SP-1 Remote

Firmware that turns a Teenage Engineering SP-1 stem player into a generic 4-fader / 4-button MIDI controller. It sends MIDI CCs out the SP-1's TRS sync jack, and also over USB MIDI, so the puck can drive hardware from the jack and a computer from the cable.

The shipped default profile targets the PopGoblin string synth on a Tiliqua, but nothing in the firmware is specific to it: the whole control surface is one table in firmware/src/profile.c.

Status

Working on hardware. Pop (the dev puck) is a functioning 4-fader, 4-button USB MIDI controller.

Phase
0	toolchain	done, proved by a byte-identical rebuild of the reference firmware
1	recovery drill	passed; TRS proven later with a plain cable, no multimeter needed
2	board bring-up	done and verified: LEDs, watchdog, power-off, charging, DFU escape hatch
3	transmit path	done; USB MIDI verified live on the Mac
4	faders	done and verified: all four drive CC 102/104/107/108 across 0-127
5	buttons	done and verified: freeze toggles cleanly, shimmer steps the synth's enum
6	LEDs	done
7	presets	done and verified: save, persist across a power cycle, recall

The whole signal chain is now proven on hardware, including TRS MIDI out, which needs only an ordinary 3.5 mm cable: data is on the tip (P0.20, Type A), so it reaches a Type A input like the Tiliqua's or a USB MIDI interface directly.

Plan: docs/superpowers/plans/2026-08-08-sp1-remote-implementation.md
Design: docs/spec.md
Measurements: docs/hardware-notes.md
Flashing and recovery: docs/flashing.md
Reviews: docs/codex-review-2026-08-08.md, docs/fable-review-2026-08-08.md
Plan: docs/superpowers/plans/2026-08-08-sp1-remote-implementation.md
Design: docs/spec.md
Reviews: docs/codex-review-2026-08-08.md, docs/fable-review-2026-08-08.md

Default profile

Control	MIDI	PopGoblin function
Fader 1	CC 102	filter cutoff
Fader 2	CC 104	reverb wet
Fader 3	CC 107	delay time
Fader 4	CC 108	delay feedback
Track button 1	CC 64	freeze, toggles on press
Track button 2	CC 105	shimmer, steps off / low / full / boost
Track button 3	CC burst	preset A: press to recall, hold 2 s to save
Track button 4	CC burst	preset B: press to recall, hold 2 s to save

To change the mapping, edit firmware/src/profile.c. Nothing else in the firmware hard-codes a CC number.

Running the tests

The decision logic is Zephyr-free C and runs on any machine:

make -C tests/host test

Building the firmware

Needs Zephyr v4.3.1 and Zephyr SDK 0.17.4; see docs/toolchain.md for the install and the environment.

The repo must live at a path without spaces (Zephyr's devicetree preprocessing splits on whitespace, and a symlink does not help). The canonical location is ~/dev/sp1-remote.

source ~/zephyrproject/.venv/bin/activate
export ZEPHYR_BASE=~/zephyrproject/zephyr
export ZEPHYR_SDK_INSTALL_DIR=~/zephyr-sdk-0.17.4
export ZEPHYR_TOOLCHAIN_VARIANT=zephyr
west build -p -b stem_player firmware -- -DBOARD_ROOT=$(pwd)

Flashing

Put the SP-1 in bootloader mode: power off, hold Track 1 + Track 4, plug in USB-C, release once the Track 1 LED lights. Then flash build/zephyr/zephyr.bin with the firmware utility at solderless.engineering, using Chrome or Edge.

Recovery

There is no reset pin on this hardware, so read this before flashing anything.

The bootloader is entered by the Track 1 + Track 4 hold above, from power off.
While this firmware is running, holding Track 1 + Track 4 for about 1.2 s resets straight into the bootloader. All four track LEDs light to confirm.
The firmware feeds a 4 second watchdog and uses SYSTEM_OFF as its only power-down path, so a hang reboots rather than bricking.

Credits

Built on the work of the SP-1 community: chattock and marcabisamra (sp1-tape-looper, the TRS MIDI transmitter and most of the board bring-up), timknapen (SP-1-dev, the pin map and wiki), ericlewis (sp1-midi, the board definition and USB MIDI), and the Solderless team (the browser flasher that makes any of this reversible).