 A UART will detect a "framing error" when it does not see a "stop" bit at the expected "stop" bit time.As the "start" bit is used to identify the beginning of an incoming character, its timing is a reference for the remaining bits.If the data line is not in the expected state (high) when the "stop" bit is expected (according to the number of data and parity bits for which the UART is set), the UART will signal a framing error.