When using these modules, pay attention to the parameters. Specifically those related to timing. The comments in the code will allow you to determine how to set the timer values to correspond with your clock rate. Also, if you are using the examples, the "serial.v" code is configured to operate at 115,200 baud, N:8:1 settings, using a 49.152MHz clock. You can re-configure the BAUD rate generator to work with different clock frequencies fairly easily, or else use a different serial interface if you want.