Why does an error occurs in asynchronous mode when starting RxD pin?

For the serial interface in the H8 and H8S family, when a receive pin (a RxD pin) is low, start the receive operation to receive a break signal at the time the low pin was detected.

Next, a framing error occurs when the serial interface could not detect the STOP bit as “1” during STOP bit detection. 

Therefore, the serial interface can operate BREAK signal detection by checking if the received data is all 00s within framing error interrupt processing.

Also, be sure to clear the framing error flags to 0 when a high level is input to the RxD pin.

Please note that receive operation will be restarted to detect break signals when the error flag is cleared in low level.