ModularBOT Firmware – STM32 Nucleo-F303RE

A modular robotics control platform featuring autonomous RC calibration, differential motor mixing, and FlySky i-BUS receiver support.

Quick Start

Prerequisites

• PlatformIO IDE or CLI (python -m pip install platformio)
• STM32 ST-Link debugger
• Nucleo-F303RE development board

Build

pio run

Upload

pio run -t upload

Monitor Serial Output

pio device monitor

Hardware Setup

Robot (STM32 Nucleo-F303RE)

Component	Connection	Note
ESP32 Receiver	PA9 (TX) / PA10 (RX)	UART @ 115200 baud
Motor 1 PWM	PA8, PA11 + PC7 (EN)	TIM1_CH1/CH4
Motor 2 PWM	PA6, PA7 + PB6 (EN)	TIM3_CH1/CH2
Motor 3 PWM	PB0, PB1 + PA5 (EN)	TIM3_CH3/CH4
Motor 4 PWM	PB4, PB5 + PC6 (EN)	TIM3_CH1/CH2
HuskyLens I2C	PB8 (SCL) / PB9 (SDA)	Reserved for future integration

Custom Radio Link (ESP-NOW)

Remote (LILYGO T-Display ESP32)

Component	GPIO	Note
Left Joystick (Y)	32	Throttle (ADC)
Right Joystick (X)	36	Steering (VP, ADC)
Button 1	17	Active LOW, pullup
Button 2	21	Active LOW, pullup
Button 3	22	Active LOW, pullup
Button 4	27	Active LOW, pullup
TFT Display	Built-in (ST7789)	Shows telemetry
USB-C Port	Built-in	Rechargeable via USB-C
Battery	JST 1.25mm	3.7V 1S LiPo with BMS

Receiver (LILYGO T-Display ESP32 (yes same as the remote))

Component	GPIO	Note
UART TX → STM32	17	Forwards control data
UART RX ← STM32	16	Optional feedback
Battery Sensor	34	ADC voltage divider

Firmware Behavior

1. Boot (~500 ms): Initialize Serial @ 115200, UART receiver.
2. Calibration (10 seconds): Collects RC channel min/max while motors stay stopped. Move joysticks to extremes on remote, then center.
3. Active (post-calibration):
   • Parse incoming UART packets from ESP32: < Throttle_H Throttle_L Steering_H Steering_L >
   • Map throttle + steering to motor PWM via calibrated ranges with deadzone.
   • Differential mixing: left motors = throttle + steering, right motors = throttle − steering.
   • Any communication timeout or invalid packet → all motors stop immediately.
   • Note - I plan to remove callibration after the final thing that ships...
4. Telemetry: ESP32 receiver reads battery voltage and transmits it back to the remote via ESP-NOW for display.

Project Structure

├── include/
│   └── pins.h              # Pin & protocol configuration
├── src/
│   ├── main.cpp            # Control loop & initialization
│   ├── motor_driver.cpp/h  # BTS7960 H-bridge abstraction
│   ├── rc_input.cpp/h      # i-BUS polling & frame handling
│   ├── calibration.cpp/h   # RC min/max/neutral tracking
├── docs/
│   └── architecture.md     # Design & module details
├── platformio.ini          # Build & dependency config
└── README.md               # This file

License

MIT

Author

Arnav – ModularBOT Project

Chassis Design

Here are the CAD designs for the main frame and a photo of the welded chassis so far.