DAEDALUS — CanSat Avionics Firmware

Embedded flight software for a CanSat (can-sized satellite) running on an STM32H725RG microcontroller, built with PlatformIO and the Arduino framework. The system operates under FreeRTOS, with all sensor reads, control loops, telemetry, and data logging executed as concurrent tasks.

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Hardware

Component	Interface	Purpose
STM32H725RG (Cortex-M7)	—	Main MCU, 480 MHz, hardware FPU
ISM330DHCX (ISM6HG256X)	SPI	6-axis IMU (accel + gyro)
BMP581	SPI	Barometric altimeter #1
MS5611	I2C	Barometric altimeter #2
BNO086	I2C	AHRS / magnetometer (rotation vector)
u-blox M10S	I2C	GNSS (10 Hz, airborne-4g profile)
INA236	I2C	Battery voltage and current monitor
XBee (64-bit API mode)	UART	RF telemetry downlink / uplink
SD card	SDMMC (4-bit)	Flight data logger
Servo A	PWM	Payload deployment mechanism
Servo B	PWM	Instrument deployment mechanism
Dynamixel servos (x3)	UART (half-duplex, 1 Mbaud)	Paraglider spool / steering
NeoPixel x2	GPIO	Status LED
Camera trigger x2	GPIO	External camera control

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Firmware Architecture

RTOS Task Map

All concurrent work is implemented as FreeRTOS tasks created in UserThreads(). Each task runs in a hal::rtos::interval_loop at a fixed period.

Task	Priority	Interval	Description
CB_EvalFSM	Realtime	RA_INTERVAL_FSM_EVAL	Kalman predict + flight state machine
CB_INSDeploy	High	RA_INTERVAL_FSM_EVAL	Instrument deployment logic
CB_ReadIMU	High	RA_INTERVAL_IMU_READING	ISM330 accel/gyro → Kalman update
CB_ReadAltimeter	High	RA_INTERVAL_ALTIMETER_READING	BMP581 + MS5611 → Kalman update
CB_ReadMAG	AboveNormal	RA_INTERVAL_MAG_READING	BNO086 rotation vector + calibration
CB_ReadGNSS	AboveNormal	RA_INTERVAL_GNSS_READING	u-blox PVT → GPS Kalman update
CB_ReadINA	AboveNormal	RA_INTERVAL_INA_READING	INA236 battery voltage/current
CB_Control	Normal	RA_INTERVAL_Controlling	Guidance + paraglider servo PID
CB_ConstructData	Normal	RA_INTERVAL_CONSTRUCT	Build telemetry/SD strings
CB_SDLogger	Realtime+1	Dynamic (state-dependent)	Write CSV rows to SD card
CB_Transmit	Normal	RA_TX_INTERVAL_MS	XBee RF downlink
CB_ReceiveCommand	Normal	10 ms	XBee uplink command parser
CB_RetainDeployment	Normal	100 ms	Re-assert servo deployment positions
CB_EEPROMWrite	Low	RA_EEPROM_WRITE_INTERVAL	Persist flight state to backup SRAM
CB_NeoPixelBlink	Normal	20 ms	Rainbow status LED
CB_AutoZeroAlt	High	RA_INTERVAL_AUTOZERO	Ground altitude auto-zero
CB_DebugLogger	Normal	1000 ms	USB-CDC debug print (compile-time optional)

Shared-Resource Mutexes

Fine-grained mutexes prevent concurrent bus access without blocking unrelated tasks:

Mutex	Guards
mtx_spi	SPI1 bus (IMU, BMP581)
mtx_i2c	I2C4 bus (BNO086, GPS, INA236)
mtx_sdio	SDMMC1 peripheral
mtx_uart	XBee UART
mtx_cdc	USB-CDC serial
mtx_buf	tx_buf / sd_buf shared strings
mtx_nav	NavState struct (GPS + heading snapshot)
mtx_kf	All Kalman filters, alt_agl, apogee_raw

I2C hung-bus detection is built into the MAG, GNSS, and INA tasks: if a single read takes more than 40 ms, two consecutive timeouts trigger RecoverI2C4(), which bit-bangs 9 SCL pulses to release any slave stuck mid-byte.

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Flight State Machine (FSM)

States are evaluated every RA_INTERVAL_FSM_EVAL ms in EvalFSM(), called from CB_EvalFSM after the Kalman predict step.

STARTUP → IDLE_SAFE → LAUNCH_PAD → ASCENT → APOGEE → DESCENT
                                                          |
                                                   PROBE_RELEASE
                                                          |
                                                PAYLOAD_RELEASE → LANDED

State	Transition Condition
STARTUP	Immediate
IDLE_SAFE	Startup countdown (RA_STARTUP_COUNTDOWN)
LAUNCH_PAD	Armed + acceleration OR altitude threshold (sampler vote)
ASCENT	Filtered vertical velocity drops below RA_APOGEE_VEL (+ optional time guard)
APOGEE	Fixed 1.5 s delay
DESCENT	Fixed 1.5 s delay; computes main release altitude as 80% of apogee
PROBE_RELEASE	alt_agl falls below RA_MAIN_ALT_COMPENSATED; activates servo A
PAYLOAD_RELEASE	alt_agl falls below RA_LANDED_ALT; activates paraglider control
LANDED	Stops Dynamixels, waits 1 min for cameras, then buzzer ON

All threshold comparisons use a statistical vote (xcore::sampler_t) with a configurable over/under ratio to reject transient sensor noise.

