From looking at the PCAL6408A datasheet I see that it is controlled by I2C (TWI). We do not have any example of using that device specifically, but you can use the TWI master driver to communicate with it. The TWI Sensor Example has a practical example of using the driver. For the details of how to control the PCAL6408A, you have to refer to it's datasheet.
In order to read and write single bytes you specify a length of 1 in the calls to nrf_drv_twi_rx() and nrf_drv_twi_tx(). Refer to the API documentation for details.
In order to read and write single bytes you specify a length of 1 in the calls to nrf_drv_twi_rx() and nrf_drv_twi_tx(). Refer to the API documentation for details.
