Posted on September 14, 2011 at 16:42USART3 is only rated up to 2.25 Mbps, unless I'm mistaken.
What baud rate do you actually want to achieve?
If you need to hit specific frequencies you will need enough design flexibility to change the primary oscillator, PLL, and bus dividers to achieve your goals. Most of us tend to write software sufficiently transparently that we can change frequencies at a single point, and have the application/drivers compute appropriate downstream peripheral/clock settings.
