Gamepad rumble support for the Commodore 64

I discovered a way to send 1-bit of data from the Commodore 64 to the Joysticks. And I use this one-bit of data to turn on/off the rumble in the gamepads through the Unijoysticle.

(Note: Spanish version of the video is here)

How does it work

I strongly suggest to view the video (see above) which contains technical info starting at 1:24

As a summary it works like this:

• The SID generates pulses that goes from 0 to ~1.3v every ~520us
• Those pulses go to the Joystick POT lines
• The pulses can be filtered from the CIA #1, Port A, Pins 6 and 7.
• The Unijoysticle receives the pulses and with the help of a timer and pull-up resistors it does the following:
  • If the line is High, turn on Rumble. Otherwise turn it off.

So, in order to turn on/off the rumble, we just need to do this in our games:

;
; Example that shows all 4 possible combinations
;
lda #%00000000   ;turn off rumble in both ports
sta $dc00

lda #%01000000   ;turn on rumble in port 1. Port #2 is Off
sta $dc00

lda #%10000000   ;turn on rumble in port 2. Port #1 is Off
sta $dc00

lda #%10000000   ;turn on rumble in both ports
sta $dc00

Note: What this discovery is about, is that by combining different well-known C64 features, we can now use to send 1-bit of data to the Joystick ports. For those who are curious about how the ADC works, read this article: Commo Pad, or just Google “Mouse 1351 internals”.

The different parts

The project consist of three parts:

• Unijoysticle Flashparty edition
• Bluepad32: The firmware that runs inside the Unijoysticle device (requires v3.5.1 or newer)
• Modified games: Rambo and Lemans

Unijoysticle 2 Flashparty edition

This board is based on the original Unijoysticle 2 board, and includes the following changes:

• Lines that connects to the POT lines
• Some external pull-up resistors
• Improved layout

Schematic and Layout files can be found here: unijoysticle2_flashparty2022

Bluepad32 v3.5.1

This is the firmware that runs in the Unijoysticle device.

• Includes the code needed to read the POT lines and turn on/off the rumble accordingly.

Pre-compiled binary can be found here: bluepad32-unijoysticle_v3.5.1

Rambo: First Blood, Part II

Things that I did to the game:

• Took the NTSC (ThunderMountain) version and cracked it
• Disassembled the game and added comments to it
• Patched it to support rumble
• Make it easier to switch weapons (Put Unijoysticle in “Enhanced” mode!). Works by pressing space as well.
• Added intro and converted it to a single-file game

While doing it I discovered Martin Galway’s Music Debug routine that can be accessed by pressing ‘P’ in the intro. While in the Music Debug routine, press letters A to Z to turn on the different songs. ‘1’ and ‘2’ switches the different music bank songs.

The complete disassembled code can be found here:

• https://gitlab.com/ricardoquesada/c64-rambo

Lemans

Things that I did to the game:

• Took the original cartridge binary, disassembled it and fully commented it
• Patched it to support rumble
• Patched other things like: Use joystick instead of paddle, fixed typo in message and other minor things
• Added intro

The complete disassembled code can be found here:

• https://gitlab.com/ricardoquesada/c64-lemans

Flashparty

This project was presented at Flashparty 2022 (a demoscene event), where it got 2nd place in the Wild category.

Many thanks for the organizers, who also setup a Commodore 64 with the Unijoysticle Flashparty edition so that the public can try it.