nespad.pio

; NES (classic) gamepad reader. To read current pad state, trigger a transfer
; by clearing PIO IRQ 0, then use blocking read from FIFO -- result appears
; there after ~100 uS. Since PIO runs concurrently, minor CPU tasks can be
; handled between trigger/read, but it's NOT recommended to immediately
; trigger another read and then catch the result in the same place on the
; next frame, as this introduces latency. Ideal time is at frame start.

.program nespad
.side_set 1

.wrap_target
  irq wait 0      side 1      ; Set IRQ and wait for CPU to clear it
  set pins, 1     side 1 [10] ; Latch high, 12 uS total
  set x, 7        side 1      ; Set bit counter
  set pins, 0     side 1      ; Latch low
bitloop:
  set pins, 0     side 1 [8] 
  in pins, 1      side 0 [2]  ; Read bit on falling clock or latch, wait 3 uS
  jmp x-- bitloop side 1      ; Clock high, repeat for 8 bits
; Autopush will write byte to FIFO at this point
.wrap