Motorola Axiom CMD11E1 Guia do Utilizador descarregar pdf (Página 91)

8.5. VARIABLE FREQUENCY GENERATOR 91

FDB 851, 804, 759, 716, 676

OFFTIMETBL

FDB 3409, 3217, 3037, 2866, 2706

FDB 2554, 2410, 2275, 2147, 2027

Note that it makes sense to enter numbers in decimal notation. Each entry in the

table requires two bytes (we use 16 bit numbers to measure time since the HC11

clock is a 16 bit quantity). Hence we use FDB instead of FCB. Note that this also

means that we have to index through memory in steps of two bytes. Suppose

we want to access the element #4 in ONTIMETBL and the value #4 is in the

B register (the number #4 is used only as an illustration. In the application the

write

LDX #ONTIMETBL

ABX

ABX ; NEED TWO ABX’S

??? 0,X

We can now modify the single tone generator to a programmable square wave

generator by making the following changes:

Before After

LDD #1000 LDX #ONTIMETBL

STD ONTIME LDY #OFFTIMETBL

LDD #3000 LDD 0,X

STD OFFTIME STD ONTIME

LDD 0,Y

STD OFFTIME

1 2 ... 86 87 88 89 90 91 92 93 94 95 96 ... 100 101

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Motorola Axiom CMD11E1 Guia do Utilizador Página 91