DECAY

; DECAY reference: an exponential fall. Start the level at the top, then HALVE it each
; step with shr (255, 127, 63, 31, 15, 7, 3, 1) so the curve drops fast and flattens.
; When the level reaches 0 the loop resets to the top. Teaches shr (halve) and jnz
; (keep looping while the level is nonzero).
top:
      mov  r0, 255
loop:
      out  p0, r0       ; draw the current level
      slp  2            ; hold each step two ticks
      shr  r0, 1        ; halve the level (shift right by one)
      jnz  loop         ; nonzero -> keep decaying
      jmp  top          ; reached 0 -> reset to the top

# DECAY. ACT 1, analog. Not a straight ramp: an EXPONENTIAL fall. Start the level high
# and HALVE it each step with shr, so the curve drops fast then flattens toward the
# floor, then resets to the top. The first SCOPE board to use a shift, so it teaches
# shr (halve) and jnz (loop while nonzero). Reference is sc_a1_09_decay.asm.
# TITLE DECAY
# OBJECTIVE An exponential fall, halving each step, then a snap back to the top.
ID sc_a1_09_decay
WINDOW 64
DIR p0 OUT
PERIPH DAC data=p0 label=DAC_on_p0
CHAN p0 ANALOG label=CH1_p0_DAC
PAR power=128 lines=8 chips=1