USART Baudrate 8 MBit @ 204MHz

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by hlsa on Fri Apr 18 03:59:19 MST 2014
Running the LPC4357 at 204 MHz, I would like to use the USART with 8 MBaud (this is the max. baudrate according to the datasheet). So far I failed to generate this baudrate. USART clock is connected to PLL1, which is also 204 MHz.

At first I tried to use the following baudrate settings:
define B8M_DLL         1
define B8M_DLM         0
define B8M_DIVADDVAL   7
define B8M_MULVAL      12

These values look fine at first glance. According to the formula given in the user manual in chapter "39.6.12 USART Fractional Divider Register", this should result in a baudrate of 8.0526 MBaud. However, it does not work. I assume that the problem is this tiny note given in the same chapter: "Important: If the fractional divider is active (DIVADDVAL > 0) and DLM = 0, the value of
the DLL register must be 3 or greater."

I tried to find any other combination of the register values, which allows to generate the 8 MBaud form the 204 MHz PLL. But there isn't any combination available. The fastest values I got were 6.375 MBaud, which works reliably. I also tried 12.75 MBaud, which also works without any problems, but it is out of specification.

Is there any way to get the 8 MBaud running at 204 MHz? Or do I need an external clock of 128 Mhz at GP_CLKIN to get the 8 MBaud running? Is there any other way?

Thanks in advance,

P.S. Please no advices to use USB or something similar. I need the UART interface.