This example reads a Sensirion SHT3x sensor (SHT30 / SHT31 / SHT35) and prints the temperature and humidity once a second. It is written for a beginner and is a nice first soldering / wiring project - this is the sensor most weather stations and room monitors start from, because it is small, accurate, and cheap.

Why this sensor?

Lots of cheap sensors drift or lie. The SHT3x is factory-calibrated and, importantly, sends a checksum (a CRC-8) with every reading. This example checks it, so a loose wire shows up honestly as "checksum failed" instead of a plausible-looking wrong number - a great habit to learn early.

What you will need

An ESP32 board with a USB cable.
An SHT3x breakout (often sold as GY-SHT31 or on a Qwiic/STEMMA board).
Four jumper wires.

Part 1 - Wire it up (4 wires)

The sensor talks over I2C (two shared data wires).

SHT3x pin	ESP32 pin	Why
`VIN`/`3V3`	`3V3`	power
`GND`	`GND`	ground
`SDA`	GPIO 21	I2C data
`SCL`	GPIO 22	I2C clock

Most breakouts fix the address at 0x44. If yours ties the ADDR pin to 3V3, it is 0x45 instead - change the pc_sht3x_begin(0x44) call.

Part 2 - Flash and read

Open Sht3x.ino, upload, and open the Serial Monitor at 115200:

SHT3x ready
temp=22.417 C   humidity=41.882 %
temp=22.430 C   humidity=41.905 %

Breathe on the sensor and watch the humidity jump; pinch it between your fingers and watch the temperature climb. Those numbers are computed from the sensor's raw 16-bit readings on the ESP32 after the checksum passes.

Where this fits

pc_sht3x_read(&temp_mc, &rh_mpct) gives you the temperature and humidity in milli-units (thousandths - so 22417 means 22.417 C, 41882 means 41.882 RH). Integers keep it heap-free and avoid floating-point formatting. From here it is a short hop to a real project: serve the reading on a web page, publish it over MQTT, chart it over WebSocket, or push it to StatsD (example 59). This is the same "read a cheap breakout, bridge it onto the network" pattern the library uses for the RTC, the radar (example 62), and the touch pad (example 63).

Troubleshooting

**"SHT3x not found".** Check SDA->21, SCL->22, power, and whether your board is at 0x44 or 0x45. An I2C scanner sketch should find it.
**"read failed (checksum or bus error)".** Usually a loose or too-long wire. That message is the CRC doing its job - the reading was rejected rather than trusted.
Humidity reads high / low near walls or breath. That is real; the sensor responds to the air right at its surface. Give it a few seconds to settle after handling it.

Build and run (PlatformIO)

The feature lives in the library, so its flag must reach the whole build:

pio ci examples/Drivers/Sht3x \
  --board esp32dev --lib "." \
  --project-option="build_flags=-DPC_ENABLE_SHT3X=1"

(The Arduino IDE reads the flag from build_opt.h beside the sketch automatically.)

How it works (for the curious)

A single-shot measurement command (0x2400) tells the SHT3x to take a reading; ~15 ms later it returns six bytes: a 16-bit temperature word and its CRC, then a 16-bit humidity word and its CRC. pc_sht3x_crc8() recomputes each checksum (the Sensirion CRC-8: polynomial 0x31, starting at 0xFF - its documented check value 0xBEEF -> 0x92 is one of the unit tests), and pc_sht3x_parse() rejects the whole reading if either fails. The raw ticks convert linearly - T = -45 + 175 * raw / 65535, RH = 100 * raw / 65535 - done in fixed-point milli-units so there is no floating point. All of that is unit-tested on a PC (see test/test_sht3x); only the command write and the six-byte read run on the ESP32.