stm32samples/F3:F303/MLX90640-allsky

IR All-Sky Camera Documentation

Project Overview

This firmware implements an infrared all-sky camera system based on up to five MLX90640 thermal imaging sensors (32×24 pixels each), designed for cloud monitoring at the Special Astrophysical Observatory of the Russian Academy of Sciences. The system automates sky quality assessment for 0.5-meter telescopes of the "Astro-M" complex.

The device continuously captures thermal images from multiple sensors (Zenith, East, South, West, North), processes environmental data from a BMP280 pressure/humidity/temperature sensor, and controls PWM heaters to prevent dew formation on optics.

Repository: https://github.com/eddyem/stm32samples/tree/master/F3%3AF303/MLX90640-allsky

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Hardware Requirements

Microcontroller

• STM32F303 series (tested on F303)
• 72 MHz system clock (HSE) or 48 MHz (HSI)

Sensors

Sensor	Interface	Purpose
MLX90640 (up to 5)	I²C (400 kHz)	Thermal imaging (32×24 pixels, -40..+300°C)
BMP280 / BME280	SPI	Ambient temperature, pressure, humidity
NTC thermistors (4)	ADC (12-bit)	Heater feedback / backup temperature

Outputs

• DAC output (PA5) – Onboard heater control (0–3.3V)
• PWM channels (TIM3) – External heaters (PA6, PA7, PB0, PB1)
• UART1 – Main debug/auxiliary interface
• USB (CDC class) – Secondary interface

Pin Assignment (STM32F303)

Pin	Function	Notes
PA0–PA3	NTC1–NTC4	ADC inputs
PA4	DAC_OUT1	Onboard heater
PA5	ADC_IN2	DAC feedback
PA6, PA7	PWM CH1, CH2	TIM3_CH1, TIM3_CH2 (external heaters)
PA9, PA10	UART1 TX/RX	Controlling interface
PA11, PA12	USB DP/DM	CDC virtual COM
PA13, PA14	SWD	Programming/debug
PA15	USB DP pullup	USB pullup management for development board
PB0, PB1	PWM CH3, CH4	TIM3_CH3, CH4 (sky-quality + T-proportional)
PB3, PB4, PB5	SPI SCK, MISO, MOSI	BMP280 interface
PB6, PB7	I²C1 SCL/SDA	MLX90640 bus
PB9	SPI CS	BMP280 chip select

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Firmware Architecture

The firmware is event-driven with a state machine for MLX90640 data acquisition and a command parser for bidirectional communication over USB and UART.

Core Modules

File	Responsibility
main.c	Main loop, SysTick, initialization, state machine orchestration
adc.c/h	ADC setup (DMA, median filtering), NTC reading, Vdd measurement
i2c.c/h	I²C low-level + DMA, multi-word transfers, bus scanning
mlxproc.c/h	MLX90640 state machine (5 sensors), parameter caching, image processing
mlx90640.c/h	MLX90640 calibration math (from Melexis datasheet)
BMP280.c/h	SPI-based BMP280/BME280 driver
heater.c/h	PWM heater control, dew-point avoidance logic
commproto.cpp/h	Command parser (USB/UART), ASCII maps, binary image export
usb_dev.c/h	USB CDC stack (bulk endpoints, ring buffers)
usart.c/h	UART driver with DMA and ring buffers
spi.c/h	SPI driver for BMP280
ringbuffer.c/h	Circular buffers
strfunc.c/h	Number/string conversions, hexdumps

Data Flow

MLX90640 sensors (I²C DMA)
       ↓
mlxproc.c (state machine, 5× parameter loading)
       ↓
mlx90640.c (pixel-by-pixel temperature calculation)
       ↓
user requests via USB/UART (ASCII map, binary, temperature map)

BMP280 (SPI)
       ↓
Temperature, Pressure, Humidity, Dew point
       ↓
Heater logic (auto or setpoint) → PWM outputs

ADC (DMA + median filter)
       ↓
NTC temperatures, MCU temperature, Vdd

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MLX90640 Processing

State Machine (mlxproc.c)

The firmware manages up to 5 sensors (addresses 0x10..0x14 left-shifted for I²C). The state machine cycles through:

1. NOTINIT – Read calibration parameters (832 words) from each sensor over I²C DMA.
2. WAITPARAMS – Wait for DMA completion, calculate parameters via get_parameters().
3. WAITSUBPAGE – Poll REG_STATUS for NEWDATA flag, skip subpage 0.
4. READSUBPAGE – Read image data (832 words) via DMA, process with process_image().

Key features:

• Automatic sensor exclusion after MLX_MAX_ERRORS (default 11) consecutive errors.
• Tlastimage[] timestamps for each sensor (used by "cartoon mode" and timeout recovery).
• Supports changing sensor I²C addresses at runtime (mlx_sethwaddr()).

