 .
To enable interrupts the code in Figure 20 uses
macros
that provide access to the Nios II status and control registers.
A collection of such macros, which can be used in any C program, are provided in Figure 21 .
The reset and exception handlers for the main program in Figure 20 are given in Figure 22 . The function called
the_reset provides a simple reset mechanism by performing a branch to the main program. The function named
the_exception represents a general exception handler that can be used with any C program. It includes assembly
language code to check if the exception is caused by an external interrupt, and, if so, calls a C language routine
named
interrupt_handler . This routine can then perform whatever action is needed for the specific application.
In Figure 22 , the interrupt_handler code first determines which exception has occurred, by using a macro from
Figure 21 that reads the content of the Nios II interrupt pending register. The interrupt service routine that is
invoked for the interval timer is shown in 23 , and the interrupt service routine for the pushbutton switches appears
in Figure 24 .
The source code files shown in Figure 16 to Figure 24 are distributed as part of the Altera Monitor Program. The
files can be found under the heading sample programs , and are identified by the name Interrupt Example .
Altera Corporation - University Program
2015
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