Interrupts, Clock Generation, Resets, and Operation Modes - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 274

Example 6.1
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Assume that the IRQ pin of the HCS12DP256 is connected to a 1-Hz digital waveform and
Port B is connected to eight LEDs. Write a program to configure Port B for output and enable the
IRQ interrupt, and also write the service routine for the IRQ interrupt. The service routine for
the IRQ interrupt simply increments a counter and outputs it to Port B. This program is to be
executed on an HCS12 demo board programmed with the D-Bug12 monitor.
Solution: The program is as follows:
#include “c:\miniide\hcs12.inc”

org $1000

count ds.b 1 ; reserve 1 byte for count

org $1500

lds #$1500 ; set up the stack pointer

movw #IRQISR,UserIRQ ; set up interrupt vector in SRAM

clr count

movb #$FF,DDRB ; configure Port B for output

bset DDRJ,$02 ; configure PJ1 pin for output (required in Dragon12)

bclr PTJ,$02 ; enable LEDs to light (required in Dragon12)

movb count,PTB ; display the count value on LEDs

movb #$C0,IRQCR ; enable IRQ pin interrupt and select edge triggering

cli ; “

forever nop
bra forever ; wait for IRQ pin interrupt
; ********************************************************************************
; This is the IRQ service routine.
; ********************************************************************************
IRQISR inc count ; increment count

movb count,PTB ; and display count on LEDs

rti

end

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6.5 Interrupt Programming in C Language

Interrupt programming in C language varies with the C compiler. In this section, we exam-
ine how interrupt programming is done in CodeWarrior, ICC12 IDE, and EGNU IDE.

6.5.1 Interrupt Programming in CodeWarrior
CodeWarrior uses the keyword interrupt to inform its C compiler that a function is an inter-
rupt service routine. Whenever the keyword interrupt appears before a function name, the Code-
Warrior C compiler generates RTI as the last instruction of the function. The template for an
interrupt service routine in CodeWarrior is as follows:
Interrupt void ISR_name (void)
{

. . . // statements to service the interrupt

}
where ISR_name is the name of the interrupt service routine.