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ARPnMIDI

🔗 GitHub Link

Open Source RP2040 Zero MIDI router / arpeggiator / looper / expression / key finder / multitool / and more! By Joseph Wozniak

with SSD1306 OLED display (2-color recommended, yellow on the bottom)

Connect multiple usb midi devices with a hub. Output din/trs or usb midi.

Control live parameters and the appegiator with:

• Optical distance sensor (VL53L0X)
• Push force pressure sensor (FSR402 sensor with LM393 module)
• Assignable CC controls
• 4 foot pedal inputs (optionally connected to a QIACHIP RX480E wireless module)

You can also add an additional RP2040 board as a usb midi device, for full usb-to-usb rounting to another usb host. Use this file for the secondary RP2040: rp2040_usb_din_midi_bridge.txt

Current Hardware Pin Map (RP2040 Zero board labels)

Use the pin numbers exactly as printed on the RP2040 Zero board:

• 0 DIN MIDI OUT (TX to DIN/TRS out + Pro Micro RX pin)
• 1 DIN MIDI IN (RX from DIN/TRS with optocoupler or Pro Micro TX pin WITH RESISTORS. 10k from tx to rx pins, 20k from tx to gnd)
• 2 I2C SDA (SSD1306 + VL53L0X on same bus)
• 3 I2C SCL (SSD1306 + VL53L0X on same bus)
• 6 Rotary encoder A
• 7 Rotary encoder B
• 8 Rotary encoder push switch
• 9, 10, 12, 13 Foot pedal 1-4 input (active high, uses pull-down)
• 14 USB host D+
• 15 USB host D-
• 26 Push/pressure analog sensor input (FSR402 sensor with LM393 module)

Pin 11 is intentionally unused, so we can put a screw there.

RP2040 / Arduino-Pico USB Host optional resistors for best performance:

• USB MIDI host D+ to 22ohm resistor to GPIO pin 14
• USB MIDI host D- to 22ohm resistor to GPIO pin 15
• The 22ohm resistors go near the RP2040.
• USB MIDI host port requires 5V VBUS power, and two 15k resistors
• One from D+ to ground, and another from D- to ground.
• Use a ground wire to the usb host thicker than 30awg.
• Twist D+ and D-, try to keep them short.

Build Notes

BOARD SETUP

IMPORTANT set CPU Speed: "240 MHz (Overclock)"

• Board: Waveshare RP2040 Zero
• USB stack: Adafruit TinyUSB
• CPU: 240 MHz ONLY
• midi USB host uses PIO-USB on pins 14/15.
• Adafruit_NeoPixel must stay disabled in this sketch while USB host is enabled (ARPNMIDI_ENABLE_RGB_LED 0), because it conflicts with PIO-USB state machines.
• Pico-PIO-USB is vendored in vendor/Pico-PIO-USB (tag 0.7.1) and must be available to Arduino.
• TinyUSB host MIDI support in the installed Arduino-Pico core must define:
  • #define CFG_TUH_MIDI (CFG_TUH_DEVICE_MAX) (ignore this if it’s confusing)

Commercial Version Coming 2026 (More Info Here)

Join the email waitlist for the first chance at the first batch! I won’t spam you.

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1. `BPM`

– Range: `20..300`.

– Sets arp tempo directly.

– Top screen: large BPM number.

2. `ARP MODE`

– Options:

  – `OFF`

  – `UP`

  – `DOWN`

  – `UP-DOWN 1`

  – `UP-DOWN 2`

  – `TRIGGER`

  – `RANDOM`

  – `UP 1-OCT`

  – `RHYTHM`

  – `OSTINATO`

  – `OCT WALK`

  – `FIFTH`

  – `BASS+CHORD`

  – `CHORD+RUN`

– Top screen: mode symbol for classic modes, pattern preview for pattern modes.