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Kalman Filters

Four independent Kalman filters run predict in CB_EvalFSM and update in the sensor tasks:

Filter	States	Measurements
filter_acc	Net acceleration, jerk	Total body acceleration
filter_alt	Altitude AGL, vertical velocity	Barometric altitude
filter_nav_n	Latitude, north velocity, north accel	GPS latitude + N velocity
filter_nav_e	Longitude, east velocity, east accel	GPS longitude + E velocity

The GPS east filter corrects its state-transition matrix on every fix using cos(lat) so the longitude coupling stays accurate across latitudes.

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Guidance and Control

During PAYLOAD_RELEASE (paraglider descent), CB_Control runs the full guidance loop at RA_INTERVAL_Controlling:

1. Navigation snapshot — locks mtx_nav briefly to copy the latest NavState.
2. Yaw smoothing — sin/cos EMA (alpha = 0.15, ~300 ms time constant) removes IMU heading noise before passing to the guidance algorithm. Physical spool offset (SPOOL_PHYSICAL_OFFSET) is applied before smoothing.
3. Guidance — controller.guidance.update() computes target spool angles from current GPS position, target waypoint, altitude AGL, and velocity components.
4. Servo PID — controller.servo_pid_update() drives three Dynamixel servos via half-duplex UART at 1 Mbaud.

A built-in kinematic simulator (SimNav namespace) can replace real GPS/IMU data for ground testing without hardware changes, activated via an uplink command.

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Instrument Deployment (INS)

EvalINSDeploy() (called from CB_INSDeploy) independently decides when to release the instrument package using redundant sensors:

• Condition 1 — ToF sensor below RA_INS_TOF_THRESHOLD AND barometric AGL below RA_INS_NEAR_THRESHOLD (both sampler-voted).
• Condition 2 (backup) — Barometric AGL below RA_INS_CRIT_THRESHOLD (sampler-voted).

Either condition activates servo B and logs a dump of the last 50 altitude samples.

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Persistent State (EEPROM / Backup SRAM)

Flight state is saved to the STM32H725 Backup SRAM (0x38800000) via EEPROM_Write() at RA_EEPROM_WRITE_INTERVAL. The store is validated on each read by a magic number and CRC-8. On power-on or watchdog reset, EEPROM_Read() restores:

• FSM state
• Packet count
• UTC time string
• Servo positions (pos_a, pos_b)
• Altitude reference (alt_ref)
• Guidance tuning parameters (bearing EMA alpha, control EMA alpha, heading deadband)

This allows the flight to resume without losing context across brownouts or watchdog resets.

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Telemetry

Outbound packets are built by ConstructString() into tx_buf and transmitted over XBee every RA_TX_INTERVAL_MS ms. The SD card CSV header format is:

DDL, PACKET_COUNT, UTC, TIMESTAMP_EPOCH, MILLIS, STATE,
TEAM_ID, MISSION_TIME, MODE, ALTITUDE, TEMPERATURE, PRESSURE,
VOLTAGE, CURRENT, GYRO_R/P/Y, ACCEL_R/P/Y,
GPS_TIME, GPS_ALTITUDE, GPS_LATITUDE, GPS_LONGITUDE, GPS_SATS,
VELOCITY_E, VELOCITY_N, CMD_ECHO, HEADING, ROLL, PITCH, YAW,
TOF, DEPLOY, ALT_REF, APOGEE_RAW, POS_A, POS_B, CPU_TEMP,
SERVO_1/2/3_ANGLE, SERVO_1/2/3_TARGET

Inbound commands arrive via the XBee AP=2 frame parser in CB_ReceiveCommand and are dispatched to HandleCommand(). Commands can also be sent over USB-CDC when RA_USB_DEBUG_ENABLED is set.

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Build Environments

Defined in platformio.ini. All environments target genericSTM32H725RGVX, use C++20, and enable the hardware FPU (-mfpu=fpv5-d16 -mfloat-abi=hard).

Environment	Source directory	Notes
MAIN	src/main/	Primary CanSat flight firmware
UAV	src/uav/	UAV variant
duty	src/test_duty/	Duty-cycle / stress test
test_path	src/test_path/	Path / guidance unit test
test_sd	src/test_sd/	SD card write test
test_unit	src/test_unit/	Unit tests
test_servoa	src/test_servoa/	Servo A test
test_servo_eeprom	src/test_servo_eeprom/	Servo + EEPROM test

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Key Source Files

File	Role
src/main/main.cpp	Top-level: hardware init, RTOS tasks, FSM, sensor reads
include/UserFSM.h	FSM state definitions
include/UserPins.h	GPIO pin assignments
include/UserSensors.h	Sensor driver wrappers
include/Controlling.h	Guidance + servo PID controller
include/custom_kalman.h	Kalman filter type aliases and init
include/XBeeDriver.h	XBee frame builder / parser
include/custom_EEPROM.h	Backup SRAM store struct + CRC
include/BNO086Cal.h	BNO086 magnetometer calibration
include/config/Main/UserConfig.h	Mission-specific tuning constants