ASM Joypad
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Revision as of 09:09, 6 July 2019 by PinoBatch (Talk | contribs) (It's more propagation delay than bouncing)
Reads the status of the joypad, saving it in a (byte) variable "_PAD", uses the definitions of gbhw.inc:
read_pad:
; init
ld a, 0
ld [_PAD], a
; read the d-pad:
ld a, %00100000 ; bit 4 = 0, bit 5 = 1 (d-pad on, buttons off)
ld [rP1], a
; read the status of the d-pad
; the first first few reads waste a few cycles to let
; the signals settle after bits 5 and 4 have changed
ld a, [rP1]
ld a, [rP1]
ld a, [rP1]
ld a, [rP1] ; keep this read
and $0F ; just keep low nibble (d-pad status)
swap a ; swap low and high nibbles
ld b, a ; save d-pad status in b register
; let's read the buttons
ld a, %00010000 ; bit 4 = 1, bit 5 = 0 (buttons on, d-pad off)
ld [rP1], a
; signal propagation...
ld a, [rP1]
ld a, [rP1]
ld a, [rP1]
ld a, [rP1]
ld a, [rP1]
ld a, [rP1]
; we have in "a" the status of the buttons
and $0F ; just keep low nibble.
or b ; OR with "b" to put in the high byte of "a"
; the d-pad status.
; now we have in "a" the status of all the controls
; make the complement of it (so activated buttons read as 1)
; and save it
cpl
ld [_PAD], a
; reset the joypad
ld a,$30
ld [rP1], a
; return
ret
