@JinGyu-Lee Fig.1: START - SLAVE ADDRESS 0x19 - WR - ACK --- DATA 0x01 - ACK - STOP Fig.2: START - SLAVE ADDRESS 0x33 - WR - NACK --- DATA 0x01 - NACK - STOP There are five conditions that lead to the generation of a NACK: No receiver is present on the bus with the transmitted address so there is no device to respond with an acknowledge. The receiver is unable to receive or transmit because it is performing some real-time function and is not ready to start communication with the master. ... /UM10204 I2C-bus specification and user manual Rev.6 by NXP/