Temperature Calculation (mlx90640.c)

Implementation follows Melexis MLX90640 Datasheet (Section 11):

• Supply voltage compensation (kVdd, vdd25)
• Ambient temperature (KvPTAT, KtPTAT, vPTAT25)
• Gain normalization
• Offset correction (per-pixel offset[], kta[], kv[])
• TGC (temperature gradient compensation)
• Pixel sensitivity (alpha[]) + KsTa, KsTo, range detection (CT3, CT4)
• Extended temperature range (up to 400°C) via alphacorr[]

Output: float array of 768 temperatures (°C) per sensor.

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Environmental & Heater Control

BMP280/BME280 Readings

• Temperature (°C)
• Pressure (Pa, also converted to hPa and mmHg)
• Humidity (%)
• Dew point (°C) – calculated via Magnus formula.

Measurements are triggered every ENV_MEAS_PERIOD (10 seconds) in forced mode.

Heater Modes

Mode	Activation	Behavior
Auto (autoheater=1)	Humidity > 90%	Sets holding temperature = ambient + dew_over_delta (7°C) + 0.5°C (min 5°C)
Hold (setheater N)	Manual command	Maintains N°C ±5°C hysteresis using PWM
Off	Default	All heaters disabled

PWM channels:

• ch0, ch1 – External heaters (controlled identically for symmetry)
• ch2 – Proportional to ambient temperature: (T_ambient + 20) × 2 %, clamped 0–100
• ch3 – Proportional to (35°C – T_ambient + T_sky) × 2.5 %, clamped 0–100

Anti-overheating: If any NTC exceeds setpoint by >5°C, PWM is reduced to PWM_HOLD_VAL (10%).

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Command Protocol

The device presents a virtual COM port (CDC) over USB and a second UART interface. Both share the same command set.

Command Syntax

<command> [parameter] [= value]

• Commands are case-sensitive (lowercase).
• Parameters are decimal (or hex with 0x, binary with b).
• Use help for full list.
• Lines end with \n (newline).

Core Commands

MLX90640 Image Commands

Command	Description	Example
ascii n	Draw ASCII-art thermal map (16 grayscale levels)	ascii 0
tempmap n	Print numeric temperature grid (°C)	tempmap 1
binary n	Dump raw float array (768×4 bytes)	binary 2
acqtime n	Show last image acquisition timestamp (ms)	acqtime 3
cartoon	Toggle live ASCII animation over USB	cartoon

Sensor Management

Command	Description	Example
listids	List active I²C addresses of all sensors	listids
mlxaddr n [= addr]	Get/set I²C address for sensor n (0–4)	mlxaddr 2 = 0x15
mlxdump n	Dump all calibration parameters for sensor n	mlxdump 0
state	Show MLX90640 state machine status	state
mlxpause / mlxcont / mlxstop	Control acquisition	–

Environment

Command	Description	Example
environ	Print T, P, H, dew point, Tsky	environ
bmereinit	Reinitialize BMP280	–
ntc [n]	Get NTC temperature (°C) for channel n (0–3)	ntc 1

Heater Control

Command	Description	Example
autoheater [= 0/1]	Enable/disable automatic dew avoidance	autoheater = 1
setheater [= N]	Set holding temperature (°C)	setheater = 25
clearheater	Disable holding, turn off heaters	–
pwm [n] [= val]	Get/set PWM duty cycle (0–100%) for channel n	pwm 0 = 50

System & Debug

Command	Description	Example
adc [n]	Get raw ADC value (0–4095) for channel n	adc 4
dac [= val]	Get/set DAC output (0–4095)	dac = 2048
mcutemp	Show MCU internal temperature (°C)	–
mcuvdd	Show Vdd (V)	–
time	Show SysTick counter (ms)	–
reset	Software reset	–
sendstr = text	Forward string to other interface	sendstr = "hello"

Raw I²C (for debugging)

Command	Description	Example
iicscan	Scan I²C bus, report found addresses	–
iicaddr [= addr]	Get/set current I²C target address (non-shifted)	iicaddr = 0x33
iicspeed [= n]	Set I²C speed: 0=10k,1=100k,2=400k,3=1M,4=2M	–
readreg reg [= n]	Read n 16-bit registers	readreg 0x8000
writedata = v1 v2 ...	Write 16-bit values	writedata = 0x1234 0x5678
hwaddr addr	Change hardware I²C address of MLX90640 (non-shifted)	hwaddr 0x15

Output Formats

ASCII Thermal Map (ascii)

RANGE=12.345
MIN=-5.67
MAX=6.78
 .':;+*oxX#&%B$@
 .':;+*oxX#&%B$@
 ...

Uses 16‑character grayscale ramp: space (coldest) → @ (hottest).

