Motorola Axiom CMD11E1 Guía de usuario descargar pdf (Pagina 88)

88 CHAPTER 8. SIGNAL GENERATION

8.3 Variable frequency signal generator

In this experiment, we will generate a square wave with frequency selected by the

user. We will ﬁx the duty cycle at 25%. To get started, we will ﬁrst write the

code for generating a single tone.

8.4 500 Hz tone generator

Here is a typical calculation to generate a 500 Hz signal:

T = 1/500 = 2 × 10

−3

seconds = (2 × 10

−3

) × (2 × 10

) = 4000 ticks

= 4000/4 = 1000 ticks

oﬀ

= 4000 − 1000 = 3000 ticks

To generate a 50 Hz, 25% duty cycle signal we use two variables called ONTIME and

OFFTIME. Every time we get an OC2 interrupt, we toggle the pin PA4 as before. We

check to see if we turned the pin ON or OFF. If we turned it on, then we schedule

the next interrupt to occur ONTIME ticks later. However, If we turned it oﬀ, then

we schedule the next interrupt to occur OFFTIME ticks later. Thus, the earlier

code that was used to generate a square wave is modiﬁed as follows:

; Various defines go here ...

ORG $3000 don’t forget the $

ME FCC /Your name/

FCB 10

FCC /ECE 372/

FCB 10

FCC /Date the program was last changed/

FCB 10, 10, 4

ONTIME RMB 2

OFFTIME RMB 2

ORG $2000 DONT FORGET THE $

LDX #ME

JSR OUTSTRG ; MAKE SURE YOU HAVE EQU FOR OUTSTRG

; Enable OC2 interrupt by setting OC2I (bit#6 in TMSK1)

; ALSO PERFORM ALL INITIALIZATION BETWEEN SEI/CLI

1 2 ... 83 84 85 86 87 88 89 90 91 92 93 ... 100 101

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Motorola Axiom CMD11E1 Guía de usuario Pagina 88