Motorola Axiom CMD11E1 Uživatelská příručka stáhnout pdf (Strana 78)

78 CHAPTER 7. INTERRUPT PROCESSING

; CLEAR THE FLAG. NO HARM IS DONE IF IT IS ALREADY CLEARED

LDAA #%10000000

STAA TFLAG2

;WAIT FOR THE FLAG TO BE SET

LOOP2

LDAA TFLAG2

ANDA #%10000000

BEQ LOOP2

;NOW TOGGLE PA4

LDAA #%00010000

EORA PORTA

STAA PORTA

;DO IT ALL OVER AGAIN

BRA LOOP1

7.3 Interrupts

If you study the above co de, most of the time is spent waiting for the clock to

rollover (the loop at LOOP2). This is a lot like sitting in front of the clock and

watching and waiting for the clock to rollover. Or, for that matter, sitting in

front of a stove and watching and waiting for kettle to boil, or watching food

being cooked in a microwave oven. A better solution would be to go about ones

job and arrange matters so that one is told when an event occurs (the clock chimes,

the kettle whistles, the microwave oven sounds an alarm etc.). When we are told

that the event has occurred, we then take appropriate action (turn oﬀ the stove,

take food out of the oven etc.). Most of the HC11 interrupts work the same way.

In essence this is what happens:

1. When an event occurs, a ﬂag is set. For example, the clock rollover sets the

TOF ﬂag.

2. Associated with the ﬂag is a masking bit. The name of the bit is the same

as the ﬂag except the ﬁnal F is replaced by I. The mask associated with TOF

is TOI.

(a) If the mask is zero, then nothing much happens. The event is ignored

by the interrupt processing structure.

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Motorola Axiom CMD11E1 Uživatelská příručka Strana 78