; Copyright (c) 2024 - 2025 kio@little-bat.de
; BSD-2-Clause license
; https://opensource.org/licenses/BSD-2-Clause

; Transmit a mono SigmaDelta audio stream 
;
; Autopull must be enabled, with threshold set to 32.
; Since we need Sigma first which is in the msbit, shift direction must be to left.
; the side set bit sets pindir for the output pin.
; before start the ISR must be loaded with the const value = 127.

; default state of the out pin is 'input' => high Z.
; the 'sigma' bit sets the out pin to low or high, but doesn't change it's direction.
; if 'count' != 0 then the delay loop is entered and the pin direction is set to 'output'.

; data format: 4 x (sigma:1 + delta:7)
;
; clocks per sample = 127*2 + 1*2 + 4  = 260
; max. on/off ratio = 127 / 3

.program sid_audio
.side_set 1 pindirs

	
l1: jmp y--, l2		side 1		; decrement y
l2:	jmp x--, l1		side 1
.wrap_target
l0: jmp y--, l0		side 0  [1]

public entry_point:
	out  pins, 1	side 0		; 1=neg, 0=pos
	out  x, 7		side 0		; delay count
	mov  y,isr		side 0		; 127 => total loops = 128
	jmp  x--, l1	side 0
.wrap