Teensy Flasher

cross-platform CLI tool for flashing firmware onto Teensy 4.0/4.1 microcontrollers via USB

usage

# flash firmware (device must be in bootloader mode)
flasher flash firmware.hex

# wait for device to appear, then flash
flasher flash -w firmware.hex

# wait with custom timeout (default 30s)
flasher flash -w -t 60 firmware.hex

# list devices in bootloader mode (prints serial numbers)
flasher list

exit codes: 0 success, 1 error

build

cargo build --release -p flasher

platform setup

Linux

echo 'SUBSYSTEMS=="usb", ATTRS{idVendor}=="16c0", MODE="0666"' | sudo tee /etc/udev/rules.d/99-teensy.rules
sudo udevadm control --reload-rules && sudo udevadm trigger

macOS

no setup required

Windows

no setup required for HID devices

entering bootloader mode

1. press the button on the Teensy board
2. LED should blink slowly
3. run flasher list to confirm detection

technical details

uses the HalfKay bootloader protocol over USB HID (hidapi):

• packet: 1 byte report ID + 64 byte header + 1024 byte data = 1089 bytes
• header: 24-bit LE address in bytes 0-2, rest zero
• boot: address bytes set to 0xFF, rest zero
• first block triggers chip erase (45s timeout)
• subsequent blocks: 500ms timeout
• automatic retry with device reopen on broken pipe

development

cargo test -p flasher

the vendored blink-test.hex blinks the LED in a 3-fast-blink pattern. source: test-firmware/src/main.cpp.