Connection signal: the outputs that are set as connection signal output, indicate whether the connection with the PC is active or not - when the connection (Ethernet link, USB connection) is dropped, this signal state will change to indicate this. It can be enabled on selected pins, but is disabled by default.
Pin 54 is used for recovering the device if it gets misconfigured. This pin should not be connected to GND on device initialization, as the device will enter the recovery mode. When the device enters normal operation, this pin may be used for any digital IO function.