The firmware can also be used to modify the EEPROM. The start bit ‘S’ is followed by the control byte with control code 1010 and the three chip select bits, A0, A1, and A2. The last bit in the control byte is the read (logic 1) and write (logic 0) bit. After the EEPROM receives the control byte, it sends a logic 0 signal back to the master. This is called the acknowledge (ACK) bit. Once the master has received the ACK bit, it sends one or more address bytes to the EEPROM. For each byte of data the EEPROM receives, it sends another ACK bit of logic 0 back to the MCU.