Flash real hardware from the browser

When your project works in the simulator, you can put it on a real board without installing anything: Velxio flashes the compiled firmware over USB, straight from the browser.

Requirements

• A Chromium-based browser (Chrome or Edge). The flasher uses the browser’s Web Serial and WebUSB APIs, which Firefox and Safari do not ship. Pico-family boards still get a Download .uf2 button there (see below).
• A data-capable USB cable to your board.
• Close anything else using the port first (serial monitors, IDEs, picotool): the browser needs exclusive access.

Flashing

1. Right-click the board on the canvas and pick Flash to real board.
2. Click Connect & flash. The browser asks which USB device to grant; pick your board.
3. Velxio uses the build it already made for that board (the same binary the simulator was running). If the code changed since, it recompiles first and the compiler output streams into the dialog.
4. Watch the progress bar; when it finishes, the board reboots into your project.

The dialog picks the protocol for the target:

Family	How it is written	The board must be
ESP32, S3, C3, C6	esptool over the serial port, the merged .bin	plugged in; hold BOOT if it does not answer
Arduino Uno, Nano, Mega, ATtiny85	STK500 against the board’s bootloader, the .hex	plugged in (ATtiny85: through an Arduino running ArduinoISP)
Raspberry Pi Pico, Pico W, Pico 2, Pimoroni RP2040 / RP2350 boards	PICOBOOT over WebUSB, the .uf2 picotool built	in BOOTSEL mode (next section)

Pico-family boards: BOOTSEL first

An RP2040 or RP2350 is programmed by its bootloader, a separate USB personality the chip only shows in BOOTSEL mode. Two ways to get there:

• By hand: hold the BOOTSEL button while plugging the board in, then release it. The board mounts as a USB drive named RPI-RP2 (RP2040) or RP2350.
• From the dialog: the flash dialog for these boards has a Reboot into bootloader over USB button. It works when the board is running a sketch Velxio built (the Arduino core reboots on a 1200 baud open) or MicroPython (the REPL runs machine.bootloader()). The browser asks for the board’s serial port, the board drops off and comes back as the bootloader. Then click Connect & flash and pick the RP2 Boot / RP2350 Boot device.

Two clicks, two permission prompts: the serial port for the reboot and the USB device for the write. Once the board is in BOOTSEL, later flashes need only the second one.

Two revisions of the same board

Pimoroni sold the Stellar and Galactic Unicorn with a Pico W (RP2040) until January 2025 and with a Pico 2 W (RP2350) since. The simulator runs the current one; the flash dialog has a Real board revision select for these boards. Pick “Pico W aboard” for the older unit: the dialog builds a second image for that chip, flashes or downloads it, and the simulator keeps running its own build. The choice is remembered per board. The label on the back of the board (or the drive name in BOOTSEL, RPI-RP2 versus RP2350) tells you which one you have.

The dialog refuses an image that does not match the chip that answered (an RP2350 build on an RP2040, a RISC-V build on an ARM configuration) before anything is erased, verifies every byte after writing, and reboots the board into the program.

Windows and an RP2040: install WinUSB once

The RP2040 bootloader ships no Windows driver descriptor, so the browser cannot claim it until WinUSB is bound to it. One-time setup:

1. Put the board in BOOTSEL and plug it in.
2. Download and run Zadig.
3. Pick RP2 Boot (Interface 1) from the list (Options, List All Devices if it is hidden), select WinUSB as the driver and click Install Driver.

RP2350 boards (Pico 2, Pico 2 W, the Pimoroni “Pico 2 W Aboard” Unicorns, Badger 2350) need nothing: their bootloader carries the descriptor and Windows binds WinUSB by itself. macOS needs nothing on either chip.

Linux: a udev rule

Linux gives USB devices to root by default. Create /etc/udev/rules.d/99-velxio-rp2.rules with:

SUBSYSTEM=="usb", ATTRS{idVendor}=="2e8a", MODE="0666", TAG+="uaccess"

then sudo udevadm control --reload-rules && sudo udevadm trigger and re-plug the board. The serial port used for the reboot step needs the usual dialout group membership as well.

Any browser: download the .uf2, or copy it to the drive

The flash dialog for a Pico-family board always offers Download .uf2 (on Firefox and Safari, where the browser cannot flash, that is the whole dialog). Save the file, put the board in BOOTSEL and drop the file on the RPI-RP2 / RP2350 drive: the board reboots into your sketch the moment the copy ends.

On Chrome and Edge there is also Copy to the board’s drive: the browser asks you to pick the drive and writes the file there itself. No driver is involved, so it is the way to program an RP2040 on Windows without installing WinUSB. The dialog checks that the folder you picked is a BOOTSEL drive (it carries INFO_UF2.TXT) before writing anything.

MicroPython projects on a Pico

The dialog uploads the project’s .py files over the REPL and reboots into main.py. MicroPython itself has to be on the board first:

• Pico and Pico W: the dialog installs it. If no REPL answers, it asks you to put the board in BOOTSEL and click Retry; that click writes the same MicroPython build the simulator runs, and one more Retry uploads your files.
• Pimoroni RP2350 boards (Badger 2350, Pico Plus 2W): they ship with Pimoroni’s own MicroPython. If yours lost it, download the .uf2 from pimoroni-pico-rp2350 and drop it on the BOOTSEL drive once, then flash from the dialog.

Troubleshooting

• “No board in BOOTSEL mode was found”: the device picker was empty. Use the reboot button or hold BOOTSEL while plugging in, then connect again.
• “The board in BOOTSEL is an RP2040 but this project is built for RP2350”: an older Unicorn with a Pico W aboard. Pick “Pico W aboard” in the dialog’s Real board revision select and flash again.
• “Could not claim the USB device” on Windows with an RP2040: the Zadig step above. On Linux: the udev rule above.
• The serial reboot did nothing: a sketch built with the USB stack disabled cannot be rebooted over USB. Hold BOOTSEL while plugging in.

Simulate first, flash second

This closes the loop that makes Velxio useful for real work: iterate fast in the simulator (no cable, no wear on the hardware, instant resets), then flash the exact same build artifact when it behaves.