3. `DIVISION`

– Options: `1/1`, `1/2`, `1/2T`, `1/4`, `1/4T`, `1/8`, `1/8T`, `1/16`, `1/16T`, `1/32`, `1/32T`, `1/64`.

– Changing this gives you different trap high-hat rolls, among other tricks.

– Top screen: pie visualization + division text.

4. `VELOCITY`

– Range: `1..127`.

– Applies to arp note output velocity.

– `VEL DOWN` sensor/push mode can reduce this live while in range.

– Top screen: bar + numeric value.

5. `LENGTH`

– Range: `1..100` percent of step duration.

– Applies to arp gate length.

– `LEN DOWN` sensor/push mode can reduce this live while in range.

– Top screen: bar + percent.

6. `INPUT CH`

– Options: `CH 1..CH 16`.

– Main processing input channel for arp/thru/bass and mapped CC/PB/program routing.

7. `ARP CH`

– Options:

  – `OFF`

  – `CH 1..CH 16`

  – `1+10`

  – `1+10-A`

  – `1-10 24`

  – `1-10 36`

  – `1-10 48`

– `1+10`: main arp on ch1 path plus drum-chord arp lane on ch10.

– `1+10-A`: same as `1+10`, plus ch10 channel aftertouch controls new ch10 drum-lane arp pulse velocity (33% minimum, `42..127`).

– `1-10 24/36/48`: same as `1+10`, plus 8-note split from input channel to ch10 drum lane:

  – `1-10 24`: input notes `24..31` remap to ch10 `36..43`

  – `1-10 36`: input notes `36..43` remap to ch10 `36..43`

  – `1-10 48`: input notes `48..55` remap to ch10 `36..43`

– Drum lane velocity is forced to max (`127`) for `1+10` and `1-10 24/36/48`.

– The ch10 drum lane still pulses like an arp lane even when `ARP MODE` is `OFF`.

8. `BASS CH`

– Options: `OFF`, then channel groups for `CH1..CH12`.

– Per channel, octave offsets are: `-2`, `-1`, `0`, `+1` oct.

– Behavior: lowest-note-priority mono bass voice.

– If bass channel equals arp channel, bass output is suppressed.

9. `THRU OUT`

– Options: `OFF`, `CH 1..CH 16`.

– Thru path for notes from input channel.

– If thru channel equals bass channel while bass is enabled, thru is suppressed for that channel.

10. `RNDRBN` (Multiple monophonic synths can be used polyphonically, or use each channel for unexpected note timbres)

– Checkbox list for output channels `CH 1..CH 16`, `[ ]CH10-1+`, `[ ]CH10-2+`, plus `CLEAR` and `BACK`.

– Click a channel while editing to toggle its checkmark.

– Checked channels are used as the round-robin rotation for the arp output channel path.

– `[ ]CH10-1+` maps final routed ch10 notes 36..51 into full-velocity note 60 on channels 1..16 after ch10 arp routing is complete.

– `[ ]CH10-2+` maps final routed ch10 notes 36..49 into channels 2..15 (note 12 on CH2, note 48 on CH3, note 60 on others).

– In select mode, the top screen shows the checked channel list in small text.

– If no channels are checked, round-robin is off.

– Checking either `CH10-1+` or `CH10-2+` unchecks the other.

11. `ROUTER`

– List entries are formatted like `16>16+12` or `01>02=00`.

– Select an input channel row, choose its output channel, then choose note transpose in semitones.

– Transpose affects note and poly-aftertouch note numbers; other channel messages only change channel.

– `CLEAR` restores every channel to its default `CH n > CH n =00` mapping.

– When all mappings are default, the runtime router is bypassed with only a single active-mask check.

12. `DIV NOTE` (notes give you drum rolls, and other magic)

– Cursor options:

  – Division slots: `1/4`, `1/4T`, `1/8`, `1/8T`, `1/16`, `1/16T`, `1/32`, `1/32T`, `1/64`

  – `+NOTE`

  – `RESET`

  – `BACK`

– In edit mode:

  – On a division slot, play a note (any channel) to map that note+channel to the division.

