Mouse

The Super NES Mouse (SNS-016) is a peripheral for the Super NES that was originally bundled with Mario Paint. It can be used with an NES through an adapter, made from an NES controller extension cord and a Super NES controller extension cord, that connects the respective power, ground, clock, latch, and data pins.

As with the standard controller, the mouse is read by turning the latch ($4016.d0) on and off, and then reading bit 0 of $4016 or $4017 several times. But its report is 32 bits long as opposed to 8 bits. Some documents about interfacing with the mouse recommend reading the first 16 bits at one speed, delaying a while, and reading the other 16 bits at another speed, following logic analyzer traces from a Super NES console. But these different speeds are merely an artifact of the main loop of Mario Paint, and the mouse will give a correct report when read at any reasonable speed.

The first byte of the report is all zeroes and may be ignored. The other three bytes are sent most significant bit first:

76543210  Second byte of report
||||++++- Signature: 0001
||++----- Current sensitivity (0: low; 1: medium; 2: high)
|+------- Left button (1: pressed)
+-------- Right button (1: pressed)

76543210  Third byte of report
|+++++++- Vertical displacement since last read
+-------- Direction (1: up; 0: down)

76543210  Fourth byte of report
|+++++++- Horizontal displacement since last read
+-------- Direction (1: left; 0: right)

The displacements are in sign-and-magnitude, not two's complement. For example, $05 represents five mickeys (movement units) in one direction and $85 represents five mickeys in the other. To convert these to two's complement, use negation:

  ; Convert to two's complement
  lda third_byte
  bpl :+
  eor #$7F
  sec
  adc #$00
:
  sta y_velocity

  lda fourth_byte
  bpl :+
  eor #$7F
  sec
  adc #$00
:
  sta x_velocity

The mouse can be set to low (x/4), medium (x/2), or high (x) sensitivity. To change the sensitivity, send a clock while the latch ($4016.d0) is turned on:

  ldy #1
  sty $4016
  lda $4016,x
  dey
  sty $4016

A program MUST NOT play samples and read the mouse at the same time. On the NES, sample playback causes occasional double reads on $4016 and $4017, which the program sees as bit deletions from the serial stream. Ordinarily, one would read each controller twice, compare the data, and use the previous frame's data if they don't match. This works because the extra latch pulse to set up the second read has no side effects on the standard NES or Super NES controller. But an extra latch pulse sent to a mouse will clear the mouse's count of accumulated mickeys.