STM32F411 6-DOF Robot Arm Controller

Firmware for a 6-degree-of-freedom robot arm based on an STM32F411CEU6 (Black Pill). The controller is designed to operate four Dynamixel joints over a half-duplex serial bus and two conventional PWM servos for the end effector.

The project uses STM32Cube HAL with PlatformIO and is uploaded/debugged through ST-Link.

System overview

• 4 × Dynamixel joints — controlled through USART2 using Dynamixel Protocol 1.0
• 1 × wrist/gripper rotation servo — PWM output on PA8
• 1 × gripper servo — PWM output on PA6
• USB-to-TTL data logger — connected through USART1
• ESP32-C3 Super Mini — planned wireless interface through USART6
• 74LS241 buffer — converts the Dynamixel UART connection into a controlled half-duplex bus

Hardware

• STM32F411CEU6 Black Pill development board
• Dynamixel AX-series actuators (the driver currently targets the AX-12A control table)
• 74LS241 tri-state buffer for the Dynamixel half-duplex bus
• Two PWM-compatible hobby servos
• USB-to-TTL serial adapter
• ST-Link programmer/debugger
• ESP32-C3 Super Mini (planned)
• Suitable external power supplies for the Dynamixel and hobby servos

Controller PCB

The repository includes the editable KiCad source for the custom controller PCB. The board combines the STM32F411 Black Pill, ESP32-C3 Super Mini, 74LS241 Dynamixel interface, power input, actuator connectors, and development headers on one carrier board.

Open pcb/6dof.kicad_pro in KiCad 9.0 or newer to view or modify the complete project.

Schematic

Editable source: pcb/6dof.kicad_sch

PCB layout

Editable source: pcb/6dof.kicad_pcb

3D preview

The KiCad files are active design sources. Review the schematic, complete the design-rule checks, and verify all footprints and routing before ordering a PCB. Gerber files, drill files, and a bill of materials are not included yet.

Pin assignment

Function	STM32 peripheral	Pin	Direction	Notes
Dynamixel TX	USART2_TX	PA2	Output	1 Mbps, 8-N-1
Dynamixel RX	USART2_RX	PA3	Input	Receives Dynamixel status packets
74LS241 direction	GPIO	PB0	Output	High = transmit, low = receive/bus released
Data logger TX	USART1_TX	PA9	Output	Connect to USB-to-TTL RX
Data logger RX	USART1_RX	PA10	Input	Connect to USB-to-TTL TX
ESP32-C3 TX	USART6_TX	PA11	Output	Planned wireless link; connect to ESP32 RX
ESP32-C3 RX	USART6_RX	PA12	Input	Planned wireless link; connect to ESP32 TX
Gripper rotation servo	TIM1_CH1	PA8	PWM output	Rotates the gripper
Gripper servo	TIM3_CH1	PA6	PWM output	Opens and closes the gripper
Built-in status LED	GPIO	PC13	Output	Active-low on the Black Pill

The USART1 pin assignment above follows the target hardware pinout. The generated firmware currently maps USART1 to PB6/PB7; update the alternate-function configuration to PA9/PA10 before wiring the logger to those target pins.

Dynamixel bus

Dynamixel Protocol 1.0 uses a single bidirectional data wire. USART2 provides separate TX and RX signals, so the 74LS241 is used to control when the STM32 drives the bus.

STM32 PA2 (USART2_TX) ──> 74LS241 input
STM32 PB0 (DIR)       ──> 74LS241 output-enable/direction control
74LS241 bus output    ──> Dynamixel DATA
STM32 PA3 (USART2_RX) <── Dynamixel DATA / receive path

STM32 GND ─────────────── Dynamixel interface GND

The firmware sets PB0 high while transmitting and returns it low before waiting for a servo response. USART2 is configured for 1,000,000 baud, 8 data bits, no parity, and 1 stop bit.

