8bit-quad

Firmware for a custom-built quadcopter based on the ATmega328 microcontroller.

The project implements the complete flight controller firmware, including attitude estimation, stabilization, PID control and radio communication. It was designed around a resource-constrained 8-bit MCU and uses a simple sequential execution model.

This revision (2022-04) is the last one that flew. It takes its commands from a standard PPM receiver, runs an MPU6050 with Mahony fusion, and flies in acro and stabilize (angle-hold) modes.

Earlier commits carry the nRF24L01+ revisions and the matching custom remote transmitter, which this one retires - see doc/changelog.md.

Features

• Attitude estimation (Mahony sensor fusion)
• Acro and stabilize (angle-hold) flight modes
• Cascaded PID control (outer angle loop, inner rate loop)
• Standard PPM receiver input, 8 channels
• Flight mode, lights and calibration selected from the transmitter's switches, with the potentiometers supplying values
• Accelerometer and gyro offset trimming from the air
• PID gains persisted to EEPROM
• Battery voltage monitoring with a low/warning/error status ladder

Hardware

Flight controller

• MCU: ATmega328
• Framework: Arduino (custom core, see hardware/)
• IMU: MPU6050 (MPU9255 also supported, selected at compile time)
• Receiver: any PPM receiver, signal on D2

Flight controller implementation

Single-threaded, fixed-period main loop (no RTOS/scheduler): a busy-wait keeps the loop cycling at a fixed period, subdivided into sub-cycles so that slower tasks (receiver read, battery read, indication LEDs, quaternion conversion, angle controller) each run on their own sub-cycle instead of every iteration.

• Sensor reading: IMU (gyro + accel) read every cycle, with zero-reads and I2C failures counted rather than ignored.
• Sensor fusion: Mahony filter (gyro + accel only, no magnetometer), producing a quaternion converted to pitch/roll. A complementary filter is available as a compile-time alternative.
• Attitude representation: Euler pitch/roll from the fusion output; yaw rate taken directly from the gyro.
• Stabilization: cascaded PID - an outer angle loop (pitch/roll) feeds setpoints to an inner rate loop (pitch/roll/yaw), which drives the motor mix. acro mode drives the rate loop directly from the sticks, skipping the angle loop.
• Motor mix: quad-X mix, clamped, written straight to the timer compare registers (OCR0A/B, OCR1A/B) - no ESC command protocol, just PWM duty cycle.
• Receiver: one PPM stream carries four sticks, four switches encoded across two channels, and two potentiometers. The switch combination selects flight mode and the configuration or calibration to apply.

Development environment

• Arduino IDE (or arduino-cli)
• Custom Arduino core, see hardware/ and the Building section below
• Libraries vendored inside the core, under hardware/8bit-quad/avr/libraries/ (MPU6050, MPU925x_I2C, UART_atmega328) - these are no longer distributed through Arduino's Library Manager, or carry local edits, so they're frozen here rather than assumed to be installed. PPMReader is vendored inside the sketch instead, since it is GPL-3.0.

Building

This uses a custom Arduino core, so it won't show up under a stock "Arduino Uno" board.

Boards Manager (recommended):

1. In Arduino IDE File > Preferences > Additional boards manager URLs, add: https://raw.githubusercontent.com/VasilKalchev/8bit-quad/main/package_8bit-quad_index.json
2. Install "8bit-quad AVR Boards" from Tools > Board > Boards Manager.

The package is the core with the vendored libraries inside it, so there is nothing else to install.

This revision needs core 1.1.0. Boards Manager offers more than one version; they carry different sets of vendored libraries, and a core meant for another revision will not necessarily compile this one. The version is in hardware/8bit-quad/avr/platform.txt, which is the same line hardware/package.sh reads when it builds the archive.

arduino-cli can pin it for you. Each sketch carries a sketch.yaml naming this revision's core, so

arduino-cli compile --profile fc

fetches exactly that core and builds against it with nothing installed beforehand.

Manual:

The repo is already laid out as an Arduino sketchbook (hardware/8bit-quad), so point File > Preferences > Sketchbook location at this repo and restart. Nothing needs copying. The IDE only auto-detects hardware/ inside the sketchbook location, so having it next to the sketch isn't enough on its own.

Either way, an "8bit-quad" section appears under Tools > Board with 8bit-quad_fc and 8bit-quad_rc entries. Open fw/8bit-quad-fc/8bit-quad-fc.ino with File > Open, select the fc board, and build/upload as usual. The sketches live under fw/, so they won't appear in the Sketchbook menu.

To cut a new release of the core, run hardware/package.sh and put the checksum and size it prints into package_8bit-quad_index.json, then attach the archive to a hardware-v<version> release.

Repository

This repository was reconstructed retroactively from archived development snapshots that originally existed as standalone project folders.

The snapshots have been converted into Git commits representing the development timeline. Abandoned development paths have been preserved as Git branches.

The detailed reconstruction history and version changes are documented in:

• /doc/changelog.md
• /doc/history.md

Every revision is tagged with its date, fw-2018.05.04 through fw-2022.04.03. The firmware was never versioned while it was being written, so the dates are all there is. The Arduino core is versioned separately, as hardware-v1.0.0 and hardware-v1.1.0.

Gallery

The /gallery/ directory contains photos documenting the hardware and development process.

Videos:

• First flights (pre 2018)
• Etching the flight controller PCB
• Ground view while flying

Related repositories

• 8bit-quad.hw - the 8bit-quad-fc board design. The remote never had one, it was hand wired on protoboard.