The crystal oscillator used in UART is of 11.059MHZ, why?
This small chart should help a lot:



There are any number of crystal frequencies that can be divided down by simple integer values to support standard UART Baud rates. Since many more low cost microcontrollers these days support higher frequencies of operation (such as 25MHz) I like to design with the 22.1184MHz crystal to achieve results a in this table.



Another popular crystal frequency for those MCUs that are rated for up to 20MHz operation is the 19.6608MHz frequency. This frequency however does not exactly support the 115.2K baud rate made very popular by the advent of the standard PC architecture when IBM first came out with their PC. Those early PCs used a frequency of 18.432MHz or 1.8432MHz for their UARTs. Using this latter frequency with a 20MHz rated CPU does mean that you give up more processing performance than using the 19.6608MHz frequency.