Adafruit Neotrellis 8x8 Feather M4 Kit running Sequencer Firmware

Project: SAMD51 MIDI Sequencer & Clock Generator Firmware

for Adafruit Neotrellis 8×8 Feather M4 Express Kit

It’s been a while since I last delved into the Neotrellis code. Back when I purchased it, I was annoyed that the sketches for the 8×4 TrellisM4 weren’t easily adaptable. I’d bought the TrellisM4 first, played with it’s sketches, and decided I was capable of expanding them out to work with the Neotrellis 8×8, but design changes got in the way and code wasn’t cross compatible.

Instead of writing the standalone sequencer I wanted, I ended up writing an interface for Trowasoft’s MultiSeq module in VCV rack, which inevitably involved chaining both the TrellisM4 and the Neotrellis 8×8 together.

It worked, but it wasn’t what I _wanted_ when I set out on the original mission.

FINALLY, after 18 months, I’ve found a gap from the usual stresses to sit and digest things, and wrapped my head around it. Thanks to todbot’s PicoStepSeq code – an 8 step sequencer for the RP2040 – and a sun-thru-the-clouds moment, I’ve managed to throw together a firmware that runs as an 8 track 32 step MIDI (over USB) gate/trigger sequencer and MIDI Clock Generator (with swing) with a few other bells and whistles 😉

You can grab the Arduino IDE C++ source for yourself and throw it up onto the Neotrellis Feather M4 Express from my Github repo at https://github.com/PatchworkBoy/Neotrellis-Gate-Sequencer/

The Treloquencer

A Neotrellis MIDI & Analogue CV/Gate Sequencer & Arpeggiator

An 8 track 32 step MIDI (over USB & Serial) & Analog note/modulation/gate/trigger sequencer with multi-mode arpeggiators available on 4 tracks, 2 tracks of Analog CV (control voltage) Output and MIDI Clock Generator (with swing) for Feather M4 Express / Neotrellis 8×8, featuring per-step per-track per-pattern note & velocity & probability & gatelength & per-track clock division layers, performance mutes and per-track loop-length (both start and endpoint) control.
16 storable preset patterns per track (all layers stored). Customisable note-per-track (Trigger/Gate mode) and channel-per-track.

Tracks 5 thru 8 can be assigned as arpeggiators for live arpeggiation of incoming MIDI notes / chords.

3 octave CV (v/oct, switchable to 2 octave Hz/V) Output for track 7 & 8 on pins A0 & A1 when tracks in CC or NOTE. Track 1-8 Gates always output on digital io pins D4/5/6/9/10/11/12/13 (sending a 0-3.2v trigger/gate).

Analog CV/Gate outputs are NOT regulated or protected in any way. Whack a 1k resistor between pin and 3.5mm TRS socket tip. Analog output is merely proof of concept. There’s something squonky going on with the Feather M4’s DACs (when used with my Neutron and K2) where they cannot hold an output voltage for long unless retriggered. Keep Release of your gates short, else you’ll hear drift-down to 0v oddities. Could just be the current load is too high from the outboard gear.

INSTALLATION

If, after uploading, your Neotrellis appears blank, it’s because the Feather M4’s QSPI Flash isn’t formatted. Quickest way to do this is double tap the upload button so FEATHERBOOT shows up on your desktop, grab latest circuitpython UF2 and throw that onto it and wait for board to reboot, then kick it back into upload mode with another button double tap & re-upload the sequencer firmware via Arduino IDE. (CircuitPython install formats the flash by coincidence much quicker than doing the SDFat example method)

Designed for use with the Adafruit 8×8 Neotrellis Feather M4 Kit, no additional hardware required (unless you want serial midi / analog outs) –

NeoTrellis Surface

Row 1 – 4: Steps 1 thru 32 (pattern edit: on/off, velocity edit: cycle thru velocity 40 / 80 / 127, Length: select end step, Probability: 10% – 100% in 10% increments)

  • Row 5: Track Select – Trk1 | Trk2 | Trk3 | Trk4 | Trk5 | Trk6 | Trk7 | Trk8
  • Row 6: Track Mutes – Trk1 | Trk2 | Trk3 | Trk4 | Trk5 | Trk6 | Trk7 | Trk8
  • Row 7: Pattern Edit | Velocity Edit | Probability Edit | Gate Length Edit | SHIFT (^) | Global Octave 0/+1/+2 (only while stopped) / ^ Channel Config (stopped) ? ^ Pattern Clock Division (running) | Loop-End / ^ Loop-Start | toggle step size – quarter / eighth / sixteenth / ^swing
  • Row 8: Toggle Play/Stop | Stop | Reset | SAVE | PRESETS / ^Factory Reset | MIDICLOCK Send On/Off | param – | param +

PARAM -/+

  • Pattern Edit – param = tempo (unless track is in ARP mode), step = on/off
  • Velocity Edit – param = all velocities (+/- 10), step = step velocity cycle (40/180/127)
  • Probability Edit – param = all probabilities (+/- 10%), step = step probability cycle (+10%)
  • Gate Length Edit – param = all gate lengths (+/- 10%), step = gate length cycle (+10%)
  • SHIFT – param = note (+/- 1)
  • Swing (SHIFT + StepSize) – param = +/- 1% (30% max)

PRESETS mode

  • Row 1 & 2 – change preset for selected track, 1-16
  • Row 3 & 4 – change ALL tracks to selected preset, 1-16
  • SAVE: store all patterns, velocity, probability & gate length maps, current step-size, track notes, track midi channels and tempo to flash. DO NOT power down whilst saving. Wait for button to cycle from Red back to Cyan.
  • FACTORY RESET (SHIFT + Presets): resets all patterns & velocity & probability & gate maps (both in memory & on disk (flash)) to default, step size to sixteenths, tempo to 120, transpose to 0. DO NOT power down whilst saving. Wait for button to cycle from Red back to Cyan.

