ADC & Joystick

ADC & Joystick — MOS Interface

For chip-level details see upd7002-adc. This page covers MOS-level access (OSBYTEs + ADVAL semantics).

OSBYTE summary

OSBYTE	Function
&10 (16)	Select number of channels to auto-cycle (X = 1-4, or 0 to disable)
&11 (17)	Force conversion on channel X (1-4)
&80 (128)	Read ADC channel (X = 1-4) or buffer status (X = &FF-&F7) — see below
&BC (188)	Read current channel (channel currently converting)
&BD (189)	Read max channel (set by &10)
&BE (190)	R/W conversion resolution (&00 default / &08 8-bit / &0C 12-bit)

OSBYTE &80 — the dual-purpose call

X selects mode:

X	Function
0	Returns last-converted channel + fire button states
1-4	Read ADC channel X (= BASIC ADVAL(X))
&FF-&F7	Read buffer status (see buffers)

The X=0 path returns (in X on exit; A is preserved by OSBYTE):

• X = channel number of last completed conversion (0 if none yet — i.e. immediately after &10 or &11).
• Low 2 bits of X = fire button states (bit 0 = J1 fire, bit 1 = J2 fire). Active-low — bit clear means button pressed.

This is BASIC’s ADVAL(0). The channel-number and fire-button bits share the same byte; mask appropriately to separate them (X AND &03 for fire bits, X AND &FC for channel).

For X = 1-4: returns the 16-bit ADC value in (X, Y) — X = low byte, Y = high byte. Combined: (Y << 8) | X = 0..&FFFF.

Reading a joystick — standard pattern

LDA #&80 : LDX #1 : JSR &FFF4    ; ADVAL(1) = joystick 1 X
; X = LSB, Y = MSB of joystick 1 X position
STX joy1_x_lo
STY joy1_x_hi
 
LDA #&80 : LDX #2 : JSR &FFF4    ; ADVAL(2) = joystick 1 Y
LDA #&80 : LDX #3 : JSR &FFF4    ; ADVAL(3) = joystick 2 X
LDA #&80 : LDX #4 : JSR &FFF4    ; ADVAL(4) = joystick 2 Y
 
LDA #&80 : LDX #0 : JSR &FFF4    ; fire buttons
STX fire_buttons                 ; bit 0 = J1, bit 1 = J2 (active low)

For per-frame game polling, run this once per vsync (events event 4 or interrupts CA1 hook).

Faster sampling: 8-bit mode

LDA #&BE : LDX #&08 : LDY #0 : JSR &FFF4    ; 8-bit conversions

Halves the latency per channel from ~10 ms to ~4 ms. For game purposes, 8-bit is usually more than enough — joystick precision is limited by the pot quality anyway.

Per-channel ADVAL is still returned as 16-bit (the chip left-justifies into the high byte). High byte alone gives 8-bit precision.

Single-channel high-rate sampling

To poll one channel as fast as possible (ignoring the others):

LDA #&10 : LDX #1 : JSR &FFF4         ; auto-cycle only channel 1
LDA #&BE : LDX #&08 : LDY #0 : JSR &FFF4   ; 8-bit
.sample_loop
    LDA #&11 : LDX #1 : JSR &FFF4     ; force a conversion
    ; wait ~4ms or hook CB1 IRQ
    LDA #&80 : LDX #1 : JSR &FFF4
    ; X = result low, Y = result high
    JMP sample_loop

Theoretical max: ~250 samples/sec on a single channel in 8-bit mode. Good enough for tracking analogue pots or single-axis controllers; not enough for audio.

Fire buttons via System VIA — direct

OSBYTE &80, X=0 is the canonical path. The fire buttons are physically connected to System VIA PB4 (J1) and PB5 (J2), active-low.

The naive “read &FE40 directly” doesn’t necessarily work as expected — see via-6522 “IRA vs IRB asymmetry”. When PB4/PB5 are configured as inputs, the 6522’s behaviour for reading IRB is hardware-revision-dependent: some sources say IRB always returns the last value written to ORB (no live pin read), others say input-mode pins do reflect live state. MOS’s joystick read uses the slow-bus protocol (set DDRB appropriately, then read), which works regardless.

Verify against real hardware before relying on direct LDA. The safest fast path is to hook the OSBYTE-internal poll instead of replacing it — or use the keyboard-event mechanism if all you want is “did the user press fire”.

Faster than OSBYTE if you can do it: typically a hand-rolled read of the System VIA via the proper input-mode sequence costs ~20 cycles vs OSBYTE’s hundreds. Worth doing for per-frame polling in tight game loops.

Master Compact joystick simulator

The Compact has no real ADC. MOS synthesises ADVAL values from User VIA (user-via, base &FE60) port-B pins on the combined joystick/user-port connector:

User VIA PB0 = joystick FIRE
User VIA PB1 = joystick LEFT
User VIA PB2 = joystick DOWN
User VIA PB3 = joystick UP
User VIA PB4 = joystick RIGHT

(These are distinct from the System VIA’s PB4/PB5 fire-button pins on non-Compact machines.)

OSBYTE &BE on Compact has a completely different bit-layout to configure how the switches map to ADVAL values:

Bit	Effect
7	0 = MOS updates ADVAL from switches; 1 = ROM-provided values
6	If set, joystick movement injects ASCII into keyboard buffer (cursor keys + COPY)
5	0 = “varying” emulated values that change over time; 1 = fixed &0000/&7FFF/&FFFF extremes
3-0	Speed of varying emulation (7 = fastest, 0 = slowest)

With bit 6 set, Compact joystick acts as cursor keys (no extra game code needed for keyboard-driven games):

Joystick	Key
RIGHT	cursor right (&89)
UP	cursor up (&8B)
DOWN	cursor down (&8A)
LEFT	cursor left (&88)
FIRE	COPY (&87)

Conversion-complete event (event 3)

events event 3 fires when an ADC conversion completes. X on entry = channel that just finished.

Useful pattern: enable event 3 + ADC auto-cycle, then handle each result as it arrives. Lower latency than polling OSBYTE &80:

LDA #&0E : LDX #3 : JSR &FFF4    ; enable event 3
LDA #&10 : LDX #4 : JSR &FFF4    ; auto-cycle 4 channels
; ... install EVNTV handler that switches on A=3 ...

See also

• upd7002-adc — Chip-level reference.
• system-via — CB1 = EOC IRQ; PB4/PB5 = fire buttons.
• events — Event 3 = ADC conversion complete.
• buffers — OSBYTE &80 X-range collision with buffer status.

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