Reading a temperature

Everything from the last seven lessons comes together in one read. To read the temperature from an LM75 at 0x48, the ESP32:

1. Sends a START: SDA falls while SCL is high. No data bit ever does that, so every chip knows a message is beginning.
2. Sends the address, 0x48, and a 0: write.
3. Gets an ACK from the chip on the ninth clock.
4. Writes the register number, 0x00, and gets another ACK.
5. Sends a repeated START, keeping the bus, then 0x48 with a 1: read. The chip acknowledges.
6. Clocks in two bytes that the chip drives onto SDA. The ESP32 ACKs the first and sends a NACK after the second: that is enough.
7. Sends a STOP: SDA rises while SCL is high, and the bus is free.

From two bytes to degrees

At 25.5 °C the two bytes are 0x19 and 0x80. Side by side they are 0x1980, which is 6528; divide by 256 and you get 25.5. Read as a signed 16-bit number, the same arithmetic works below zero.

Wire.beginTransmission(0x48);
Wire.write(0x00);                 // the temperature register
Wire.endTransmission(false);      // false: a repeated start, not a stop
Wire.requestFrom(0x48, 2);
int16_t raw = (Wire.read() << 8) | Wire.read();
float celsius = raw / 256.0;

The int16_t matters. On an ESP32 an int is 32 bits, and without the cast a reading below zero comes out as a large positive number.

Every I2C sensor is read like this: write which register, then read bytes. What changes from chip to chip is the address, the register numbers, and the arithmetic, and those are in its datasheet.