USB relay firmware for STM32 and RP2040 boards that exposes a simple serial command interface over USB CDC-ACM.

• Control up to 8 relay outputs from a host computer over USB serial.
• Turn relays on/off, toggle, pulse for a duration, or set all channels with a bitmask.
• Identify devices by unique MCU-based serial number.
• Reboot directly into on-chip USB bootloader mode from software.

This firmware currently targets these board families (default TinyUSB board in parentheses):

• STM32F0 (stm32f072disco)
• STM32F1 (stm32f103_bluepill)
• STM32F4 (stm32f411blackpill)
• RP2040 (raspberry_pi_pico)

Hardware behavior is family-port specific (GPIO mapping, relay polarity, UID base, DFU ROM address).

Connect each USB relay module input (IN1..IN8) to the corresponding MCU GPIO below, and connect grounds together.

• STM32F0 (stm32f072disco): IN1=PA0, IN2=PA1, IN3=PA2, IN4=PA3, IN5=PA4, IN6=PA5, IN7=PA6, IN8=PA7
• STM32F1 (stm32f103_bluepill): IN1=PB12, IN2=PB13, IN3=PB14, IN4=PB15, IN5=PB8, IN6=PB9, IN7=PB10, IN8=PB11 (active-low outputs)
• STM32F4 (stm32f411blackpill): IN1=PB0, IN2=PB1, IN3=PB2, IN4=PB10, IN5=PB3, IN6=PB4, IN7=PB5, IN8=PB7
• RP2040 (generic): IN1=GP2, IN2=GP3, IN3=GP4, IN4=GP5, IN5=GP6, IN6=GP7, IN7=GP8, IN8=GP9

1. Build firmware for your board family.
2. Flash with ST-Link/J-Link (STM32) or UF2/picotool (RP2040).
3. Connect over USB CDC serial from your host.
4. Send line-based commands and parse machine-readable responses.

After USB enumeration, open the device serial port (for example with picocom, minicom, or screen) and send one command per line.

Examples:

help
id
version
state
set 1 on
set 1 off
toggle 1
pulse 1 1000
pulse-on 1 250
pulse-off 1 250
setmask 0xff
all off
reboot-dfu

Pulse command behavior:

• pulse <relay> [duration-ms]: invert current state immediately, then restore previous state after duration.
• pulse-on <relay> [duration-ms]: force on immediately, then force off after duration.
• pulse-off <relay> [duration-ms]: force off immediately, then force on after duration.

Typical responses:

OK
ERR invalid-relay
ERR invalid-arg
STATE 0x03
ID 045101780587252555FFC660
VERSION usb-relay 0.1.0
UPTIME 12345

Notes:

• Relay indices are 1..N where N is board-configured (up to 8).
• Commands are line-oriented (\n or \r\n).
• On USB mount, firmware emits OK.
• Default pulse duration is 1000 ms when omitted.

• ARM GCC toolchain
• CMake 3.20+
• Python 3
• TinyUSB checkout at tinyusb (or vendored in firmware/tinyusb)

Run from the project root:

git clone https://github.com/hathach/tinyusb.git
python3 tinyusb/tools/get_deps.py stm32f0 stm32f1 stm32f4 rp2040

cmake -S firmware -B build-f0 -DBOARD_FAMILY=STM32F0
cmake --build build-f0

cmake -S firmware -B build-f1 -DBOARD_FAMILY=STM32F1
cmake --build build-f1

cmake -S firmware -B build-f4 -DBOARD_FAMILY=STM32F4
cmake --build build-f4

cmake -S firmware -B build-rp2040 -DBOARD_FAMILY=RP2040
cmake --build build-rp2040

Output artifacts include .elf, .bin, .hex in the selected build directory.

Use TinyUSB family flash targets after configure/build.

cmake --build build-f0 --target usb-relay-stlink
cmake --build build-f1 --target usb-relay-stlink
cmake --build build-f4 --target usb-relay-stlink

If you prefer flashing directly (without CMake flash targets), use STM32CubeProgrammer CLI. Linux CLI example:

# STM32F0 build output
STM32_Programmer_CLI --connect port=swd \
  --write build-f0/usb-relay.bin 0x08000000 \
  --verify --go

# STM32F1 build output
STM32_Programmer_CLI --connect port=swd \
  --write build-f1/usb-relay.bin 0x08000000 \
  --verify --go

# STM32F4 build output
STM32_Programmer_CLI --connect port=swd \
  --write build-f4/usb-relay.bin 0x08000000 \
  --verify --go

Optional alternative with st-flash (from stlink):

st-flash --reset write build-f0/usb-relay.bin 0x08000000
st-flash --reset write build-f1/usb-relay.bin 0x08000000
st-flash --reset write build-f4/usb-relay.bin 0x08000000

cmake --build build-f0 --target usb-relay-jlink
cmake --build build-f1 --target usb-relay-jlink
cmake --build build-f4 --target usb-relay-jlink

RP2040 boards expose a ROM USB mass-storage bootloader (RPI-RP2) in BOOTSEL mode.

Enter BOOTSEL mode:

1. Hold BOOTSEL.
2. Plug in USB (or press reset while holding BOOTSEL).
3. Release BOOTSEL after the board appears as RPI-RP2.

Flash by copying UF2 from the shell:

# Optional: verify the RP2040 boot volume is present
lsblk -o NAME,LABEL,MOUNTPOINT | grep RPI-RP2

# Copy firmware UF2 to the mounted RP2040 drive
cp build-rp2040/usb-relay.uf2 /media/$USER/RPI-RP2/
sync

Optional picotool workflow (Linux CLI):

# If the board is already running this firmware, ask it to reboot to USB bootloader:
picotool reboot -u

# Program flash directly, then reboot
picotool load -f build-rp2040/usb-relay.elf
picotool reboot

If the board is already running this firmware, you can enter bootloader mode over USB serial:

# Replace with your CDC device (for example /dev/ttyACM0)
printf "reboot-dfu\n" > /dev/ttyACM0

After reboot-dfu, the MCU jumps to STM32 ROM DFU bootloader. Flash with dfu-util:

# STM32F0
dfu-util -a 0 -s 0x08000000:leave -D build-f0/usb-relay.bin

# STM32F1
dfu-util -a 0 -s 0x08000000:leave -D build-f1/usb-relay.bin

# STM32F4
dfu-util -a 0 -s 0x08000000:leave -D build-f4/usb-relay.bin

After reboot-dfu, RP2040 enters USB boot mode (RPI-RP2). Flash either UF2 or with picotool:

# UF2 copy method
cp build-rp2040/usb-relay.uf2 /media/$USER/RPI-RP2/
sync

# Optional picotool method
picotool load -f build-rp2040/usb-relay.elf
picotool reboot

This repo includes a udev rule for VID:PID cafe:4010.

Install:

./udev/install-udev-rules.sh

If you change USB VID/PID in firmware/src/usb_descriptors.c, update udev/99-usb-relay.rules accordingly.

• TinyUSB device stack with CDC-ACM transport.
• Line-based command parser with machine-readable status/error responses.
• USB serial descriptor string is generated from board unique ID (STM32 96-bit UID; RP2040 board ID zero-padded to 12 bytes).
• Relay ports are split by family:
  • firmware/boards/ports/port_stm32f0.c
  • firmware/boards/ports/port_stm32f1.c
  • firmware/boards/ports/port_stm32f4.c
  • firmware/boards/ports/port_rp2040.c
• reboot-dfu jumps to STM32 system-memory DFU ROM on STM32, and enters USB boot mode on RP2040.