STM32U031C8 I2C Sensor Slave Project

Overview

This project implements an I2C slave interface on the STM32U031C8 MCU to control and read multiple sensors. The MCU acts as a slave device, receiving commands from a master over I2C and performing actions such as turning sensors on/off, reading sensor data, and configuring thresholds.

Supported sensors and modules:

-LED

-Infrared (IR) sensor

-Accelerometer

-GPS

-Battery charger status

-Real-time clock (RTC)

-Interrupts (MCU, IR, ACC)

Table of Contents:

1. Command Definitions

   1.1 Command & Response Format

   1.2 Examples

2. Sensor Thresholds & Configuration

   2.1 Accelerometer

   2.2 Infrared Sensor (IR)

   2.3 Battery Charging

1. Command Definitions:

Protocol Header

1.1 Command & Response Format:

Command structure: | CMD (1B) | SENSOR_ID (1B) | DATA_LEN (1B) | DATA (N Bytes) |

Response structure: | STATUS (1B) | DATA_LEN (1B) | DATA (N Bytes) |

1.2 Examples:

Command Description	Command	Expected Response
Turn ON LED	{0x10,0x01,0x00,{}}	{0x00,0x00,{}}
Turn OFF LED	{0x11,0x01,0x00,{}}	{0x00,0x00,{}}
Read LED status	{0x12,0x01,0x00,{}}	{0x00,1,{0x01}} (0x01=ON, 0x00=OFF)
Read IR data	{0x12,0x02,0x00,{}}	{0x00,5,{INT, int16 presenceVal, int16 motionVal}}
Set IR threshold	{0x13,0x02,0x01,{THS}}	{0x00,0x00,{}}
Read acceleration	{0x12,0x03,0x00,{}}	{0x00,1,{INT}}
Set ACC threshold	{0x13,0x03,0x01,{THS}}	{0x00,0x00,{}}
Read GPS data	{0x12,0x04,0x00,{}}	{0x00,12,{lat, lon, hMSL}}
Read battery status	{0x12,0x05,0x00,{}}	{0x00,3,{power_source, battery_soc, charge_status}}
Read current time	{0x12,0x06,0x00,{}}	{0x00,6,{YY,MM,DD,HH,MM,SS}}
RTC with GPS	{0x13,0x06,0x00,{}}	{0x00,0x00,{}}
Read MCU/IR/ACC interrupts	{0x12,0x07,0x00,{}}	{0x00,3,{MCU, IR, ACC}}

2. Sensor Thresholds & Configuration:

2.1 Accelerometer:

Interrupt code:

Code	Meaning
0x00	No motion detected
0x01	Motion detected

Default threshold: ACC_THS_DEFAULT = 0x04

Trigger values: 0–63 (1 LSB = fraction of ±2g full scale)

Threshold (mg) = FS(g) × (threshold / 64) × 1000

Threshold	FS(g) = ±2g (mg)
1	31 mg
2	62 mg
4	125 mg
8	250 mg
16	500 mg
32	1 g
63	1.97 g

Recommended values:

1–3: Very sensitive (tiny motion)

4–8: Medium motion (walking, light shake)

10–20: Strong motion (hit, fall)

2.2 Infrared Sensor (IR):

Interrupt code:

Code	Meaning
0x00	No motion detected
0x02	Motion detected
0x04	Presence detected

Frequency [Hz]= 1Hz = 1000ms Hysteresis Default (HYST = 32)

Detection:

Compares two internally filtered signals

Event flag set if difference > threshold

Flag cleared when signal < (threshold − hysteresis)

Threshold	Hysteresis	Approx. Signal Change	Use Case
100	32	≥0.05°C	Very sensitive, long range
150	32	≥0.075°C	Sensitive, moderate range
200	32	≥0.1°C	Balanced sensitivity/stability
250	32	≥0.125°C	Less sensitive, shorter range
300	32	≥0.15°C	Low sensitivity, short range
400	32	≥0.2°C	Minimal sensitivity, very stable

2.3 Battery Charging:

Power Source Code	Meaning
000b	Not powered from VBUS
100b	Unknown adapter
111b	Boost OTG

Charge Status Code	Meaning
000	Not charging
001	Trickle charge
010	Pre-charge
011	Fast charge (CC mode)
100	Taper charge (CV mode)
110	Top-off timer active charging
111	Charge termination done