I also try to test with EVAL Board LPC54608 with Flash magic the baud rate can be run 115200.
Now I found the problem come from RS232 driver use part MAX232 (5V). Change to MAX3232 (3.3V to 5V) then can solve this issue.
1.MAX232 can used with LPC1766 (Customer prototype board)
2.MAX232 can used with LPC54608 (NXP EVAL board)
3.MAX232 can't use with LPC54606 (Customer prototype board)
Can you explain MAX232 (5V) why can't use with LPC54606? Please advise.