Check the clock frequency & that the baud rate required can be achieved. Error of <1 is never a problem, <2 will work but is undesirable, 5 will not work. Note that this assumes that the other end of the link has perfect timing, which will normally be true for a PC or USB serial port. If the other end of the link is another microcontroller check its exact baud rate and try to match that. Check the serial link configuration which must match the other end - most common is 8 bit, no parity, 1 stop bit.-Check the clock frequency & that the baud rate required can be achieved. Error of <1　is never a problem, <2　will work but is undesirable, 5　will not work. Note that this assumes that the other end of the link has perfect timing, which will normally be true for a PC or USB serial port. If the other end of the link is another microcontroller check its exact baud rate and try to match that. Check the serial link configuration which must match the other end - most common is 8 bit, no parity, 1 stop bit.