Hello,
The limitation for oscillator/crystal frequency is mainly Percent error difference between desired and actual UART baud rate.
For example: we select 8MHz crystal. The baud rate divisor = (crystal frequency/prescaler)/(desired baud rate * 16) as mentioned on chapter 7.8 from the datasheet. So, the baud rate divisor = 8.68 (or 2.17 for prescaler=4). Since the baud rate divisor is an integer number, we have to round it to 9 and the real baud rate will be 5555,56 bps. That presents more than 3.5% baud rate error. In that case, data over UART may be wrongly interpreted.
The acceptable percent error difference between desired and actual UART baud rate depends on a number of bits, but I would like to recommend to keep it definitely below 2.5%. This is the general issue of almost all UART transfers and you may found plenty of web pages with baud rate error calculations.
The optimum solution is using oscillator/crystal frequency which may be divided by 16 and desired baud rate without any reminder. For example 1.8432MHz/(16*57600bps) = 2.
The next solution is searching the oscillator/crystal frequency which will be at least close. For example:
Crystal = 12MHz and the baud rate divisor = 13 will result in 57692.3bps baud rate which presents just 0.16% error.
I hope this information helps.
Regards,
David
