How to read 32-bit I2C register using Arduino Wire library: A minimal example

The following code demonstrates how to read a register that is 4 bytes (32 bits) long over I2C. It will work with almost all I2C devices like EEPROMs, ADCs and others, provided you have the correct. Note that some devices like the LAN9303 have a slightly different addressing scheme or other peculiarities. In my opinion, it’s most efficient to just try out the standard way of reading a register and start from there.

Note that this code does not implement error handling for the sake of simplicity. Additionally, we wait for data using delay() instead of Wire.available(). This is a minimal example so it creates minimal confusion for the reader. We will provide a full example with error handling in a followup post.

Option 1: Reading the register into an uint32_t (recommended)

const uint8_t SLAVE_I2C_ADDRESS = 0b1010;
const uint16_t SLAVE_I2C_REGISTER_ADDRESS = 0x50;

Wire.beginTransmission(SLAVE_I2C_ADDRESS);
Wire.write(SLAVE_I2C_REGISTER_ADDRESS);
Wire.endTransmission();
Wire.requestFrom(SLAVE_I2C_ADDRESS, 4); // This register is 32 bits = 4 bytes long
delay(5); // Wait for data to be available
// Read directly into an uint32_t
uint32_t buf;
size_t actually_read = Wire.readBytes((uint8_t*)&buf, 4);
// Print register value
Serial.printf("Register value: %08lx\r\n", __builtin_bswap32(buf));

For an explanation on why we need __builtin_bswap32(), see How to print 32-bit uint32_t as eight hex digits in Arduino

Option 2: Reading the register into an uint8_t array

const uint8_t SLAVE_I2C_ADDRESS = 0b1010;
const uint16_t SLAVE_I2C_REGISTER_ADDRESS = 0x50;

Wire.beginTransmission(SLAVE_I2C_ADDRESS);
Wire.write(SLAVE_I2C_REGISTER_ADDRESS);
Wire.endTransmission();
Wire.requestFrom(SLAVE_I2C_ADDRESS, 4); // This register is 32 bits = 4 bytes long
delay(5); // Wait for data to be available
// Read into a 4-byte buffer
uint8_t buf[4];
size_t actually_read = Wire.readBytes(buf, 4);
// Print register value
Serial.printf("Register value: %02x%02x%02x%02x\r\n", buf[0], buf[1], buf[2], buf[3]);

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