Piano LED Visualizer — firmware

Bare-metal firmware that lights a WS2812B strip above an 88-key keybed in response to MIDI. Two platforms, one shared implementation:

Platform	Board	USB MIDI	Strip
Pico	RP2040 / RP2350	TinyUSB device	PIO
Circle	Pi Zero / Zero 2	CUSBMIDIGadget	SPI

All the visualizer logic lives in src/ and is shared verbatim. Each platform supplies two small things: an ILEDStrip implementation, and a main loop that feeds MIDI packets to CPianoLEDs::OnMIDIPacket().

The design rationale, bill of materials, electrical notes and project phases live in PIANO-LED-CIRCLE-PLAN.md. This file covers only how to build and run the firmware (plan Phase 1).

What this is

The board is a USB MIDI device. The PC is the host and owns everything else — the piano connection, the learning software, the song library. From the PC's side this firmware is just another ALSA MIDI output port:

Casio PX-S7000 --USB-B--> PC --USB--> board (this firmware) --> WS2812B strip

There is no network stack, no shell, and nothing writable at runtime. The board boots into this firmware in about a second and does one job.

On the Circle build the piano cannot be plugged into the board directly: Circle has no OTG support, so its USB controller is gadget-only and all MIDI must arrive from the PC. RP2040/RP2350 can do USB host, so that restriction is a Circle property rather than an architectural one — but nothing here uses it.

Layout

Path
src/config.h	Every tunable. Start here.
src/pianoleds.cpp	Note-to-LED mapping, colour, brightness clamps. Platform-independent.
src/ledstrip.h	ILEDStrip — the entire hardware surface the logic depends on.
pico/	RP2040 / RP2350 backend: TinyUSB MIDI + PIO WS2812.
firmware/	Circle backend: CUSBMIDIGadget + CWS28XXStripe.
tests/	Host-side tests. No toolchain or SDK needed.
circle/	Circle as a submodule, pinned to Step51.

Building — Pico (RP2040 / RP2350)

sudo apt-get install gcc-arm-none-eabi cmake
git clone --recursive https://github.com/raspberrypi/pico-sdk   # if you lack one
PICO_SDK_PATH=/path/to/pico-sdk ./pico/build.sh

Produces pico/build/pianoled.uf2. Hold BOOTSEL while plugging the board in and copy the .uf2 onto the drive that appears.

BOARD selects the target (pico, pico2, pico_w, pico2_w; default pico2). All four run this firmware unchanged — the radio variants simply leave their radio unused.

The strip data pin is WS2812_PIN in src/config.h, default GPIO2. PIO can drive it from any GPIO, so this is a free choice.

Building — Circle (Raspberry Pi Zero)

Circle is a submodule pinned to Step51, so clone recursively:

git clone --recursive <this repo>
# or, in an existing clone:
git submodule update --init

Then:

sudo apt-get install gcc-arm-none-eabi     # Debian/Ubuntu
./build.sh

The pin is deliberate. A pinned Circle tree still builds in five years; nothing here tracks a moving upstream.

This produces two images in boot/, which coexist on one card:

Image	RASPPI	Model
kernel.img	1	Pi Zero / Zero W (ARM1176)
kernel7.img	2	Pi Zero 2 / Zero 2 W (Cortex-A7)

The Pi picks the right one at boot, so the same SD card runs on either model.

SD card (Circle only)

The Pico boots from internal flash and needs none of this.

FAT32, single partition. Copy in:

• boot/kernel.img and boot/kernel7.img
• From the Raspberry Pi firmware repo boot/ directory: bootcode.bin, start.elf, fixup.dat
• cmdline.txt (optional). Circle reads kernel options from it; useful for raising the log level while bringing the board up.

Nothing is ever written to the card at runtime, so pulling the power mid-note cannot corrupt it.

Wiring

On Pico, the WS2812B waveform comes from a PIO state machine, so the data line is any GPIO you like — WS2812_PIN in src/config.h, default GPIO2. Both hardware SPI blocks stay free.

On Circle, verified against circle/addon/WS28XX: CWS28XXStripe clocks the waveform out over SPI at a fixed 6.4 MHz, encoding one LED bit per SPI byte. On SPI master device 0 that fixes the data line at MOSI = GPIO10 (BCM) = physical pin 19, and it occupies the only SPI master a Pi Zero exposes.

Three things from plan section 7 that are not optional:

• Level shifter. WS2812B wants logic high at 0.7 × VDD = 3.5V on a 5V rail; the Pi's GPIO is 3.3V. Put a 74AHCT125 on the data line. Skipping this is the single most common cause of "the strip flickers intermittently".
• Common ground. The Pi's ground and the LED supply's ground must be tied.
• Power injection. Feed 5V at both ends of the strip.

On a Pi Zero, connect the PC to the USB port, not PWR, with a data cable. On a Pico, the single USB connector is the one.

Configuring

Everything is in firmware/config.h, and every value there is a Phase 0 question. Two are load-bearing:

• STRIP_REVERSED — whether pixel 0 sits at the bass or treble end. Decide this after the strip is physically mounted; it is one flag to flip.
• GLOBAL_BRIGHTNESS and MAX_LIT_KEYS — the power clamps. 176 LEDs at full white would draw ~10.5A against a 6A supply. Real playing never comes close, but these two make a whited-out strip unreachable rather than merely unlikely. Do not raise them without redoing the arithmetic in plan section 7.

Any of them can also be overridden at build time without editing the file:

make -C firmware EXTRADEFINE=-DSTRIP_REVERSED=1          # Circle
cmake -B pico/build -S pico -DCMAKE_CXX_FLAGS=-DSTRIP_REVERSED=1   # Pico

Known limitation: the note-to-LED mapping is linear in semitone index, but a real keybed is not — 52 white keys span the same 1222mm, so one white key is ~3.38 LEDs rather than 2. This drifts within each octave, worst at F, by up to ~0.87 LEDs (~6mm) even after an optimal offset and scale. A geometric map derived from white-key positions would remove it. Not yet implemented.

MIDI behaviour

• Notes 21–108 (A0–C8) map to the strip; anything outside is dropped.
• Note On with velocity 0 is treated as Note Off.
• CC 120 (All Sound Off) and CC 123 (All Notes Off) clear the strip.
• Notes on HINT_MIDI_CHANNEL (default channel 16) light in a separate colour, for the plan's Phase 3 "light the next key to play". A key actually being played takes precedence over a hint on the same key.
• Notes held when the USB host suspends are cleared, so nothing stays lit.

Tests

./tests/run.sh

Compiles the real src/pianoleds.cpp against a capture backend that records pixels in memory, and exercises the mapping, the note-off paths, the range clamping and both power clamps across nine configuration variants.

Because the logic depends only on ILEDStrip, this needs no ARM toolchain, no Circle, and no pico-sdk — just g++. It checks the arithmetic, not the wiring, and does not replace bench-testing on real hardware.