Numeric Map (tempmap)

-2.3 1.2 3.4 ...
 5.6 7.8 9.0 ...
...

Binary Image (binary n)

BINARY0=<768×4 bytes raw float array>ENDIMAGE\n

Little-endian IEEE 754 float values. Exact size: 3072 bytes per image.

Environment Output

TEMPERATURE=22.34
SKYTEMPERATURE=-15.67
PRESSURE_HPA=1013.25
PRESSURE_MM=759.98
HUMIDITY=65.20
TEMP_DEW=15.60
T_MEASUREMENT=12345678

Sensors location

N	ID	Cardinal direction
0	0x10	Zenith
1	0x11	East
2	0x12	South
3	0x13	West
4	0x14	North

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Building & Flashing

Prerequisites

• ARM GCC toolchain (arm-none-eabi-gcc)
• Make
• st-flash, stm32flash or OpenOCD (for flashing MCU)

Build

git clone --depth=1 https://github.com/eddyem/stm32samples.git
cd stm32samples/F3:F303/MLX90640-allsky
make

Configuration

• EBUG flag in Makefile enables debug output (slower, but verbose).
• NOCAN – define if CAN peripheral not used (increases USB buffer).
• BUILD_NUMBER and BUILD_DATE in version.inc are auto-generated.

Flashing

make flash # for flashing over st-link
# make boot - for flashing over USART1 bootloader
# make dfuboot - for flashing over USB-DFU

or manually (over st-link):

openocd -f interface/stlink.cfg -f target/stm32f3x.cfg -c "program build/firmware.elf verify reset exit"

udev Rule (Linux)

Create /etc/udev/rules.d/99-USBiinterface.rules:

ACTION=="add", ENV{USB_IDS}=="0483:5740", ATTRS{interface}=="?*", PROGRAM="/bin/bash -c \"ls /dev | grep $attr{interface} | wc -l \"", SYMLINK+="$attr{interface}%c", MODE="0666", GROUP="tty"

After replug, device appears as /dev/ir-allsky0.

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Calibration & Tuning

NTC Thermistors

The firmware includes a lookup table (LUT) for NTC with B=3950, R25=1000Ω, covering -10..+59°C. If a sensor reads <5 ADC → short circuit; >4090 ADC → open circuit.

To calibrate your NTCs, modify Rlut[] in adc.c.

MLX90640

Calibration data is per-sensor and stored in EEPROM. The firmware reads it automatically on startup. No user calibration is required – the factory data is used.

Vdd Measurement

Internal Vrefint (1.2V typical) and factory calibration value *VREFINT_CAL_ADDR are used:

Vdd = (VREFINT_CAL * 3.3) / ADC_VREF

MCU Temperature

Internal sensor (ADC1_IN16) uses factory calibration TEMP30_CAL_ADDR and TEMP110_CAL_ADDR.

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Troubleshooting

No USB device detected

• Check USBPU_ON() in main.c – some boards need pull-up control, others constant.
• Verify PA11/PA12 are configured as AF14.
• Try different USB cable / port.

I²C communication errors

• Reduce speed: iicspeed = 1 (100 kHz).
• Check pull-up resistors (2.2k–10kΩ on SCL/SDA).
• Use iicscan to verify sensor addresses.

BMP280 not responding

• Check SPI wiring (PB3–PB5, PB9 CS).
• Try software reset: bmereinit.
• Verify chip ID: should be 0x58 (BMP280) or 0x60 (BME280).

Heater not working

• Check autoheater status: autoheater.
• Verify NTC readings: ntc.
• For manual mode: setheater = 25, observe pwm.

Cartoon mode stops updating

• Cartoon mode works only over USB, not UART.
• If no new images, check state – may be paused or in error.

Watchdog resets

• IWDG period is 2 seconds. Long operations (e.g., full calibration dump) may trigger reset.
• Disable in debug builds (remove IWDG_REFRESH calls or undefine EBUG).

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Performance & Limitations

Parameter	Value
Image update rate (5 sensors)	~1–2 Hz (depends on I²C speed)
RAM usage	~39 kB (image buffers + parameters)
Flash usage	~26 kB
Maximum sensors	5 (limited by I²C address space 0x10–0x14)
Temperature range	-40..+300°C (MLX90640)
Accuracy	±1°C (typical after calibration)

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License

All source files are licensed under GNU General Public License v3.0 unless stated otherwise.
The BMP280 driver includes MIT-licensed code from sheinz (2016).

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Credits & References

• Author: Edward V. Emelianov edward.emelianoff@gmail.com
• MLX90640 Datasheet: Melexis (Document REV 10)
• BMP280 Datasheet: Bosch Sensortec
• STM32F303 Reference Manual: STMicroelectronics (RM0316)

For questions, bug reports, or contributions, please use the GitHub issue tracker.