CONFIG mode

  • Row 1 & 2 – set MIDI channel 1 to 16 for selected track
  • Row 4 – set selected tracks mode: Trigger/Gate, CC, NOTE, or ARP (buttons 1 – 4, ARP for trk 5 thru 8 only)
  • Row 4 – set v/oct (white) & hz/v (purple) when in NOTE or CC mode with button 8.

Analog gates are sent in all modes. Analog CV is sent only for track 7 & 8 when in CC or NOTE mode.

Track Modes (over MIDI)

  • Trigger/Gate – Outputs fixed MIDI Note for all steps, Velocity, Gate On/Off
  • CC – Outputs CC, Value, Gate On/Off
  • NOTE – Outputs per-step note, Velocity, Gate On/Off
  • ARP – available on tracks 5 thru 8 – Outputs per-step note, Velocity, Gate On/Off

For the currently selected track…

In Trigger/Gate mode, with SHIFT toggled on, incoming MIDI is realtime mapped to step on/off.

In CC or NOTE mode, Velocity pane allows step selection… and then:

  • In CC mode, param +/- changes CC Value, and incoming MIDI note is captured to selected step as value.
  • In NOTE mode, param +/- changes Note, MIDI Input is captured to selected step (both velocity and note). SHIFT = MIDI Input is listened to and notes/velocity captured to current playing step in realtime.
  • In ARP mode with SHIFT toggled on…held incoming midi notes (played live via external source) are arpeggiated in accordance with chosen pattern over chosen number of octaves.
  • the arp engine’s notestack can hold a maximum of 10 notes (cos 8 fingers, 2 thumbs). Once full, oldest note shuffles off the pile to make way for newest note.
    • On Pattern Edit view, shift on, param +/- cycles thru number of octaves (1-4)
    • On Pattern Edit view, shift off, param +/- cycles thru patterns (1-7) Arp Patterns:
    • 1 – Up – Notes are played from lowest to highest
    • 2 – Down – Notes are played from highest to lowest
    • 3 – Inclusive – Notes are played from lowest to highest, then highest to lowest; the bottom and top notes are played twice
    • 4 – Exclusive – Notes are played from lowest to highest, then highest to lowest; the bottom and top notes are played only once
    • 5 – Outside In – Notes are played lowest then highest, then second lowest and second highest, and so forth until they meet in the middle
    • 6 – Order – Notes are played in the order that they come in
    • 7 – Random – Notes are played randomly
    • To LATCH the arpeggio, change selected track to a non-arp track then let go of the keys.
      Currently latched arpeggio will remain latched until you return to arpeggio track, put it in shift mode – it is now armed unlatch when you hit the next note / chord and start a new arpeggio. Each arpeggiator is clocked by the selected track’s sequencer pattern, and respects all the layers (velocity / probability / gatelength) & any clock division. Each arpeggiator is independent, and can have it’s own pattern and octave range.

MIDI Clock
Outputs optional self-generated MIDI Clock (24 PPQN), Play/Stop/Reset (ideal for use in VCV rack with MIDI > CV module)

  • Default BPM: 120, adjustable via param buttons in -/+ 1 increments. Swing (+/- 30% max) is also applied to clock output.
  • OR can be driven with a 24PPQN external midi clock (eg: VCV’s Impromptu Clocked at x24 to CV>MIDI module clock input)

Default Mapping for VCVRack MIDI > Gate module:

  • Trk1: Note C2 (36) – MIDI Ch1 (Analog gate on D4)
  • Trk2: Note C#2 (37) – MIDI Ch1 (Analog gate on D5)
  • Trk3: Note D2 (38) – MIDI Ch1 (Analog gate on D6)
  • Trk4: Note D#2 (39) – MIDI Ch1 (Analog gate on D9)
  • Trk5: Note E2 (40) – MIDI Ch2 (Analog gate on D10)
  • Trk6: Note F2 (41) – MIDI Ch3 (Analog gate on D11)
  • Trk7: Note F#2 (42) – MIDI Ch4 (Analog gate on D12, val as CV on A0)
  • Trk8: Note G2 (43) – MIDI Ch5 (Analog gate on D13, val as CV on A1)
  • Play: Note C0 (12) – MIDI Ch16
  • Stop: Note C#0 (13) – MIDI Ch16
  • Reset: Note D#0 (14) – MIDI Ch16

Each track is transposable to set custom note (CC number in CC mode). Each track can be set to any MIDI Channel.

Octave button globally shifts ALL track notes up +1ve or +2ve ON TOP of any existing per-track transposition.

FAR from perfect. Open to improvements – throw me a pull request.

TO DO:

  • CHORD mode
  • Song mode