Motorola Axiom CMD11E1 Uživatelská příručka stáhnout pdf (Strana 81)

7.4. THE REAL TIME INTERRUPT 81

; INTERRUPT SERVICE

SERVICE

; TOGGLE PA4

LDAA #%00010000

EORA PORTA

STAA PORTA

; TURN OFF THE FLAG!

LDAA #%10000000

STAA TFLG2

; END WITH AN RTI

RTI

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; Connect the service to the interrupt ;

ORG $00D0 ; $00D0 WHERE THE SERVICE STARTS

JMP SERVICE ; JUMP TO WHERE THE SERVICE CODE ACTUALLY IS

How does this code diﬀer from the previous one? We don’t wait for the clock

to rollover. Instead, the main program goes about its task (in this case not a

very interesting one!). Note that although the service routine uses the A register,

this does not aﬀect the value the main routine has stored in the A register (the

character Z).

7.4 The Real time interrupt

The real time interrupt, RTI (not to be confused with the RTI instruction) acts

exactly like the timer overﬂow interrupt, except you can control the time between

interrupts using the last two bits (0 and 1) of

PACTL

at location

$1026

. If the

both the bits are set, the time between the interrupts is is 32.768 ms, i.e. same

as timer overﬂow. However, you can decrease the time between the interrupts

(increase the rate) by changing the last two bits as PACTL as shown below:

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 100 101

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Motorola Axiom CMD11E1 Uživatelská příručka Strana 81