🛠 Lire un capteur BME280 en I2C sur le CM3

Objectives

Enable and use the CM3's I2C bus (the BCM2837 exposes two on the SODIMM) to read a BME280 temperature/humidity/pressure sensor. You will learn how to enable the bus via config.txt, scan the bus with i2cdetect, and read registers in Python using smbus. Note: the CM3 has no internal ADC (datasheet RP-008161-DS §2) — any analog sensor requires an external converter over I2C or SPI.

Hardware

• CM3 on CMIO3, Raspberry Pi OS booted
• BME280 sensor (I2C, address 0x76 or 0x77 depending on the SDO pin)
• Wires: SDA to GPIO2, SCL to GPIO3, VCC to 3.3 V, GND (3.3 V logic if bank0 is set to VREF 3.3 V as on the CMIO3)

Complete code

# 1. Activer I2C : ajouter dans /boot/firmware/config.txt (ou via raspi-config)
#    dtparam=i2c_arm=on
sudo reboot
# 2. Outils et scan du bus
sudo apt install -y i2c-tools python3-smbus
sudo i2cdetect -y 1        # doit afficher 76 (ou 77)

#!/usr/bin/env python3
# bme280.py — lecture brute des registres
import time, smbus

BUS = smbus.SMBus(1)          # /dev/i2c-1 (contrôleur « arm »)
ADDR = 0x76

BUS.write_byte_data(ADDR, 0xF4, 0x2F)   # ctrl_meas : temp+pression, oversampling x16
time.sleep(0.1)
hi, lo = BUS.read_i2c_block_data(ADDR, 0xFA, 2)   # regs 0xFA/0xFB : température 20 bits
print('code brut température :', ((hi << 8) | lo) >> 4)

# En pratique, utiliser la couche haute niveau (paquet pip bmp280 par exemple).

Expected verification

sudo i2cdetect -y 1 displays the sensor's address 76 (or 77). The script prints a raw code that changes when you warm the sensor with your finger. With the high-level library, temperature, pressure, and humidity are displayed in physical units.

Common errors

• Forgetting dtparam=i2c_arm=on and rebooting: /dev/i2c-1 does not exist, smbus raises a FileNotFoundError.
• Address 0x77 not found: the sensor module's SDO pin is wired incorrectly (some modules change the default address).
• Powering the sensor with 5 V on a 1.8 V GPIO bank: I2C pins are tolerant to their bank's voltage, not beyond it — check the VREF rail.
• Confusing i2c0 (GPIO0/1) and i2c_arm (GPIO2/3): the "arm" bus corresponds to pins 2/3 on the CMIO3.
• Expecting a direct analog sensor: without an ADC on the SoC, use an MCP3008 (SPI) or a digital sensor.