Serial communication protocols

Summary¶

A drone uses four serial communication protocols, each matched to its use case: UART for point-to-point asynchronous data (GPS, RC receiver, VTX, payload), SPI for high-speed synchronous sensor reads (IMU gyroscope), I2C for multi-device low-bandwidth sensor buses (payload I2C chain), and MAVLink for structured bidirectional mission and telemetry data (ArduPilot on the Bandit platform). Understanding which protocol is appropriate for a new connection — and why — prevents common integration errors like using I2C for a high-bandwidth sensor or UART for a multi-device bus.

Concept¶

UART — Universal Asynchronous Receiver-Transmitter¶

UART is asynchronous: the transmitter and receiver agree on baud rate in advance, then exchange data without a shared clock signal. A start bit signals the beginning of each byte; a stop bit signals the end. The receiver synchronises its clock to the start bit.

This clock-free design makes UART simple and universally supported. Every microcontroller ever manufactured has at least one UART. Point-to-point — one transmitter, one receiver per wire pair. Not suitable for multi-device buses.

Uses on libdrone: GPS (UART2, 57,600 baud), ELRS receiver (UART3, 420,000 baud), VTX OSD (UART1, 115,200 baud), payload UART4 and UART5.

SPI — Serial Peripheral Interface¶

SPI is synchronous: a shared clock (SCK) times each bit. The master drives SCK, MOSI (Master Out Slave In), and chip-select (CS); the slave drives MISO (Master In Slave Out). All four signals run simultaneously — full duplex.

The shared clock eliminates baud-rate negotiation and allows much higher speeds than UART: 10–50 MHz is common. At 10 MHz, a 16-byte sensor read takes ~13 µs — fast enough for 32 kHz gyro sampling.

Uses on libdrone: ICM-42688-P IMU gyroscope (SPI, 10 MHz). Not used for payload connections — connector pin count is limited.

I2C — Inter-Integrated Circuit¶

I2C uses just two wires (SDA + SCL) shared by multiple devices. Each device has a 7-bit address. The master initiates all transactions by sending an address; only the addressed device responds. Open-drain outputs allow multiple devices without bus contention.

Speed: 100 kHz standard, 400 kHz fast mode (libdrone payload standard), 1 MHz fast-plus. Slower than SPI, but adequate for sensors that update at 1–100 Hz. Multi-device capability allows many sensors on one bus with simple two-wire wiring.

Uses on libdrone: payload I2C bus (Connector A PIN 5/6), Sensirion SEN66 at address 0x6B.

MAVLink¶

MAVLink (Micro Air Vehicle Link) is a message-based protocol designed for flight controller to ground control station communication. Each message has a system ID, component ID, message type, and payload. MAVLink runs over UART (or UDP/TCP for ground station links) and supports hundreds of defined message types covering: arm/disarm, navigation waypoints, mode changes, sensor telemetry streaming, parameter read/write, and mission management.

Uses on libdrone: ArduPilot on the Bandit platform uses MAVLink on UART6 for the companion Pi Zero 2W (LCM-1). Betaflight on Core/Pro uses MSP (a simpler FC-specific protocol) rather than MAVLink. Not used in standard Core/Pro configuration.

Reference¶

Protocol selection guide¶

Protocol parameters used on libdrone¶

Procedure¶

Rationale¶

Why the IMU uses SPI instead of I2C¶

The ICM-42688-P gyroscope samples at 6400 Hz in libdrone's configuration — a new 16-byte reading every 156 µs. At I2C 400 kHz fast mode, transmitting 16 bytes plus protocol overhead takes approximately 400 µs — more than twice the 156 µs sample interval. I2C cannot keep up with the gyro's sample rate. At SPI 10 MHz, the same read takes ~13 µs — 12 times faster than the sample interval. SPI is the only reasonable choice for high-rate gyro data.

Connections¶

requires: [] related: - crsf-protocol - payload-software-protocol - flight-controller-hardware leads_to: - crsf-protocol - payload-software-protocol