Guitar Pedal Firmware — Ryan Williamson

Rust firmware for a Daisy Seed effects pedal, with the DSP in a no_std crate that still runs under cargo test on a laptop.

Personal effects-pedal firmware built on the Cleveland Music Co. Hothouse platform, which runs on the Electrosmith Daisy Seed — an STM32H750 Cortex-M7 handling mono audio at 96 kHz.

Two pedals share one platform library. Cassette LoFi Junky does Gen-Loss style tape degradation. Driftwood is a dual-engine ambient machine with independent time and space sections. Earlier C++ builds remain in the repository as bench references.

DSP architecture

Embedded DSP is awkward to work on because the interesting code is the hardest to reach. A wrong coefficient sounds wrong, and the debugging loop is flash, listen, guess, repeat.

So the maths lives in its own crate that is no_std on the target and an ordinary Rust library on a development machine. Bucket-brigade delay emulation, PT2399-style reverb, saturation curves, pitch shifting, wow and flutter — all of it is pure functions over buffers, with no allocator, no hardware types, and no reference to the board. That crate has its own unit tests and they run in milliseconds on a laptop.

What stays on the device is the part that genuinely needs to be there: the control-surface HAL reading six knobs, three toggles and two footswitches, the audio callback, and the DMA plumbing.

Separating the DSP from the hardware layer shortens the development loop and makes the algorithms testable before flashing the board.

Constraints that shape the code

There is no allocator, and the 96 kHz audio callback has a fixed real-time budget. Expensive DSP operations still need choosing carefully because missing that deadline produces an audible glitch rather than a slow response, so transcendentals and interpolation use approximations tested against their exact forms.

Knob behaviour needed as much attention as the DSP. A linear taper across a 50 ms to 1600 ms delay range puts every musically useful value in the first eighth of the sweep, so the tapers are shaped to spend travel where a player actually listens.

It is a different discipline from the enterprise work. There is no observability, no rollback, and the only integration test is whether it sounds right through an amplifier.