************************************************************
* TAPEBITS
* Read cassette data, record duration of each bit phase.
*
* The buffer contains 2-byte ints, each of which is the
* number of loops during a bit phase (ie: how long it was
* 0, or how long it was 1).  Each loop is 28 clock cycles,
* or about one 31960th of a second.  So a '1' bit at 2400Hz
* should be about 13.32 loops for the entire bit, or 6.66
* for each phase; and a '0' bit at 1200Hz should be about
* 26.63 loops per bit, or 13.32 loops per phase.
* So when recieving valid cassette data, the numbers for
* each phase should be 6, 7, 13, or 14.
************************************************************

PIA1AD	equ	$FF20	; PIA 1, side A, Data register
IRQ	equ	$10
FIRQ	equ	$40
ACTIVE	equ	$05FF	; lower right character on screen

buffer equ $4000	; where to store results
endbuf equ $8000

	org $3000

start equ *
	bra start2
	fcb $99,$08,$25,$18,$10
	fcc "TAPEBITS V0.1"

start2 equ *
	orcc #IRQ|FIRQ	; disable interrupts
	sty tmp_y	; save .y
	ldx #buffer
	clr over	; no overflows yet
	lda PIA1AD
	anda #1		; look at cassette input bit

new equ *
	sta prev	; 5 ; remember if it is 0 or 1
	ldy #1		; 4 ; initialize counter
* Since each phase has an overhead of 32 cycles (new + diff),
* we count one additional loop. Loops take 28 cycles each,
* so this is pretty close.

same equ *
	leay 1,y	; 5
	cmpy #$FFFF	; 5 ; counter overflow?
	beq overflow	; 3
	lda PIA1AD	; 5
	anda #1		; 2 ; look at cassette input bit
	cmpa prev	; 5 ; compare with previous
	beq same	; 3 ; not changed yet; count more
*			 --
*			 28

diff equ *
	inc ACTIVE	; 7 ; show that we're waiting
	sty ,x++	; 9 ; phase changed, save count
	cmpx #endbuf	; 4 ; buffer full?
	bne new		; 3 ; if not, count more
*			 --
*			 23

done equ *
	stx last
	ldy #tmp_y	; restore .y
	andcc #~(IRQ|FIRQ)	; re-enable interrupts
	rts

overflow equ *
	lda over
	inca		; another overflow
	cmpa #30	; too many?
	bhs done	; if so, exit
	sta over	; not too many yet
	lda prev	; still the same bit value
	bra diff	; continue counting

	org $3100

last	fdb	0	; last address filled + 2
prev	fcb	0	; last known value of cassette bit
over	fcb	0	; number of counter overflows
tmp_y	fdb	0	; .y saved here on entry

	end start