  – When mapped note is held, arp temporarily switches to that division.

  – If `+NOTE` is blank, mapped trigger note is swallowed.

  – If `+NOTE` has a note value, mapped trigger note is replaced by `+NOTE` and then processed normally.

– `RESET` clears all division-note mappings and `+NOTE` when you exit edit mode on `RESET`.

– `BACK` exits edit mode without clearing mappings.

13. `MAP CC`

– Cursor options:

  – Param slots: `BPM`, `ARP`, `DIV`, `VEL`, `LEN`, `IN`, `ACH`, `BCH`, `TCH`, `INCC`, `EYCH`, `EYMD`, `PSMD`, `KEY`, `SCL`, `LOOP`, `LOAD`

  – `CLEAR`

  – `CH:SET` / `CH:ALL`

– In edit mode on a param slot, send a CC to learn that CC number and channel for that slot.

– `EYMD` / `PSMD` changes the Eye mode and Push mode. Mapping excludes `CC 1..CC 19`, `CC103`, and Loop options.

– `CLEAR` clears all MAP CC assignments when you exit edit mode on `CLEAR`.

– `CH:SET`/`CH:ALL` toggles when you exit edit mode on that row:

  – `CH:SET`: mapped CC must match both learned CC and learned channel.

  – `CH:ALL`: mapped CC number works from any incoming channel.

– `LOAD` map target switches and loads preset immediately.

– `LOOP` map target changes loop length across `1 BAR`, `2 BAR`, `4 BAR`, `8 BAR`; mapped CC does not select `FREE`.

– Parameter changes from MAP CC are live and persist to the current preset (same autosave behavior as normal edits).

– MAP CC assignments are stored per preset and restore on boot/load with that preset.

– For stepped parameters, incoming CC is thresholded to target steps to avoid jitter.

– Display jumps to the changed parameter screen after movement settles, to avoid over-refreshing while CC is moving quickly.

14. `IN CC >`

– Options: `CH 1..CH 16`, `ALL3`.

– Routes your knobs. Destination for input-channel `CC`, `Pitch Bend`, `Program Change`, and `Channel Aftertouch`.

– `ALL3` fans these messages to arp/thru/bass channel outputs (de-duplicated).

15. `LEGATO` (A sampler can simulate the keyboard response of an analog synth)

– Options: `OFF`, `CH 1..CH 16`.

– Applies to note messages on the selected non-`INPUT CH` lane.

– Behavior: monophonic last-note priority with explicit handoff:

  – newest pressed note becomes active output

  – releasing that note re-activates the previous still-held note

  – when no held notes remain, output note is released

16. `REMOTE`

– Options: `CH 1..CH 16`.

– Output channel used by both foot pedal remote actions (`REMOTE 1` and `REMOTE 2`).

17. `REMOTE 1`

– Action sent when pedal 1 goes low.

– Value mapping:

  – `0..127` => note number pulse

  – `128..254` => CC `1..127` pulse

– Pulse length uses project pedal pulse timing.

18. `REMOTE 2`

– Same action model as `REMOTE 1`, for pedal 2.

19. `USB HOST`

– Options: `OFF`, `IN ONLY`, `IN/OUT`.

– `IN/OUT` enables USB host TX fanout as well as input.

20. `SCRNSVR`

– Options: `OFF`, `AUTO`, `NOW`.

– `AUTO`: enters after idle timeout.

– `NOW`: immediate manual activation (not persisted as always-on).

– Redraws procedurally and refreshes on interval.

21. `EYE/PUSH`

– Options: `CH 1..CH 16`.

– Shared output channel for both `EYE MODE` and `PUSH` generated notes/cc/pitch/loop messages.

