PCA9555: Function of SDA pull low FET in device block diagram

The SCL line is unidirectional (the PCA9555 does not do clock stretching).
The SDA line is bidirectional, as required by the I²C protocol. This FET is just the output driver.

Hi Ben,

As Clemen's pointed out the SDA FET is required for a bi directional signal since our device needs to drive low during I2C ACK's and during read transactions where the device will drive SDA low to send data to the I2C controller.

-Bobby