To write on the I²C bus, the controller sends a START condition on the bus with the address of the target, as well as the last bit (the R/ W bit) set to 0, which signifies a write. After the target sends the acknowledge bit, the controller then sends the register address of the register to which it wishes to write. The target acknowledges again, letting the controller know it is ready. After this, the controller starts sending the register data to the target until the controller has sent all the data necessary (which is sometimes only a single byte), and the controller terminates the transmission with a STOP condition.

See the Section 8.6.2 section to see list of the TCA9539-Q1s internal registers and a description of each one.

Figure 8-7 shows an example of writing a single byte to a target register.

Figure 8-7 Write to Register

Figure 8-9 shows the Write to the Polarity Inversion Register.

Figure 8-8 Write to the Polarity Inversion Register

Figure 8-9 shows the Write to Output Port Registers

Figure 8-9 Write to Output Port Registers