For UART asynchronous serial I/O mode, the IO Wizard did not allow me to select external clock for baud rate generator. Why ?
IO Wizard does not allow external clock source selection for UART asynchronous serial I/O mode due to following reason. For UART, IO Wizard would like to illustrate the correct setting of bit rate register as according to baud rate. In order to run the baud rate calculation, a known count source frequency value is required. Calculation cannot be carried out for external clock because it is an unknown frequency to IO Wizard engine.