I2C Protocol in Depth: Addressing, ACK, Pull-ups and Bus Speed

Addressing and a Complete Register Read

After START, the master sends a 7-bit address followed by a read/write bit (0 = write, 1 = read). The address byte on the wire is therefore the 7-bit address shifted left by one, plus R/W. Addresses 0x00 to 0x07 and 0x78 to 0x7F are reserved, leaving 112 usable 7-bit addresses.

MPU-6050 with AD0 tied low: 7-bit address 0x68
Write address byte: (0x68 << 1) | 0 = 0xD0
Read address byte:  (0x68 << 1) | 1 = 0xD1

Reading a register needs two phases: write the register number, then read the data. Using a repeated start keeps another master from taking the bus between them.

S  0xD0  A  0x75  A  Sr  0xD1  A  [data]  N  P
|  addr+W   reg      |  addr+R      byte    |  stop
start      WHO_AM_I  repeated start         master NACKs last byte

A = ACK from slave, N = NACK from master
Expected data for MPU-6050 WHO_AM_I: 0x68

#include <Wire.h>

const uint8_t MPU_ADDR = 0x68;   // AD0 pin low
const uint8_t WHO_AM_I = 0x75;

void setup() {
  Serial.begin(115200);
  Wire.begin();                  // Uno: SDA = A4, SCL = A5
  Wire.setClock(400000);         // Fast-mode, 400 kHz

  Wire.beginTransmission(MPU_ADDR);
  Wire.write(WHO_AM_I);          // register pointer
  uint8_t err = Wire.endTransmission(false);  // false = repeated start, no STOP
  if (err != 0) {
    Serial.print("No ACK, error code ");
    Serial.println(err);
    return;
  }

  Wire.requestFrom(MPU_ADDR, (uint8_t)1);      // read 1 byte, then STOP
  if (Wire.available()) {
    uint8_t id = Wire.read();
    Serial.print("WHO_AM_I = 0x");
    Serial.println(id, HEX);     // expect 0x68
  }
}

void loop() {
}

Wire.endTransmission() returns 0 on success, 2 when the address was not acknowledged and 3 when a data byte was not acknowledged. Checking it turns a silent failure into a useful message.