When using the UART (RS-232 Serial Port) why is my baud rate incorrect?

Description

You may experience this problem when using the UART (RS-232 Serial Port) component in a Qsys design with Quartus® II software version 11.0.   This may result in the UART not synchronizing or communicating with the other end.

To work around this issue calculate the correct divisor_constant, by dividing the UART input clock frequency assigned to the UART, by the assigned Baud rate(bps), then verify this value is assigned to the divor_constant simulation varible in the Qsys generated <Qsys_system_name>_uart_0 file.  If needed change the assigned value to match your calculation.

For example, the input clock to the Qsys UART is 83MHz.  The assigned Baud rate(bps) = 115200.  The divisor should be 83,000,000 / 115200 = 720.  When verifying this value in the _uart_0 file you find the following lines of code:

//synthesis translate_on
//synthesis read_comments_as_HDL on
//  assign divisor_constant = 434;
//synthesis read_comments_as_HDL off

Change the divor_constant = 720 as shown below.

//  assign divisor_constant = 720;

This problem is fixed in Quartus II software version 11.0 SP1.