DCLK MIDI Clock

DCLK — Dual MIDI Clock

.axs instruments · Firmware v1.6.3 · Hardware v1.0

DCLK is a dual MIDI clock built around an STM32F411 Black Pill. It sends MIDI clock to two TRS (Type A) outputs and over USB MIDI, with quantised start/stop and a separate clock divider for each output, so two machines can be dropped in and out of a jam on the bar.

Update the firmware

➜ DCLK Web Flasher & Editor

Flash the latest release straight from Chrome or Edge on a computer. No software to install. Your saved settings are kept. Flasher source: DCLK-Flasher.

1. Plug DCLK into the computer with a USB‑C data cable.
2. Put DCLK into update mode: click Enter update mode on the flasher, or choose Settings > UPDATE and push the encoder. The screen shows DFU READY. (Firmware 1.5 and later. On older firmware, or if DCLK won't start: hold BOOT0, tap NRST, keep holding BOOT0 for about a second. The screen and LEDs stay off in that mode.)
3. Click Flash Firmware and choose STM32 BOOTLOADER.

Windows needs a one-time WinUSB driver (Zadig); the flasher's Troubleshooting section walks through it.

Features

• Tempo from 30 to 300 BPM, set with the encoder (24 PPQN)
• Two TRS Type A MIDI outputs plus USB MIDI (class compliant, shows up as DCLK, no drivers)
• Quantised start: outputs wait for the next bar boundary of the wait length (1, 2, 4, 8 or 16 bars) before sending Start
• Optional quantised stop, landing on the same boundary
• Per-output clock divide: /1 /2 /3 /4 /6 /8
• USB MIDI clock follows either output
• Two 5 V analogue sync outputs (3.5 mm TRS): clock pulse on the tip (1, 2, 4, 8, 12 or 24 PPQN per output, default 24, after the channel's divider), run gate on the ring. SYNC 1 follows CH1, SYNC 2 follows CH2 (firmware 1.6 and later; PPQN setting from 1.6.1)
• Screen saver: dims while a clock runs, switches off when stopped, with a slow breathing BEAT LED in standby
• Capacitive touch buttons (CAP1206) for CH1, CH2 and Settings
• Settings saved to flash and kept across power cycles and firmware updates

Using DCLK

Clock, Start and Stop are MIDI System Real-Time messages, so they carry no MIDI channel: "CH1" and "CH2" mean the two output jacks.

LEDs

• CH1 / CH2: solid while running, slow fade while waiting to start, blink on the beat while waiting for a quantised stop, off when stopped.
• BEAT: flashes on every beat.

Main screen: a white CH1 / CH2 tab appears along the top while that output plays, and disappears when it stops (firmware 1.6.2 and later). Below them are the tempo and four beat bars; the current beat stands taller. The bars and the BEAT LED move while an output plays, and (from 1.6.3) while a USB host plays in HOST mode even with both outputs off.

Menus

Connections

Hardware

KiCad 9 project in build/DCLK-MIDI-Clock/, with JLCPCB-ready Gerbers, BOM and placement files in production/. Custom footprints are in lib/DCLK.pretty/.

Main parts: STM32F411CEU6 Black Pill, SSD1306 128×32 I²C OLED, CAP1206 touch controller, Alps EC11E encoder with switch, 2× CUI SJ1‑3525N TRS jacks, GCT DCJ200 DC jack, 3× 5 mm LEDs.

Building the firmware

The sketch is src/dclk_midi_clock/dclk_midi_clock.ino (Arduino IDE).

• Core: STM32duino with the custom board entry DCLK Dual MIDI Clock, which gives a clean 48 MHz USB clock from the Black Pill's 25 MHz crystal. The sketch warns at compile time if another board is selected.
• Board options: USB support None (TinyUSB provides USB) · U(S)ART support Enabled (generic 'Serial').
• Libraries: Adafruit GFX, Adafruit SSD1306, Adafruit TinyUSB.
• Upload method: STM32CubeProgrammer (DFU or SWD). The image must be linked at 0x08000000, with no HID or UF2 bootloader offset.

To publish a release, use Sketch > Export Compiled Binary, copy the .hex into the flasher repo's firmware/ folder and update its manifest.json.

Repository layout

src/dclk_midi_clock/   firmware sketch, compiled builds, font
legacy/                every firmware release (hex, bin, source)
build/DCLK-MIDI-Clock/ KiCad project and production files
lib/DCLK.pretty/       custom footprints
assets/                artwork (polarity marking)

License

MIT. See LICENSE. © 2026 .axs instruments