Do not power the actuators from the STM32 board. Use correctly rated external supplies and connect the grounds of the STM32, servo supplies, Dynamixel bus, USB-to-TTL adapter, and ESP32 interface.

Joint IDs and calibration

The earlier arm configuration stored in this repository uses the following IDs and zero offsets:

Joint	Dynamixel ID	Home position	Approx. offset	Positive direction
Base	1	493	144.57°	Counterclockwise
Shoulder	9	308	90.32°	Counterclockwise
Elbow	13	481	141.35°	Counterclockwise

The current test program in src/main.c controls Dynamixel ID 4, moving it between raw positions 350 and 650. Confirm every actuator ID and mechanical zero before enabling torque on the assembled arm.

For AX-series position control, raw values 0..1023 represent approximately 0..300°:

angle = raw_position × 300 / 1023

Firmware status

Currently implemented:

• USART2 Dynamixel Protocol 1.0 communication at 1 Mbps
• Half-duplex direction control through PB0
• Ping, 8/16-bit read and write, torque, LED, goal-position, speed, and present-position commands
• Packet validation for ID, length, checksum, timeout, and servo-reported errors
• TIM1 channel 1 configuration for PA8
• TIM3 channel 1 configuration for PA6
• USART1 configuration at 115200 baud

Still to be integrated into the active application:

• Initialize USART1 and move its GPIO mapping from PB6/PB7 to PA9/PA10
• Add and initialize USART6 on PA11/PA12
• Initialize TIM1 and TIM3 and start both PWM channels
• Add coordinated control for all six axes
• Define the command protocol between the STM32 and ESP32-C3
• Add data-logger output to the active control loop

At present, main() initializes only GPIO and USART2. The other peripheral configuration files exist but are not yet called by the active program.

Building and uploading

Requirements

• Visual Studio Code with the PlatformIO extension, or PlatformIO Core
• ST-Link programmer connected to SWDIO, SWCLK, GND, and 3.3 V reference

PlatformIO commands

Build the firmware:

pio run

Upload through ST-Link:

pio run --target upload

Open the serial monitor for logger output once USART1 logging is enabled:

pio device monitor --baud 115200

The PlatformIO environment is genericSTM32F411CE, using the STM32Cube framework and ST-Link for upload and debugging.

Project structure

.
├── include/
│   ├── dynamixel.h       # Dynamixel driver API and control-table addresses
│   ├── gpio.h
│   ├── main.h
│   ├── tim.h
│   └── usart.h
├── docs/
│   ├── 3dpcb.png         # Render of the assembled controller PCB
│   ├── pcb_footprint.png # PCB routing and footprint preview
│   ├── schematic.png     # Controller schematic preview
│   └── stm32pinout.png   # STM32 peripheral pin assignment
├── pcb/
│   ├── 6dof.kicad_pcb    # Editable PCB layout
│   ├── 6dof.kicad_pro    # KiCad project settings
│   └── 6dof.kicad_sch    # Editable schematic
├── src/
│   ├── dynamixel.c       # Protocol 1.0 packet transport and commands
│   ├── gpio.c            # LED and 74LS241 direction GPIO setup
│   ├── main.c            # Active test application
│   ├── tim.c             # PA8 and PA6 PWM timer configuration
│   └── usart.c           # USART1 and USART2 configuration
├── main_lama/            # Earlier application code kept for reference
└── platformio.ini        # PlatformIO target and build configuration

Safety notes

• Test one actuator at a time before operating the complete arm.
• Verify Dynamixel IDs and baud rates before enabling torque.
• Keep the arm clear of people and objects during first motion tests.
• Use a current-limited actuator supply and provide an accessible emergency power cutoff.
• Set conservative position and speed limits in software to prevent mechanical collisions.
• Never connect 5 V logic directly to a non-5-V-tolerant STM32 input; verify the complete 74LS241 interface and supply arrangement before powering it.

License

This project is open-source software available under the MIT License.