22. `EYE Distance Sensor`

– Modes:

  – `OFF`

  – `DIV +2`, `DIV -2`, `DIV +3`, `DIV -3`, `DIV FULL`, `DIV3`

  – `VEL DOWN`, `LEN DOWN`

  – `ARP LATCH`, `ARP LATCH+`

  – `ARP FREEZE`, `ARP FREEZ+`

  – `PITCH UP`, `PITCH DOWN`

  – `NOTES C0..NOTES C7` (2-oct note ranges)

  – `CC 1..CC 19`

  – `CC103`

  – `Loop Rec/ Play/Over`

  – `Loop Stop/ Delete`

– `DIV3` profile: far zone most-slowing, middle less-slowing, close small zone speed-up.

– `ARP LATCH`: latch arp notes; wave over sensor to clear latch phrase.

– `ARP LATCH+`: latch arp plus thru/bass behavior.

– `ARP FREEZE`: wave over sensor to lock notes into frozen arp set.

– `ARP FREEZ+`: freeze mode with thru path freeze support as well.

– `NOTES Cx`: quantized note output with edge padding and range trims.

– `CC103`: sends a CC 103 pulse.

– `Loop Rec/ Play/Over`: if no loop exists, arms loop recording; recording starts on the first note played after the trigger. If a loop exists and is stopped, it starts playback immediately. If a loop is already playing, it toggles overdub on/off. Holding this trigger for more than 2 seconds cancels and deletes the loop.

– `Loop Stop/ Delete`: first trigger stops loop recording/playback; the next distinct trigger deletes the stored loop.

23. `PUSH Pressure Sensor`

– Same mode list and behavior family as `EYE MODE`.

– Uses calibrated thresholds and curved response so light press is less aggressive.

– Shares channel with `EYE/PUSH` setting.

24. `LOOP`

– Options: `1 BAR`, `2 BAR`, `4 BAR`, `8 BAR`, `FREE`.

– Sets the note-only loop length used by `Loop Rec/ Play/Over`.

– Change the length in real time to record a longer loop over a repeating shorter one.

– Fixed bar loops start playback automatically after the selected length is recorded.

– `FREE` records until `Loop Stop/ Delete` stops it.

– While editing this screen, pressing the push sensor toggles `Auto Overdub` on/off.

– Loop status icon in the upper-right: open circle = armed/recording/overdub, open play triangle = playing, pause bars = loop saved and stopped.

25. `KEY` (WARNING while on key/guitar screens like this, rapid incoming notes will slow the arp due to refreshing the display)

– Options:

  – `OFF`

  – standard key roots: `C..B` (an off-key note will trigger the next on-key one above it)

  – alternate white-key mapper roots: `CKEY C..CKEY B` (these map the white keys to each scale note instead)

26. `SCALE`

– Options:

  – `OFF`, `MAJOR`, `MINOR`, `MAJ+MIN`, `BLUES`, `MAJ BLUES`, `BLUES+BOTH`, `HARM MIN`, `MEL MIN`

– In `CKEY` mode, combo scales are skipped (`MAJ+MIN`, `BLUES+BOTH`).

27. `GIT/KEYS`

– Options: `GUITAR`, `PIANO`.

– Chooses visualization style for key/scale pages.

– Saved in preset and restored on boot/load.

– LIVE NOTES DISPLAY in real-time so you can see where to play along.

28. `LOAD`

– Select preset slot `1..16` in edit mode.

– Preset is actually loaded when you click back to select mode.

– Grid view shows slot location.

– NOTE all changed settings will AUTO SAVE to the slot you last loaded from.

29. `SAVE`

– Select destination slot `1..16` in edit mode.

– Slot is written when you click back to select mode.

– Grid view shows slot location.

– The next changed settings will AUTO SAVE to this slot.

30. `PANIC`

**WARNING if you stay on this screen, rapid incoming notes will slow the arp due to refreshing the display**

  – device counts

  – queue depth and drop counters

  – overload risk flag

  – last incoming channel/note and on/off state

  – RX age timing

Stay tuned and subscribe to my socials for ongoing developments!