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Figure 5-6 shows the protocol to writing to a device that supports 10-bit addressing. At the beginning of a 10-bit address write, the controller sends a START condition. Then, the first byte is sent with the reserved address for 10-bit I2C addressing and is followed by a 0 for a write. The reserved address last two bits includes the first two bits of the 10-bit address. At this point there can be multiple targets that have the same first address byte for 10-bit addressing, so multiple devices can ACK this address. The second byte includes the target address. This byte is the eight least significant bits of the 10-bit target address.
With the second byte, there is presumably only one device with the unique 10-bit address. This is the only device that ACKs the communication. With the two bits of the reserved address and the eight bits of the second byte target address, this totals 10-bits of addressing.
This target stays in communication until the controller sends a STOP condition or until the controller sends a repeated START condition to communicate with a different target address.