Listing for the Frequency Generating subroutine

*Define variables and some setup registers for the frequency generating program before calling the program, 
*Store the frequency generation constant in the variable frec initialize the output of PORTA so that 
*all outputs except PORTA 4 are high (and thus the filters are off. The output filters to be turned on
*are then pulled low. Then the fgen program can be called.
*This example will generate 9500Hz from a 3.58MHz Ceramic Oscillator which will last ~250mS.
*The frec variable is calculated by the number of loops it takes to wait for a half cycle of the period of the
*(half on, half off) required output frequency ( Time=1/(FoutHz * 2halfnodes/cycle), the number of loops is how
*long the micro has to wait to achieve this time) and then the timer will allow for the half cycles 
*to repeat until the timer signals that ~250mS have passed. The program then returns to the 
*previous routine. This timer allows for frequency-independent tone output time.

$base 10T                       ;decimal default
tscr    equ $08                 ;timer interrupt register
output  equ $00                 ;output port definition (PA0-7)

     org        $00C0           ;RAM origin
temp1   rmb 1                   ;temporary variable
frec    rmb 1                   ;frequency variable
        ...
        lda #$FF                
        sta output              ;sets the output porta high so all filters are off
        ...
        lda #11                 ;constant for 12kHz, can be changed to have different frequencies
        sta frec                ;store in variable frec
        ...
        jsr fgen                ;call frequency generation subroutine
        ...
*Frequency generating subroutine. Optimize the number of instructions to output the proper frequencies.
fgen    sta temp1               ;store accumulator in a temp variable 
        lda #04                 ;number of interrupts for 250mS (5*65mS)
timer   bset 2,tscr             ;clear real time interrupt flag
innr3   bset 4,output           ;turn on oscillator output
        ldx frec                ;number for one half cycle
innr1   decx                    ;count down for half cycle
        bne innr1               ;if not finished, keep counting
        bclr 4,output           ;turn off oscillator output
        ldx frec                ;number for other half of cycle
innr2   decx                    ;count down for half cycle
        bne innr2               ;if not finished, keep counting
        brclr 6,tscr,innr3      ;if 65mS not passed (see timer/counter section of 
docs) do another cycle
        deca                    ;subtract from the five  65mS for 250mS total
        bne timer               ;have 250mS gone by? If not go back and repeat all
        lda temp1               ;restore accumulator
        rts                     ;